Chapter 5

Macronutrients and energy balance

Macronutrients: introduction

Protein

Fats

Carbohydrate

Energy balance

Macronutrients: introduction

The macronutrients are protein, fat, and carbohydrate, and they are required in gram amounts. They are major sources of energy as well as providing essential nutrients such as some fatty and amino acids.

Protein

Protein provides approximately 10–15% of the energy in the diet. Protein is essential for numerous structural and functional purposes and is essential for growth and repair of the body. In adults, approximately 16% of body weight is protein. 43% of this is muscle, 15% skin, and 16% blood. Protein is in a constant state of flux in the body with protein being synthesized and degraded continuously.

Protein flux (Q) can be described by the following equation:

image

where I = intake, D = degradation, S = synthesis, and O = oxidation to CO2 and urinary nitrogen.

Function

Protein has numerous functions in the body. Recent research has increasingly been focused on the role of amino acids in metabolism, e.g. the role of branch chain amino acids in glucose homeostasis, particularly insulin resistance. Examples of the different functions of protein are as follows.

Structural: Protein is important for the structure of the body and about half of the body’s protein is found in structural tissues such as skin and muscle. These structural proteins are collagen (25% of the body’s protein), actin, and myosin.

Transport: Proteins act as transport carriers in the blood and body fluids for many molecules and nutrients, e.g. haemoglobin, lipoproteins.

Hormonal: Hormones and peptides are proteins or amino acid chains, e.g. insulin, pancreatic polypeptide.

Enzymes: All enzymes are proteins. Extracellular enzymic proteins include the digestive enzymes, e.g. amylase. Intracellular enzymes are involved in metabolic pathways, e.g. glycogen synthase.

Immune function: Antibodies are protein molecules. Proteins are also involved in the acute phase response to inflammation.

Buffering function: The protein albumin acts as a buffer in the maintenance of blood pH.

Structure

Proteins are macromolecules consisting of amino acid chains. Amino acids are joined to each other by peptide bonds (Fig. 5.1). Amino acids form peptide chains of various lengths from two amino acids (dipeptide), four to ten peptides (oligopeptides) and more than ten amino acids (polypeptides). Reactive side groups of the amino acids combine to form links between amino acids in the chain and other peptide chains. The polypeptides form pleated sheets or helices. Polypeptides fold and cross-links form between amino acids to stabilize the folds. Proteins are formed by the combination of polypeptides. These cross-links give the peptide a distinctive function and shape (Fig. 5.2). There are approximately 20 amino acids and each has a different side group, size, and different properties, e.g. pH, hydrophilic or hydrophobic. These properties are used in the analysis of amino acids.

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Fig. 5.1Formation of a polypeptide.

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Fig. 5.2Formation of a protein.

Indispensable (essential) amino acids

Some amino acids can be synthesized by the body but others must be supplied by the diet. These are known as indispensable or essential amino acids; there are eight essential amino acids (Table 5.1). Some amino acids are only essential in specific circumstances and are classified as conditionally indispensable. In childhood, other amino acids are essential that are not essential in adults (arginine, histidine, cysteine, glycine, tyrosine, glutamine, proline). These amino acids are essential in children because they are required in amounts larger than can be synthesized because of high demand, immature biological pathways, or a combination of these. Conditionally indispensable or essential amino acids only become essential in circumstances when the requirement is i, e.g. glutamine in certain clinical conditions.

Table 5.1 Classification of amino acids

Indispensable/essential amino acids Indispensable (conditionally essential) amino acids Dispensable (non-essential) amino acids
Leucine (Leu) Tyrosine (Tyr) Glutamic acid (Glu)
Isoleucine (Ile) Glycine (Gly) Alanine (Ala)
Valine (Val) Cysteine (Cys) Aspartic acid (Asp)
Phenylalanine (Phe) Arginine (Arg)
Threonine (Thr) Proline (Pro)
Methionine (Met) Histidine (His)
Tryptophan (Trp) Glutamine
Lysine (Lys) Serine (Ser)
Asparagine (Asn)

Requirements

The amino acid content of a protein determines its biological value. Proteins that contain all the indispensable amino acids in sufficient quantities have high biological value. High biological value proteins are from animal sources, e.g. meat, eggs, milk, dairy products, and fish. If one, or more, indispensable amino acid(s) is absent from a protein, it will have low biological value. Generally, plant proteins are of low biological value. The indispensable amino acid that is in shortest supply is known as the limiting amino acid. By combining foods with low biological value, it is possible to provide all indispensable amino acids in the diet; this is important in vegan diets. For example, the limiting amino acid in wheat is lysine and in pulses it is methionine. A diet combining wheat products such as bread with pulses will provide all the indispensable amino acids, e.g. pitta bread and dhal.

As already stated, protein is constantly being turned over; 3–4 g of protein are turned over per kg of body weight per day. Each day 10–15 g of nitrogen are excreted in urine (6.25 g protein is equivalent to 1 g nitrogen). Small amounts are lost in faeces and skin. When nitrogen (protein) intake equals nitrogen excretion, the body is said to be in nitrogen balance. Healthy adults will be in positive nitrogen balance. Nitrogen balance studies have been used to derive the recommended requirements listed in Table 5.2.

Table 5.2 Recommended nutrient intake of protein for all age groups and average daily intakes of protein of adult men and women in the UK*

Age Weight, kg RNI, g/day
Children (both sexes)
0–3 months 5.9 12.5
4–6 months 7.7 12.7
7–9 months 8.8 13.7
10–12 months 9.7 14.9
1–3 years 12.5 14.5
4–6 years 17.8 19.7
7–10 years 28.3 28.3
Males
11–14 years 43.0 42.1
15–18 years 64.5 55.2
19–50 years 74.0 55.5
50+ years 71.0 53.3
Females
11–14 years 43.8 41.2
15–18 years 55.5 45.4
19–50 years 60.0 45.0
50+ years 62.0 46.5
Additional RNI required for females
During pregnancy +6.0
Lactation: 0–6 months +11.0
Lactation: 6+ months +8.0
Adults (19-64 years) Average daily intake UK, g/d
Men 87.4
Women 66.6

* Source: data for RNIs, Department of Health (1991). Dietary reference values for food and nutrients for the United Kingdom. HMSO, London. Source: data for average daily intakes for adults, Public Health England & Food Standards Agency (2018). The National Diet and Nutrition Survey: Results from Years 7 and 8 (combined) of the Rolling Programme (2014/2015–2015/2016). Public Health England, London.

Deficiency

If energy intake is insufficient, protein will be degraded to produce energy; ∴ protein deficiency can occur when the diet does not provide enough protein or energy or a combination of both. Protein energy malnutrition (PEM) is a major cause for concern in developing countries (see image…… Chapter 20, ‘Global nutrition’, p. 435), but does occur in the UK amongst at risk groups. These include immunocompromised individuals (e.g. AIDS), people with anorexia nervosa, and cancer patients with cachexia. Mild PEM is fairly common amongst surgical or elderly hospital patients. Protein deficiency can also occur as the result of ↑ losses in renal disease, ↑ catabolism in trauma, burns, or sepsis, or malabsorption. Protein deficiency results in muscle wasting, stunted growth, poor wound healing, and susceptibility to infection, oedema, and fatty liver.

Sources of dietary protein

In the typical UK diet, 60% of protein intake has high biological value. High biological protein is supplied by meat and meat products, fish, eggs, and milk and dairy products (Table 5.3). Plants such as cereals and pulses supply proteins of low biological value.

Table 5.3 Contribution of food sources to protein intake in UK adults*

Food group % Daily intake
Cereals and cereal products (including bread) 22
Red meat (all meat excluding chicken and turkey) 20
Chicken, turkey and dishes 17
Milk and milk products 13
Bread 11
Fish and fish dishes 7

* Source: data from Public Health England & Food Standards Agency (2018). The National Diet and Nutrition Survey: Results from Years 7 and 8 (combined) of the Rolling Programme (2014/2015–2015/2016). Public Health England, London.

Fats

Fats are often referred to as lipids. Lipids are described by chemists as substances that are poorly soluble or insoluble in water but are soluble in organic solvents. Fat is the term most often used when discussing foods and lipid metabolism. Over 95% of dietary fats are triglycerides (triacylglycerols); other types of fat include cholesterol, phospholipids, and sterols.

Function

The functions of fat in the diet are:

Energy source—fat provides 37 kJ (8.8 kcal) per gram.

Fat provides essential fatty acids.

Fat is a carrier for fat soluble vitamins A, D, E, and K.

↑ Palatability by improving taste perception and appearance of food.

Some fats are important constituents of cell membranes and can be converted to biologically active compounds, such as steroid hormones, interleukins, thromboxanes, and prostaglandins.

Cholesterol is converted to bile acids, which are important in digestion.

Fatty acids

Fats consist of fatty acids that have carbon chains containing up to 22 carbon molecules in the chain. The type of fatty acid attached to the glycerol molecule determines its physical properties, nutritional function, and physiological function. Hydrogen is added to unsaturated fatty acids to make them more solid when manufacturing some food products such as vegetable spreads; this process is known as hydrogenation.

Fatty acids are carbon molecules with a methyl group at one end and a carboxyl acid at the other (Fig. 5.3). They can have chains of 4–22 carbon molecules, although most have 16–18. Hydrogen atoms are attached to the carbon chain; the number of hydrogen atoms determines the degree of saturation (with hydrogen atoms) of the fatty acid. A fatty acid with hydrogen atoms on every arm is ‘saturated’. Unsaturated fatty acids contain double carbon bonds where there is no hydrogen (Fig. 5.3). If there is only one double bond, the fatty acid is monounsaturated. When more than one double bond is present, the fatty acid will be polyunsaturated.

Fatty acids have a common name, e.g. linoleic acid, a systematic name, and a notational name. The systematic name reflects the number of carbon atoms, and the number of double bonds, so that linoleic acid becomes octadecadienoic acid. This represents 18 carbons (octadeca-) and two double bonds (di-). The notational name for linoleic acid is 18:2 n6 or 18:2 6; again this represents 18 carbon atoms and two double is also represented as :2 after 18. The position is relative to the methyl (or omega) end of the carbon chain. Linoleic acid has its first double bond between the sixth and seventh carbons. Common names, systematic names, and notational names are shown in Table 5.4. Table 5.5 shows the average intakes of fat and DRVs. Table 5.6 shows the main sources of fat in the UK adult diet.

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Fig. 5.3Structure of fatty acids.

Saturated fatty acids

Saturated fatty acids (SFA) contain carbon atoms linked by single bonds and hydrogen on all available arms; they have a relatively high melting point and tend to be solid at room temperature. SFA are obtained from animal storage fats and their products, e.g. meat fat, lard, milk, butter, cheese, and cream. Fats from plant origin tend to be unsaturated with the exception of coconut oil and palm oil. Some manufactured margarines and spreads contain significant amounts of SFA. Plasma low-density lipoprotein (LDL) cholesterol, and ∴ plasma cholesterol, tends to be raised by dietary SFA. High intakes of SFA are associated with atherogenesis and cardiovascular disease.

Monounsaturated fatty acids

Monounsaturated fatty acids (MUFA) contain only one double bond and are usually liquid (oil) at room temperature. Olive oil and rapeseed oil are the most concentrated dietary sources of MUFA. MUFA are present in many foods, including meat fat and lard. Dietary MUFA do not raise plasma cholesterol, and lower LDL without a detrimental effect on high-density lipoproteins (HDL).

Polyunsaturated fatty acids

Polyunsaturated fatty acids (PUFA) contain two or more double bonds and are liquid at room temperature. They are easily oxidized in foods and in the body. PUFA are involved in the metabolism of cholesterol, are components of phospholipids in cell membranes, and are precursors of biologically active compounds such as prostaglandins, interleukins, and thromboxanes. Therefore, they have a vital role in the immune response, blood clotting, and inflammation. PUFA are derived from the essential fatty acids linoleic acid (n6 or 6) and -linoleic acid (n3 or 3), and are ∴ divided into omega 3 (3) or omega 6 (6) groups of PUFA. Essential fatty acids (EFA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) are important in neural development of the fetus and infant. PUFA occur as cis or trans forms depending on the way the hydrogen atoms are arranged. In cis formation the hydrogen atoms are bonded to either end of the double bond on the same side; whereas, in the trans form the hydrogen atoms are on opposite sides (Fig. 5.3). Most naturally occurring fats are in the cis form.

Omega () 3 PUFA

3 PUFA (and parent essential fatty acid -linoleic acid) are found in fish and fish oils and their health benefits are being more fully explored. The health benefits of ↑ consumption of oily fish include improved cardiovascular risk factors. The Western diet contains a high ratio of 6:3 PUFA; a lower ratio is recommended. Research studies have shown benefits in cognitive function but epidemiological studies are required.

Table 5.4 Nomenclature of fatty acids

Common name Notational name Systematic name
Saturated fatty acids
Butyric 4:0 Tetranoic
Caproic 6:0 Hexanoic
Caprylic 8:0 Octanoic
Capric 10:0 Decanoic
Lauric 12:0 Dodecanoic
Myristic 14:0 Tetradecanoic
Palmitic 16:0 Hexadecanoic
Stearic 18:0 Octadecanoic
Arachidic 20:0 Eicosanoic
Behenic 22:0 Docosanic
Monounsaturated fatty acids
Palmitoleic 16:1n7 cis-9 hexadecenoic
Oleic 18:1n9 cis-9 octadecenoic
Elaidic 18:1n9 trans-9 octadecenoic
Eicosenoic 20:1n9 cis-11 eicosaenoic
Erucic 22:1n9 cis-13 docosaenoic
Polyunsaturated fatty acids
Linoleic 18:2n6 cis, cis-9,12 octadecadienoic
-linolenic 18:3n3 cis-9,12,15 all octadecatrienoic
-linolenic 18:3n6 trans-5, cis-9, cis-12 octadecatrienoic
Arachidonic 20:4n6 cis-5, 8, 11, 14 eicosatetraenoic
EPA 20:5n3 Eicosapentaenoic
DHA 22:6n3 Docosahexaenoic
Trans fatty acids

Trans fatty acids are rare in naturally occurring fats. Some are made in the rumen of cows and sheep, and ∴ are found in lamb, beef, milk, and cheese. However, the most significant source of trans fatty acids in the diet is through hydrogenation of PUFA to produce more solid forms of vegetable oils for spreads, margarines, and some food products. Trans fatty acids have been associated with adverse effects on lipoprotein status by elevating LDL and depressing HDL, although further research is required. It is recommended that intake should not exceed 1.2% of total energy intake.

Essential fatty acids

Linoleic and -linoleic acids are essential fatty acids. Other longer chain fatty acids such as arachidonic, EPA, and DHA are physiologically important but can be synthesized to a limited extent from linoleic and -linoleic acids. These longer chain fatty acids are not essential fatty acids but their intake may become critical in fatty acid deficiency. EFA are most commonly found in plant and fish oils. Deficiency of linoleic acid has been demonstrated in children, although a deficiency of -linoleic acid has not been seen in healthy people. This has → debate about the essentiality of -linoleic acid. Deficiency is characterized by a scaly dermatitis. The recommended intake of linoleic acid is at least 11% of total energy and 0.2% for -linoleic acid.

Sterols

Sterols are relatively simple molecules; the most common sterol is the wax-like cholesterol. Cholesterol and cholesterol ester (cholesterol to which a fatty acid is attached) are only found in animal foods. Phytosterols are found in plant foods. Cholesterol has structural roles in lipoproteins and membranes and is a precursor for bile acids, steroid hormones, and vitamin D. Dietary cholesterol has little influence on plasma levels as most circulating cholesterol is endogenous. Reduction of intake of saturated fat results in lower plasma cholesterol levels.

Lipid transport

Fat digestion and absorption are covered in image…… Chapter 1, ‘Digestion’ p. 16. Lipids are not soluble in water, and ∴ complex with apolipoproteins to form water-miscible compounds. Approximately 2% of total plasma lipids are free fatty acids and are transported as compounds of albumin. The remainder of the lipids are carried in the blood as lipoproteins. Lipoproteins are identified by the apoliprotein that is present (apo A, apo B, apo C, apo D, apo E, apo F, apo H, and apo L). There are five classes of lipoproteins, which vary in density:

Chylomicrons.

Very low-density lipoproteins (VLDL).

Low-density lipoproteins (LDL).

Intermediate density lipoprotein (IDL).

High-density lipoproteins (HDL).

In addition, lipoprotein (a) complex of LDL with apolipoproteins (A), may be formed; this particle is highly atherogenic.

High and low levels of the lipoproteins have adverse effects on health. High levels of LDL are associated with ↑ health problems and LDL is colloquially known as ‘bad cholesterol’. HDL is colloquially known as ‘good cholesterol’ (see Table 5.7 for dietary sources).

Table 5.5 Average intake for adults compared with DRVs for fat for adults (as a percentage of daily food energy intake) in the UK*

Average intake (% daily food energy) DRV (% food energy intake)
Men Women
Total fat 34.4 35.0 35
Saturated fat 12.3 12.7 11
Trans fatty acids 1.1 0.9 <1.2

* Source for average daily intakes for adults, Public Health England & Food Standards Agency (2018). The National Diet and Nutrition Survey: Results from Years 7 and 8 (combined) of the Rolling Programme (2014/2015–2015/2016). Public Health England, London.

Table 5.6 Sources of total fat, saturated and trans fatty acids in the diet of adults in the UK (NDNS)*

Food Total fat, % Saturated fatty acids, % Trans fatty acids, %
Meat, meat products, and meat dishes 22 23 28
Cereal and cereal products 21 21 18
Milk and milk products 12 21 30
Vegetables, potatoes 12 5 7
Fat spreads 8 8 9

* Source for average daily intakes for adults, Public Health England & Food Standards Agency (2018). The National Diet and Nutrition Survey: Results from Years 7 and 8 (combined) of the Rolling Programme (2014/2015–2015/2016). Public Health England, London.

Chylomicrons

Chylomicrons mainly consist of triglycerides as they transport dietary lipids. Plasma levels rise after eating and are negligible in the fasting state. Chylomicrons leave the enterocytes of the small intestine and enter the lymphatic system before transferring to blood vessels. The triglycerides are hydrolysed by lipoprotein lipase, so releasing fatty acids that are used for energy or stored in adipose tissue. The life cycle of a chylomicron is 15–20 minutes and the liver clears the remnant from the blood. Fat-soluble vitamins reach the liver as part of the remnant.

Very low-density lipoproteins

VLDL are synthesized in the liver and are large particles that are rich in triglycerides. They deliver fatty acids to adipose tissue, muscles, and heart where lipoprotein lipase facilitates their release from triglycerides. The enzyme in the heart has a high affinity for triglyceride and, when triglyceride concentrations are low, they are preferentially released into heart tissue. Following release of triglycerides the remaining remnants are intermediate-density lipoproteins (IDL), which are the precursors of LDL (see Table 5.8).

IDL

IDL are formed from the degradation of VLDL; they are either degraded further to LDL or are rapidly cleared by the liver.

LDL

LDL contain mainly cholesterol and cholesterol ester as they are the end product of VLDL metabolism. They carry approximately 70% of plasma cholesterol and are taken up by the liver and other tissues.

HDL

The liver and intestine synthesize and secrete HDL. HDL is involved in the reverse transport of cholesterol from tissues, especially arteries and arterioles, to the liver or transfers it to other lipoproteins.

Table 5.7 Dietary sources of cholesterol

Cholesterol content Food
High Liver, offal
Eggs, mayonnaise
Shellfish
Fish roe
Medium Meat fat
Full fat milk and dairy produce, e.g. cream, cheese, butter
Meat and fish products
Manufactured meat products, e.g. pies
Low Skinless poultry
Skimmed milk and dairy products, e.g. cottage cheese, low fat yoghurt
Cholesterol free Fruit (including avocados and olives) and vegetables
Vegetable oils
Cereals, pasta
Rice
Egg white
Sugar

Table 5.8 Functions of plasma lipoproteins

Lipoprotein Function
Chylomicrons Transport dietary lipids to peripheral tissues and liver
VLDL Transports lipids from liver to peripheral tissues
LDL Transports cholesterol to peripheral tissues and liver
HDL Removes cholesterol from peripheral tissues to the liver

Carbohydrate

Carbohydrates are the most significant source of energy in the diet (see image this Chapter ‘Energy balance’, p. 86). In developing countries up to 85% of energy in the diet is provided by carbohydrate; this figure is as low as 40% in some developed countries. The relationship between dietary carbohydrates and fat is usually reciprocal. Diets rich in fat will have low levels of carbohydrates and vice versa.

Structure and classification

The empirical formula for carbohydrates is Cx(H2O)y; glucose is the simplest carbohydrate (C6H12O6 or C6(H2O)6) (Fig. 5.4). Simple carbohydrates (monosaccharides) can combine to form disaccharides, e.g. sucrose (C12H22O11) from two disaccharides, oligosaccharides, e.g. raffinose which is formed from three to nine monosaccharides, or polysaccharides, which form from ten or more saccharides, e.g. starches.

image It is important to recognize that the physical effects (food matrix) of a carbohydrate may influence its nutritional properties.

Carbohydrates that can be digested and absorbed in the small intestine and → ↑ in blood glucose levels are referred to as glycaemic carbohydrates (Table 5.9). Plant polysaccharides that cannot be digested (non-glycaemic) are referred to as fibre or non-starch polysaccharides (NSP). Sugar alcohols, e.g. sorbitol, are also classified as carbohydrates, although their empirical formula is slightly different.

image

Fig. 5.4Carbohydrate molecules.

Table 5.9 Classification of carbohydrates in the diet (FAO/WHO 1998)*

Glycaemic Non-glycaemic
Monosaccharides Oligosaccharides
Glucose
Fructose
Galactose
Raffinose, stachyose, verbascose
Human milk oligosaccharides
Fructo-oligosaccharides
Inulin
Disaccharides
Sucrose
Lactose
Maltose
Trehalose
Polysaccharides Non-starch polysaccharides
Starch—amylopectin, amylose, modified food starches Cellulose (insoluble)
Hemicellulose (soluble and insoluble forms)
-glucans (mainly soluble)
Fructans, e.g. inulin (not assayed by current methods)
Gums (soluble)
Mucilages (soluble)
Algal polysaccharides (soluble)
Sugar alcohols
Sorbitol
Xylitol
Mannitol

* Source: data from WHO/FAO (1998). Carbohydrates in human nutrition, FAO food and nutrition paper no.66. FAO, Rome.

Sugar alcohols are only partially absorbed.

Sugars (mono- and disaccharides)

Monosaccharides include glucose, fructose, and galactose. The monosaccharide glucose is found in small amounts in fruit and vegetables, but is not abundant in natural foods. It is made from starch and used commercially. Fructose is found in honey, fruit, and vegetables and is manufactured from fructose-rich corn syrup for the food industry. Sucrose is the commonest disaccharide and is extracted from sugar beet or sugar cane. Table sugar is 99% sucrose and the major dietary source of disaccharides. Sucrose is hydrolysed into glucose and fructose. Lactose is found in milk and milk products. It is hydrolysed to glucose and galactose. Maltose is present in malted wheat and barley. Malt extract is used in brewing and in malted products.

Oligosaccharides

Raffinose, stachyose, and verbascose are oligosaccharides that are made of galactose, glucose, and fructose. They are found in legumes and seeds. Humans do not have the enzyme needed to digest them but they may be fermented in the colon. Fructo-oligosaccharides and inulin have been shown to stimulate growth of the potentially beneficial bifidobacteria in the colon.

Polyols

Sorbitol, inositol, and mannitol are polyols (sugar alcohols) that are only partially absorbed and ∴ provide less energy than the corresponding sugars. They have been used as sugar substitutes. Small amounts occur naturally, but significant amounts in the diet come only from manufactured foods. Large amounts can cause osmotic diarrhoea.

Starch

Starch is the main storage polysaccharide in plant cells and is found in large quantities in cereal grains, potatoes, and plantains. Starch is the largest source of carbohydrate in the diet. Starch consists of two glucose polysaccharides: amylose and amylopectin. The linkages between the glucose molecules are hydrolysed by the action of saliva and pancreatic amylases. Many factors affect the rate at which hydrolysation occurs so that some starches are readily digested while others pass undigested into the colon. This has resulted in the classification of starches (Table 5.10) into rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS). Both RDS and SDS are digested in the small intestine while RS passes undigested into the colon where it is available for fermentation.

Fibre

Dietary fibre as defined by the Scientific Advisory Committee on Nutrition (SACN)1 ‘…encompasses all carbohydrates that are naturally integrated components of foods and that are neither digested nor absorbed in the small intestine and have a degree of polymerisation of three or more monomeric units, plus lignin’. SACN also recommended that the term fibre be used rather than non-starch polysaccharide. Fibre can be classified as soluble (in water at pH 7.0) or insoluble, and it is this classification that categorizes the function of these polysaccharides. Insoluble fibre consists mainly of cellulose and some hemicelluloses. Insoluble fibre binds to water in the colon and swells. This stimulates peristalsis so ↑ transit time in the colon thereby reducing the risk of constipation and possibly reducing the risk of colon cancer. Soluble fibre blunts the response of blood glucose to ingestion. The reabsorption of bile acids is slowed by soluble fibre so ↑ cholesterol losses in faeces and reducing blood cholesterol levels. Diets high in fibre are associated with ↑ risk of bowel cancer, type 2 diabetes, and cardiovascular disease (CVD). Table 5.11 lists sources of soluble and insoluble fibre in the diet.

Intrinsic sugars

These are sugars that are present in intact cells, e.g. fructose in whole fruit and sugars in milk, i.e. lactose and galactose.

Free sugars

Sugars that are in a free or readily absorbable state, e.g. added sugars (usually sucrose), or released from disrupted cells, e.g. fructose in fruit puree or juice. Previously known as non-milk extrinsic sugars (NMES). Diets high in free sugars are associated with ↑ risk of dental caries, obesity, and type 2 diabetes, and contribute to the development of dental caries.

Recommended intakes

Sugar and starch

SACN recommended that the average intake of free sugars should be <5% of the daily food energy. Starches, intrinsic sugars, and milk sugars should provide the balance of dietary energy not provided by alcohol, protein fat, and free sugar, which is on average 36.2% in the UK (Tables 5.125.14).

Fibre

It is recommended that the adult diet contain 30 g/day (see Table 5.15).

Table 5.10 Classification of starch

Class Glycaemic response Food source
Rapidly digestible starch Large Cooked starchy cereals, warm potatoes
Slowly digestible starch Small Muesli, oats, pasta, legumes
Resistant starch None Unripe bananas, whole grains, starchy foods e.g. potatoes that have been cooked and cooled

Table 5.11 Dietary sources of soluble and insoluble fibre in the diet

Soluble fibre Insoluble fibre
Apples Beans
Barley Brown rice
Citrus fruits Fruits with edible seeds
Guar gum Lentils
Legumes Maize
Oats Oats
Pears Pulses
Strawberries Wheat bran
Wholemeal breads
Wholemeal cereals
Wholemeal pasta
Whole wheat flour
Peas

Table 5.12 Daily carbohydrate and NMES intake of adults (NDNS)*

Men Women
Total carbohydrate, g/day 249 199
% food energy 47.6 47.7
DRV, % food energy 50.0 50.0
Free sugars, g/day 64.3 50.0
% food energy 11.9 11.6
DRV, % food energy <5 <5
AOAC Fibre, g/day 20.7 17.4
DRV, g/day 30 30

* Source for average daily intakes for adults, Public Health England & Food Standards Agency (2018). The National Diet and Nutrition Survey: Results from Years 7 and 8 (combined) of the Rolling Programme (2014/2015–2015/2016). Public Health England, London.

Table 5.13 Sources of carbohydrate in the diet (NDNS)*

Food group % Daily intake
Cereals and cereal products including bread 46
Bread 18
Potatoes and savoury snacks 10
Non-alcoholic beverages 7
Alcoholic beverages 3

* Source for average daily intakes for adults, Public Health England & Food Standards Agency (2018). The National Diet and Nutrition Survey: Results from Years 7 and 8 (combined) of the Rolling Programme (2014/2015–2015/2016). Public Health England, London.

Table 5.14 Sources of free sugars in the diet (NDNS)*

Food group Specific foods % Intake
Sugar, preserves, and confectionery 25
Table sugar, preserves, and sweet spreads 16
Cereals and cereal products 24
Non-alcoholic beverages 21
Alcoholic beverages 9

* Source for average daily intakes for adults, Public Health England & Food Standards Agency (2018). The National Diet and Nutrition Survey: Results from Years 7 and 8 (combined) of the Rolling Programme (2014/2015–2015/2016). Public Health England, London.

Table 5.15 Sources of fibre in the diet (NDNS)*

Food group Selected food % intake
Cereals and cereal products 38
Breakfast cereals 6
Vegetables (excluding potatoes) 20
Potatoes and savoury snacks 12
Fruits, seeds, and nuts 10

* Source for average daily intakes for adults, Public Health England & Food Standards Agency (2018). The National Diet and Nutrition Survey: Results from Years 7 and 8 (combined) of the Rolling Programme (2014/2015–2015/2016). Public Health England, London.

Glycaemic index

The glycaemic index (GI) is a method of ranking foods and carbohydrates based on their immediate effect on blood glucose levels. The FAO/WHO (1998)2 define the GI as ‘the incremental area under the blood glucose response curve of one 50 g carbohydrate portion of a test food expressed as a percentage of response to the same amount of carbohydrate from a standard food taken by the same subject’. The standard carbohydrate is glucose, which has a GI of 100. Foods with a high glycaemic index are readily absorbed and raised blood glucose quickly. Low glycaemic index foods are digested and absorbed slowly and raise blood glucose levels slowly. The GI can only be determined by in vivo measurement. Foods are categorized as:

Low GI: ≤55.

Medium GI: 56–69.

High GI: ≥70.

Table 5.16 lists examples of GI of these categories. A list of foods that have been tested was published by Foster-Powell et al. (2002)3; more information is available at the Glycemic Index Foundation image…… http://www.gisymbol.com/. A list of commercially available products in the UK was published by Henry et al. (2005).4 The way a food is processed, prepared, and cooked will affect the GI of the food. The overall GI of the diet is important rather than aiming to introduce a few low GI foods. The health benefits of a low GI diet include:

improved diabetic glucose control (see image…… Chapter 22, p. 493);

improved risk factors for heart disease (see image…… Chapter 23, p. 519);

weight reduction (see image…… Chapter 21, ‘Obesity’, p. 469);

there is some evidence to suggest ↓ risk of some cancers.

Glycaemic load (GL)

GL extends the concept of GI by considering the effect that GI and the amount of a carbohydrate have on postprandial blood glucose levels.

image

Blood glucose levels rise more rapidly after a high GL meal than a low GL meal. It is recommended that a healthy diet should have a low GI and a low GL.

Table 5.16 Examples of low, medium, and high GI foods

Low GI Medium GI High GI
Apples, oranges, pears, peaches
Beans and lentils
Pasta (all types made from durum wheat)
Sweet potato, peeled and boiled
Sweet corn
Porridge
Custard
Noodles
All Bran, Special K, Sultana Bran
Honey
Jam
Shredded Wheat
Weetabix
Ice cream
New potatoes, peeled and boiled
White basmati rice, cooked
Pitta bread
Couscous
Glucose
White and wholemeal bread
Brown rice, cooked
White rice, cooked
Cornflakes
Baked potato
Mashed potato

Energy balance

To maintain body weight, energy intake must equal energy expenditure (EE). If EE exceeds energy intake, body weight will be lost. Weight loss is achieved by ↑ EE or ↓ energy intake. To gain weight the equation is reversed.

The SI unit of energy is the joule (J); the joule is a small amount of energy. Energy in food is usually expressed as kilojoules (kJ) and EE is expressed as kJ or megajoules (MJ). In practice many people continue to express energy in kilocalories (kcal). A calorie can be defined in several ways, although the most frequently used definition is:

the energy required to raise the temperature of 1 g of water from 14.5°C to 15.5°C.

EE can be expressed per unit of time, e.g. kJ per minute or MJ/day or in Watts (W) (see Box 5.1 for a summary of units).

Box 5.1 Units used in energy balance

1000 joules = 1kJ

1000 kJ = 1MJ

1 kcal = 4.184 kJ*

1 kJ = 0.239 kcal

1 W = 1 joule per second

0.06 W = 1 kJ per min

86.4 W = kJ per 24 h

* The Royal Society (London) recommended conversion factor.

EE

Total energy expenditure (TEE) has the following components:

basal metabolic rate (BMR), 50–75%;

physical activity (PA), 20–40%;

dietary induced thermogenesis (DIT), 10%.

Growth, pregnancy, lactation, injury, and fever are energy-requiring processes that will ↑ EE and → ↑ energy intake.

BMR

BMR is the amount of energy expended by the body to maintain normal physiological functions. It remains constant throughout the day, under normal conditions, and constitutes 50–75% of TEE; it is the largest component of TEE.

BMR is affected by many factors:

Body weight: BMR ↑ or ↓ with ↑ or ↓ body weight.

Body composition: Fat mass is relatively metabolically inactive and expends less energy gram for gram than fat free mass (FFM). Men have a higher FFM to fat ratio than women and ∴ have a higher BMR than women of the same age and weight.

Age: children have a higher BMR per kg than adults as a result of the energy requirement of growth. As adults age, metabolism slows and FFM ↓ ∴ ↓ BMR.

Gender: men generally have a higher BMR because of differences in body weight and body composition. The BMR of a 65 kg man will be approximately 1 MJ/day higher than a weight- and age-matched woman.

Genetic factors: BMR can vary by up to 10% between subjects of the same age, sex, and body weight. Recent research has shown that there are ethnic differences in BMR.

Physiological changes: BMR ↑ during pregnancy and lactation.

Disease and trauma: fever, sepsis, infection, and surgical and physical trauma ↑ BMR.

Nutritional status: the body adapts to changes in energy intake by altering body weight and/or body composition. An individual who is consuming more energy than is required will ↑ weight and ↑ BMR so making further weight gain impossible unless there is further intake ↑ or ↓ PA.

Environment: the energy cost of maintaining body temperature is influenced by ambient temperature, wind speed, radiant temperature of the surrounding, and clothing.

Hormonal status: several hormonal factors influence BMR, especially thyroid function. BMR is ↑ in hyperthyroidism and ↓ in hypothyroidism. There are small cyclical changes during the menstrual cycle of some women, with a rise after ovulation.

Pharmacological effects: therapeutic drugs and substances such as caffeine and capsaicin can modulate BMR.

Psychological effects: anxiety will ↑ EE in the short term. Longer term effects of stress and anxiety have not been established.

Measurement of BMR

BMR must be measured under standard conditions.

Post-absorptive state: at least 12 hours after last food or drink. This should also include other stimulants such as caffeine or smoking.

Thermoneutral environment: 20–25°C; comfortably warm.

Supine: sitting up will ↑ EE slightly.

Awake but in a state of complete physical and mental relaxation.

Heavy PA on the day before the measurement may influence the BMR and should be avoided.

In practice BMR is usually measured first thing in the morning before eating and drinking or undertaking PA. If any of the conditions are not met, the measurement is termed resting metabolic rate (RMR). RMR is slightly higher than BMR while sleeping metabolic rate is 5–10% lower than BMR.

Measurements of EE

EE can be measured directly (the measurement of heat production), indirectly (the measurement of O2 consumption), or by non-calorimetric methods, e.g. heart rate (HR) monitoring. Methods have also been developed that are indirect measures of gaseous exchange (O2 consumption), i.e. doubly labelled water technique.

Direct calorimetry

Direct calorimetry is the measurement of heat produced by the body. Subjects are placed in an insulated chamber and heat loss is measured over a period of at least 24 hours. Direct calorimetry is difficult in practice as the chamber must be capable of detecting all heat generated within the chamber and other sources of heat must be eliminated or accounted for. Direct calorimeters are very precise instruments but are expensive and difficult to build and maintain and few are available; ∴ this method is not frequently used.

Indirect calorimetry

Indirect calorimetry is based on the principle that food is oxidized in the body to produce energy and that by measuring oxygen consumption it is possible to calculate EE. The following equation demonstrates the amount of energy produced by the oxidation of 1 mole of glucose:

image

image

The energy produced by the oxidation of 1 g glucose is ∴ 15.4 kJ (2780/180) and 1L of oxygen is equivalent to the production of 20.7 kJ (2780/(6 × 22.4)). Therefore, if the amount of oxygen used is known, it is possible to calculate the amount of energy or heat produced. Similar calculations can be made for protein, fat, and alcohol.

Respiratory quotient (RQ) is the ratio of CO2 produced to O2 used. From the RQ it is possible to estimate the macronutrient composition of the diet (see Table 5.17). The energy content of a mixed diet is approximately 35% fat, 50% carbohydrate, and ∴ has an RQ of 0.87. To improve the accuracy of the calculations, an estimate of nitrogen excretion is used. Substitution into a formula yields EE. The formulae most frequently used are those of Weir (1949),5 or Elia and Livesey (1992)6 despite limitations (see Box 5.2).

Indirect calorimetry equipment

A variety of apparatuses are available to measure oxygen consumption. The simplest method is the Douglas bag where expired air is collected in a strong non-permeable bag. The volume of expired air over a set period is measured using a dry gas meter and the expired gases are analysed and compared to the ambient air. From this, it is possible to calculate O2 consumption and CO2 production rates and ∴ calculate EE. In clinical situations, a ventilated hood, canopy, or tent, e.g. Deltatrac, Gem, Sensormedics, is used which measures gaseous exchange continuously and has a processor to calculate EE. Other systems are available that can be used during exercise. Respiration chambers are used by some research units; these are small chambers in which a subject stays for several hours or days and gaseous exchange is measured continuously. These chambers are expensive to build and use, but give precise measurements.

Non-calorimetric methods

HR is related to EE and this relationship has been used to estimate EE, although the results are not very reliable, particularly at low activity levels.

Accelerometers are often used to measure PA; they are small computer motion analysers that measure duration, frequency, and intensity of PA. They are used in conjunction with log books that enable the full analysis of activities.

Box 5.2 Weir, and Elia and Livesey formulae

Weir formula

image

If nitrogen cannot be measured, protein is assumed to be 15% of the energy of the diet and the formula becomes:

image

Elia and Livesey formula

image

where VO2 = O2 consumed, VCO2 = CO2 produced, and N = urinary nitrogen excretion

Table 5.17 Energy values for oxidation of nutrients*

Nutrient O2 consumption, L/g CO2 production, L/g RQ Energy released, kJ/g Energy released, kJ/L O2
Starch 0.829 0.832 0.994 17.49 21.10
Glucose 0.746 0.742 0.995 15.44 20.70
Fat 1.975 1.402 0.710 39.12 19.81
Protein 0.962 0.775 0.806 18.52 19.25
Alcohol 1.429 0.966 0.663 29.75 20.40

* Reproduced from Garrow, J.S., James, W.P.T., Ralph, A. (1999). Human Nutrition and Dietetics, Table 3.4, p. 28. With permission from Elsevier.

CO2 is not an ideal gas, 1 mole at STP occupies 22.26 L not 22.4 L.

Doubly labelled water

Data are collected on free-living subjects over a period of 10–20 days. This does not require extensive equipment for the collection of gases and ∴ does not restrict the subject. Subjects are given an oral dose of water that has known amounts of the stable isotopes deuterium (2H) and 18O. These isotopes mix with the body’s water and, as energy is used, CO2 and H2O are produced. As 18O is in both H2O and CO2 it is lost more rapidly than 2H, which is only lost in H2O. The difference between the rate of loss of 2H and 18O reflects the rate at which CO2 is produced. From this, it is possible to calculate EE. This method requires collection of body fluid, either blood, urine, or saliva, before the test period and samples at specified times during the study. It is possible to use this method in babies, hospital patients, field work, and other groups in whom it is difficult to measure EE by other methods. Specialist equipment is required for the analysis of blood and urine samples and, because of a world shortage of 18O, this method is expensive.

Estimation of energy requirements

Energy requirements are estimated by using prediction equations such as the Henry equations (2005), see Appendix 4, p. 871. Table 5.18 shows the Henry equations with additional data on men aged 60–70 years (DH 1991).7 Regression analysis of measured BMR against gender, age, and weight was used to generate the equations. Numerous equations are available; ideally they should be population-specific. They are developed for use in healthy groups; in individuals the accuracy may be ±10–20%. If equations are extended for use in illness, the accuracy may be reduced by 50%.

Traditionally, TEE is calculated using a physical activity level (PAL) that has been derived from experimental studies, often using doubly labelled water; this is known as the factorial method.

For example, a sedentary male worker, aged 40 years, weight 90 kg, with an inactive lifestyle would have PAL of 1.4 (Table 5.19); ∴ his TEE would be

image

7 Department of Health (1991). Dietary reference values for food and nutrients for the United Kingdom. HMSO, London.

If an activity diary has been kept, it is possible to calculate TEE more accurately by partitioning time during the day spent on specific activities, and using physical activity ratios (PAR; see image…… Appendix 4, p. 871) it is possible to calculate a directly related PAL value for the day.

image

image

SACN does not recommend this approach, but recognizes that currently no viable alternative is available

Table 5.18 Formulae for the estimation of BMR*

Age, years BMR prediction equation, MJ/d
Males <3 0.255 (w) −0.141
3–10 0.0937 (w) +2.15
10–18 0.0769 (w) +2.43
18–30 0.0669 (w) +2.28
30–60 0.0592 (w) +2.48
**>60 0.0563 (w) +2.15
Females <3 0.246 (w) −0.0965
3–10 0.0842 (w) +2.12
10–18 0.0465 (w) +3.18
18–30 0.0546 (w) +2.33
30–60 0.0407 (w) +2.90
**>60 0.0424 (w) +2.38

* Henry, C.J. (2005) Basal metabolic rate studies in humans: measurement and development of new equations. Public Health Nutr. 8, 1133–1152.

** Department of Health (1991). Dietary reference values for food and nutrients for the United Kingdom. HMSO, London.

w Weight in kg.

Table 5.19 Calculated PAL values for light, moderate, and heavy activity (occupational and non-occupational)*

Non-occupational activity level Occupational activity level
Light Moderate Heavy
M F M F M F
Sedentary 1.4 1.4 1.6 1.5 1.7 1.5
Moderately active 1.5 1.5 1.7 1.6 1.8 1.6
Very active 1.6 1.6 1.8 1.7 1.9 1.7

* Source: data from Department of Health (1991). Dietary reference values for food and nutrients for the United Kingdom. HMSO, London.

Energy intake

Energy is provided by the macronutrients and alcohol.

Protein provides 4 kcal (17 kJ)/g.

Carbohydrate provides 3.75 kcal (16 kJ)/g.

Fat provides 9 kcal (37 kJ)/g.

Alcohol provides 7 kcal (29 kJ)/g.

Polyols (e.g. sorbitol) and volatile fatty acids (produced by gut bacteria by fermentation of some fibre components) contribute small, negligible amounts of energy.

Energy consumption

The average daily energy intakes for adults in UK are 8.79 MJ (2100 kcal) for men and 6.87 MJ (1641 kcal) for women; these intakes are below estimated average requirements (EARs). The sources of energy are shown in Fig. 5.5.

image In the UK adults are not energy-deficient, as demonstrated by the rising prevalence of obesity. The low percentages of EARs may be a result of under-reporting. The level of PA is also important.

Energy requirements

The SACN recommendations are shown in Table 5.20 for babies and children aged up to 10 years. These are given as EARs. EARs for men and women grouped for age, height and weight at a BMI of 22.5 kg/m2, and assuming a PAL of 1.63 are shown in Table 5.21.

image

Fig. 5.5Percentage contribution of food types to average daily total energy intake of UK adults.

Source: data from Public Health England & Food Standards Agency (2014). The National Diet and Nutrition Survey: Results from Years 1, 2, 3 and 4 (combined) of the Rolling Programme (2008/2009–2011/2012). Public Health England, London.

Table 5.20 EARs for energy of children aged 0–18 years*

Age EAR MJ/d (kcal/day)
Boys Girls
0–3 months 2.6 2.4
4–6 months 2.7 2.5
7–9 months 2.9 2.7
10–12 months 3.2 3.0
1–3 years 4.1 3.8
4–6 years 6.2 5.8
7–10 years 7.6 7.1
11–14 years 9.9 9.1
15–18 years 12.6 10.2

* Source: data for EARs from SACN (2011). Dietary reference values for energy. TSO, London.

Table 5.21 EARs according to height and weight at BMI = 22.5 kg/m2 and assuming a PAL of 1.63*

Height, cm Weight, kg, BMI = 22.5 kg/m2 EAR, MJ/d
Males
19–24 178 71.5 11.6
25–34 178 71.0 11.5
35–44 176 69.7 11.0
45–54 175 68.8 10.8
55–64 174 68.3 10.8
65–74 173 67.0 9.8
75+ 170 65.1 9.6
Females
19–24 163 29.9 9.1
25–34 163 59.7 9.1
35–44 163 59.9 8.8
45–54 162 59.0 8.8
55–64 161 58.0 8.7
65–74 159 57.2 8.0
75+ 155 54.3 7.7

* Source: data from SACN (2011). Dietary reference values for energy. TSO, London.

PA assessment

PA is the most variable component of TEE and most amenable to change. PA is a complex behaviour that includes any bodily movement produced by the contraction of skeletal muscles resulting in EE. It incorporates all daily activities and is not synonymous with exercise, which is a subcategory and tends to be structured leisure-time activity. Sedentary behaviour is independent from PA and should be considered as a separate component, but can often be measured with the same instrument as PA. The choice of measuring instrument is a balance between accuracy, reliability, detail, and practical considerations. The timing of the assessment is important and must include consideration of day-to-day variability in PA patterns, (e.g. weekday vs. weekend day), and seasons and special occasions that could influence habitual PA. The length of measurement period is dependent on these factors and the aim of the assessment. If the aim is to assess habitual PA, a longer measurement period is required and repeated measurement periods (e.g. different times of year) should be considered (see Table 5.22).

PA assessment in children

There are additional challenges in assessing PA in children as their activity patterns are more varied and movement is more sporadic and multi-dimensional than in adults and they have cognitive limitations in recalling their activity. With some of the objective instruments for PA assessment, changes in body size and energy efficiencies with growth also need to be considered. Instruments can broadly be divided into subjective and objective, and can be used in combination to provide complementary measurements.

Table 5.22 Definitions in PA assessment

Measurement Description
Intensity Intensity of activity usually defined in terms of metabolic equivalents (MET), such as light (1.1–2.9 MET) moderate (3.0–5.9 MET), and vigorous (6.0+ MET) intensity*
Frequency Frequency of time spent in specific activities or intensity levels over a set period of time
Duration Time spent in specific activities or intensity levels including total time per day, proportion of waking hours, or length of bout of activity
Patterning Occurrence of specific activities or intensity levels over set period of time, e.g. time of day or day of week
Types of activity Specific activities of interest, e.g. walking or cycling
Domains of activity Context of activities, e.g. home, work, leisure-time, or mode of transport
Sedentary behaviour Time spent in activities involving being sedentary, e.g. watching television, reading, or on computer

* METS are used when describing PA intensity multiples of an individual’s resting oxygen uptake, defined as the rate of oxygen (O2) consumption of 3.5 mL of O2/min/kg body weight in adults.

Subjective instruments

Diaries
PA recalls

Activity recalls are analogous to 24-hour diet recalls, but may cover a longer time period (1 day—1 month). Activities tend to be of moderate-vigorous intensity that are relatively easy to recall. Repeated recalls at intervals across a time period can be used to capture information on patterning or estimate habitual PA.

Questionnaires

Questionnaires are widely used, and there are many questionnaires used in different populations and age groups. Questionnaires vary from a few generic questions to detailed lists of questions on different activities. To assess total PA, questionnaires should include all domains and all common activities undertaken in the population of interest, taking into account culture-specific activities. It is important that a questionnaire is designed and validated against a criterion measure for use in the population group in which PA is being assessed (see Box 5.3).

Box 5.3 Good design features of questionnaires

Good for use in large groups

Assess patterns, frequency, duration, and type of PA

Capture context of PA

Measures of sedentary behaviour possible

Limited in ability to assess EE

Difficult for individual to quantify some activities

Subject to recall bias or social desirability in reported activities

Limited applicability in children due to cognitive stage

Can require considerable data processing

Objective instruments

Pedometers

Pedometers are motion sensors worn on the hip or waist that measure locomotor activity as steps taken, walking or running. There is a large variation in the accuracy and reliability of different pedometer models. This, in part, reflects the different mechanics of the models and also variations in stride length. Some models allow a setting of the individual’s walking stride length to improve estimation of the distance covered and steps taken (see Box 5.4).

Box 5.4 Pedometers

A simple and inexpensive measure of walking activity

Small and non-invasive for people to wear

Only assesses locomotor activities, not activities of upper body, cycling, or water activities

Unable to assess patterns, frequency, duration of activity, types of PA, or sedentary behaviour

Measurement capability varies with body placement, e.g. hip or waist

Limited application in groups of children as pedometer steps influenced by body size

Limited applicability in those with restricted mobility

Data not stored in memory of device

Best for ranking individuals or assessing change in locomotor activities, e.g. to monitor adherence to a walking intervention

Unable to estimate EE

Accelerometers

Accelerometers detect and record acceleration resulting from normal bodily movement (Box 5.5). Models can measure acceleration in one direction (usually vertical), two and three directions (triaxial; vertical, medio-lateral, and anterior-posterior), and are commonly expressed as a movement count value. The sampling period of current models vary from recording movement every second to every 60 seconds. The length of the measurement period depends on the sampling period and memory capacity of the accelerometer, but generally ranges from several days to weeks. Most models store the movement counts for downloading at the end of the measurement period into a PC via an interface. This allows the activity count data to be used to measure patterns, frequency, and duration of PA; estimate time spent in different intensities of activity with the use of appropriate cut points; and measure time spent sleeping and sleep quality.

Box 5.5 Accelerometers

Most commonly used objective instrument

Small and non-invasive for people to wear

Often used in children

Limited applicability in those with restricted mobility

Unable to assess types of PA

Measurement capability varies with body placement, e.g. hip, ankle, or wrist, as does not capture all activity across the body, e.g. if worn on hip, upper body activity is not captured

Most models are currently not waterproof so limited capability to measure water-based activities

Time is required for processing output data. Many models have software provided for these analyses

Some variation in the accuracy and reliability of different accelerometer models

Estimation equations for EE have been developed for different accelerometer models and populations, but these have limited accuracy in estimating EE on an individual basis

HR monitors

HR provides an indirect measure of PA, as it measures the individual’s physiological response to PA. Minute-by-minute HR data are recorded from a chest strap and can then be downloaded at the end of the measurement period into a PC via an interface for processing (see Box 5.6).

Box 5.6 Heart monitors

Can assess whole range of movements and activities (including water activities)

Can measure patterns, frequency, and duration of PA

Unable to assess types of PA or sedentary behaviour

Can estimate time spent in different intensities of activity with the use of appropriate cut points

At low levels of PA, HR is a less reliable measure of PA

Can estimate EE and patterning of expenditure with an individualized calibration of the O2/HR relationship

HR responses reflect not only PA but are also affected by hydration, prandial status, body position, ambient temp., humidity, emotion stress, smoking, caffeine intake, and certain drugs e.g. -blockers

Relationship between individuals’ HR and PA or EE can alter with changes in body weight, body composition, physical fitness, ageing, and illness

Downloaded data require considerable processing to estimate PA or PA EE

Combined monitors

Instruments are increasingly becoming available that combine more than one objective method to overcome some of the limitations of the individual methods (e.g. combining HR monitoring with accelerometry). Some of these monitors can estimate EE, but may have limited accuracy in estimating on an individual basis.

Doubly labelled water

Doubly labelled water does not give a direct measure of PA EE. However, when combined with measured or estimated BMR, reasonable estimates of PA EE can be derived by subtraction of BMR and DIT, averaged over the period of isotope sampling (commonly 7–10 days; see Box 5.7). See previous section in this image chapter for methodology (p. 87).

Box 5.7 Doubly labelled water

Provides no measure of day-to-day PA EE

Unable to assess patterns, frequency, duration, or types of PA or sedentary behaviour

Most applicable in healthy groups, limitations in application in some illnesses

Can be used in children and infants

Application constrained by expense of method and specialist processing


1SACN (2015) Carbohydrates and Health report and supporting documents are available at image…… www.gov.uk/government/publications/sacn-carbohydrates-and-health-report

2WHO/FAO (1998). Carbohydrates in human nutrition, FAO food and nutrition paper no.66. FAO, Rome.

3Foster-Powell, K., et al.. (2002). International table of glycaemic index and glycaemic load. Am. J. Clin. Nutr. 76, 5–56.

4Henry, C.J.K., et al. (2005). Glycaemic index and glycaemic load values of commercially available products in the UK. Br. J. Nutr. 94, 922–930.

5Weir, J.B. (1949). New methods for calculating metabolic rate with special reference to protein metabolism. J. Physiol. (Lond.) 109, 1–9.

6Elia, M., Livesey, G. (1992). Energy expenditure and fuel selections in biological systems: the theory and practice of calculations based on indirect calorimetry and tracer methods. In Metabolic Control of Eating, Energy Expenditure and the Bioenergetics of Obesity (ed. A.P. Simonopoulos), pp. 68–131. Karger, Basel.