Book: Nutrition: A Very Short Introduction (Very Short Introductions)

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Apart from water, the body’s first requirement under all conditions is for a source of energy to perform physical and chemical work. The metabolic fuels to provide this energy are derived from the diet—fats, carbohydrates, protein, and alcohol. For a few hours immediately after a meal the constituents of the meal provide these fuels directly. At the same time, reserves of fat (in adipose tissue) and carbohydrate (in liver and muscle) are laid down for use during the period of fasting between meals.

The need for energy to perform physical work and move the body is obvious. Apart from this obvious work output, even at rest the body has a considerable requirement for energy. Only about one-third of the average person’s energy expenditure is for voluntary activity; two-thirds is required for maintenance of the body’s functions, metabolic integrity, and homeostasis (maintenance of the normal state) of the internal environment.

This energy requirement at rest is the basal metabolic rate (BMR). Part of this requirement is obvious—the heart beats to circulate the blood, breathing continues, and there is considerable electrical activity in nerves and muscles, whether they are ‘working’ or not. The brain and nervous system comprise only about 2 per cent of body weight, but consume some 20 per cent of resting energy expenditure, because of the need to maintain electrical activity. Less obviously, there is also a requirement for energy for the wide variety of biochemical reactions occurring all the time in the body: laying down reserves of fat and carbohydrate; the turnover of tissue proteins; transport of compounds into and out of cells; and the synthesis and secretion of hormones.

Energy expenditure can be measured by the output of heat from the body. The unit of heat used in the early studies was the calorie—the amount of heat required to raise the temperature of 1 gram of water by 1 degree Celsius. The calorie is still used in nutrition, usually as the kilocalorie, kcal (sometimes written as Calorie with a capital C). One kcal is 1,000 calories (103 cal), and hence the amount of heat required to raise the temperature of 1 kg of water by 1 degree Celsius.

More correctly, the Joule is used as the unit of energy. The Joule is an SI unit (International System of Units), named after James Prescott Joule (1818–89), who first showed the equivalence of heat, mechanical work, and other forms of energy. In nutrition, the kiloJoule (kJ = 103 J) and megaJoule (MJ = 106 J) are used.

To convert between calories and Joules:

1 kcal = 4.184 kJ (normally rounded off to 4.2 kJ)

1 kJ = 0.239 kcal (normally rounded off to 0.24 kcal)

The average total daily energy expenditure of adults is between 1,900 to 2,400 kcal (7.5 to 10 MJ) for women and 2,000 to 2,900 kcal (8 to 12 MJ) for men.

Measurement of energy expenditure

Energy expenditure can be measured by measuring heat output from the body, but this is a tedious experimental procedure, requiring a thermally insulated room in which a small increase in temperature can be measured accurately. An indirect method of determining energy expenditure is by measurement of the consumption of oxygen.

It is relatively simple to measure oxygen consumption by sampling inspired and exhaled air and measuring the oxygen content of each—the difference is the amount of oxygen that has been consumed. Using a respirometer (a small back pack containing an air bag attached to a mouthpiece) it is possible to measure oxygen consumption in a variety of different activities, as well as at rest, for periods of an hour or so at a time. Early studies calibrated such measurements of oxygen consumption against direct measurement of heat production and showed that each litre of oxygen consumed is equivalent to energy expenditure of 4.8 kcal (20 kJ).

BMR is determined by measuring oxygen consumption over a period of about hour, with the subject completely at rest (but not asleep), in a comfortably warm room (so that energy is not being expended to maintain body temperature), some four hours after a meal, so that energy is not being expended on digestion, absorption of the products of digestion, or the synthesis of reserves of fat and carbohydrate. If the experimental conditions are not rigorously controlled, it is usual to call the resultant energy expenditure the resting metabolic rate (RMR), keeping the term BMR for studies conducted under precisely controlled conditions. Both BMR and RMR are expressed as kcal (or MJ)/24 hours.

It is important that the person is not asleep when measuring BMR or RMR. Some people show an increase in metabolic rate and become hot when they are asleep, while others show a small drop in body temperature, associated with a reduction in metabolic rate. People whose body temperature falls slightly in sleep are biologically efficient; they are conserving energy. However, those biologically inefficient people whose metabolic rate rises when they are asleep are fortunate in that this increase in metabolic rate means that they are consuming more metabolic fuel. They are increasing their energy expenditure to ‘burn off’ surplus metabolic fuels they have consumed in food. They do not gain weight as readily as people whose metabolic rate falls when they are asleep.

The energy cost of various types of physical activity can also be determined by measuring oxygen consumption. Although the results of any such measurement will be expressed in kcal (or kJ), it is more useful to express the results as a multiple of the individual’s BMR. This means that it is relatively easy to apply the results of studies in a small number of people to others whose BMR can be measured (or estimated from body weight, age, and gender), but whose energy expenditure in physical activity has not been measured. This multiple of BMR for any given physical activity is known as the physical activity ratio (PAR) for that activity, or sometimes as the metabolic equivalent of the task (MET). As shown in , gentle, sedentary activities have a PAR of up to 1.4 x BMR, while strenuous activities such as walking cross-country with a load may have a PAR of almost 8 x BMR.

If we sum the PARs for the different activities through the day, multiplied by the fraction of the 24 hours spent in each activity, we can calculate a person’s overall physical activity level (PAL). shows the ranges of PAL for people in light, moderate, and heavy occupational work, calculated through the eight-hour working day, and not allowing for any additional energy expenditure in leisure activities. At first sight, these values of PAL do not seem to fit well with the energy cost (PAR) of individual activities, but most people do not engage in the more strenuous activities for very long, and much of their day is spent in less strenuous activities or at rest.

Measurement (or calculation) of BMR and calculation of a person’s PAL does not give his or her total energy expenditure, because we have to add in the metabolic energy cost of eating—the cost of synthesizing and secreting digestive enzymes, absorbing the products of digestion, and, perhaps most importantly, the energy cost of synthesizing reserves of fat and carbohydrate, and the energy cost of the increase in protein synthesis that occurs after a meal. This is seen as an increase in heat output from the body after eating—diet-induced thermogenesis. Overall it may account for as much as 10 per cent of the energy yield of the meal.

Table 2. Physical activity ratios (PARs) (multiples of basal metabolic rate (BMR)) in different types of activity

PAR

 

1.0–1.4

Lying, standing or sitting at rest: watching tv, reading, writing, eating, playing cards and board games

1.5–1.8

Sitting: sewing, knitting, playing piano, driving

 

Standing: preparing vegetables, washing dishes, ironing, general office and laboratory work

1.9–2.4

Standing: mixed household chores, cooking, playing snooker or bowls

2.5–3.3

Standing: dressing, undressing, showering, making beds, vacuum cleaning

 

Walking: 3–4 km/h, playing cricket

 

Occupational: tailoring, shoemaking, electrical and machine tool industry, painting and decorating

3.4–4.4

Standing: mopping floors, gardening, cleaning windows, table tennis, sailing

 

Walking: 4–6 km/h, playing golf

 

Occupational: motor vehicle repairs, carpentry and joinery, chemical industry, bricklaying

4.5–5.9

Standing: polishing furniture, chopping wood, heavy gardening, volley ball

 

Walking: 6–7 km/h

 

Exercise: dancing, moderate swimming, gentle cycling, slow jogging

 

Occupational: labouring, hoeing, road construction, digging and shovelling, felling trees

6.0–7.9

Walking: uphill with load or cross-country, climbing stairs

 

Exercise: jogging, cycling, energetic swimming, skiing, tennis, football

From data reported by Department of Health (1991). Dietary Reference Values for Food Energy and Nutrients for the United Kingdom. HMSO, London; and FAO/WHO/UNU (1985). ‘Energy and Protein Requirements: Report of a Joint FAO/WHO/UNU Expert Consultation, WHO Technical Reports Series 724, WHO, Geneva

Table 3. Classification of types of occupational work by physical activity ratio (PAR) (multiples of basal metabolic rate (BMR)): figures show the average PAR through an eight-hour working day, excluding leisure activities

Work intensity

PAR1

Light

1.7

professional, clerical, and technical workers, administrative and managerial staff, sales representatives, housewives/-husbands

Moderate

2.2–2.7

sales staff, domestic service, students, transport workers, joiners, roofing workers

Moderately heavy

2.3–3.0

machine operators, labourers, agricultural workers, bricklaying, masonry

Heavy

2.8–3.8

labourers, agricultural workers, bricklayers, masonry workers where there is little or no mechanization

(1) Where a range of PAR is shown, the lower figure is for women and the higher for men From data reported by Department of Health (1991). Dietary Reference Values for Food Energy and Nutrients for the United Kingdom. HMSO, London

Metabolic fuels

The dietary sources of metabolic energy (the metabolic fuels) are carbohydrates, fats, protein, and alcohol. As can be seen from , fat yields more than twice as much energy per gram as carbohydrates and proteins.

Table 4. Energy yield and oxygen consumption in oxidation of metabolic fuels

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Although there is a requirement for energy sources in the diet, it does not matter unduly how that requirement is met. There is no requirement for a dietary source of carbohydrate because the body can synthesize as much carbohydrate as is needed from the amino acids derived from proteins. Similarly, there is no requirement for a dietary source of fat, apart from the essential fatty acids that are required in relatively small amounts, and there is certainly no requirement for a dietary source of alcohol. Diets that provide more than about 35–40 per cent of energy from fat are associated with increased risk of heart disease and some cancers, and there is some evidence that diets that provide more than about 20 per cent of energy from protein are also associated with chronic diseases. Therefore, the general consensus is that diets should provide about 55 per cent of energy from carbohydrates, 30 per cent from fat, and 15 per cent from protein.

Although there is no requirement for fat in the diet, fats are nutritionally important, for a number of reasons. First, it is difficult to eat enough of a very low-fat diet to meet energy requirements. The problem in many less developed countries, where under-nutrition is common, is that diets provide only 10 to 15 per cent of energy from fat, and it is difficult to consume a sufficient bulk of food to meet energy requirements. By contrast, the problem in Western countries is an undesirably high intake of fat, contributing to the development of obesity and chronic diseases. In addition, four of the vitamins—A, D, E, and K—are fat-soluble, and are found in fatty and oily foods. They are absorbed dissolved in fat, so with a very low-fat diet the intake and absorption of these vitamins may be inadequate to meet requirements. There is a requirement for small amounts of two essential fatty acids that cannot be synthesized in the body and must be provided in the diet. Finally, in many foods a great deal of the flavour (and hence the pleasure of eating) is carried in the fat; also, fat lubricates food, making it easier to chew and swallow.

Metabolic fuels in the fed and fasting states

In the fed state, during three to four hours after eating a meal, the main fuel for muscle and other tissues is glucose, which is produced by the digestion of dietary carbohydrates. Glucose in excess of immediate requirements will be used for synthesis of the storage carbohydrate glycogen in liver and muscle, and also for synthesis of fatty acids, and then fat, in liver and adipose tissue. Dietary fat will mainly be used to add to fat reserves in adipose tissue. Amino acids from dietary protein in excess of requirements for tissue protein synthesis will be used as metabolic fuels, or used to synthesize glucose (and hence glycogen) or fatty acids (and hence fat).

The fasting state begins some four to five hours after a meal, when the body needs to call on the reserves laid down after a meal. Glycogen reserves can provide glucose, but the total amount of glycogen in the body would only last for 12 to 18 hours, and the brain is more or less completely reliant on a source of glucose (while red blood cells are completely reliant on glucose). Muscle can use glucose (from the bloodstream or its own glycogen reserves), but it can also use fatty acids liberated from adipose tissue fat reserves. In the fasting state, muscle ceases to take up glucose from the bloodstream and uses fatty acids as its main fuel, so sparing glucose for the brain and red blood cells. During fasting, the rate of protein synthesis slows down, but breakdown continues at a more or less constant rate. This leads to the liberation of amino acids that can be used in the liver for the synthesis of glucose that can be exported for use by the brain and red blood cells. This is the process of gluconeogenesis, the new synthesis of glucose from non-carbohydrate precursors.

As fasting continues for more than about 12 hours, and muscle reserves of glycogen are more or less exhausted, fatty acids alone cannot meet the needs for muscle metabolism. At this stage, the liver takes up some of the fatty acids that have been liberated from adipose tissue, and uses them to synthesize small water-soluble compounds (the ketone bodies) that can be used by muscle and, to a limited extent, also the brain. Gluconeogenesis from amino acids liberated by protein breakdown continues.

Energy balance

In order to maintain a constant body weight, it is necessary to balance energy intake from food with energy expenditure. Most people achieve this balance very well, and indeed, as noted earlier, even grossly overweight people are in energy balance, with a stable, if excessive, body weight. Part of this ability to balance energy intake and expenditure is the result of the physiological systems of control over hunger, satiety, and appetite discussed in . In addition, there are physiological mechanisms that permit the metabolism of metabolic fuels not linked to the normal conservation of energy in the form of ATP (adenosine triphosphate). ATP is the ‘energy currency’ of the cell, and is used: to power chemical reactions; to transport compounds into and out of cells; in electrical activity of brain and nerves; and in contraction of muscle for movement and physical work. The processes of oxidation of metabolic fuels linked to the formation of ATP are normally tightly coupled, so that metabolic fuels are only oxidized when there is a need for ATP. However, there are proteins in muscle and some other tissues that can uncouple these reactions to some extent. This uncoupling is important in response to exposure to cold, as a means of generating heat to maintain body temperature. It is usually referred to as non-shivering thermogenesis, as opposed to shivering when increased muscle contraction leads to increased heat production. The increase in body temperature and metabolic rate that some people show when they are asleep is the result of such uncoupling of fuel oxidation and ATP formation.

If you deliberately overfeed someone by 10 per cent, they will gain weight initially, then stabilize at a higher body weight. Initially they will be in positive energy balance, with an intake greater than their expenditure, and laying down additional reserves of body fat. When their weight has stabilized, they will return to balance, despite still eating 10 per cent more than previously, at a higher body weight. There are four reasons for this. If you eat more food, there is a greater energy cost for digestion and absorption, and the synthesis of fat and carbohydrate reserves—an increase in diet-induced thermogenesis. As the amount of adipose tissue in the body increases, there is an increase in the secretion of the hormone leptin, which acts to increase the uncoupling of fuel oxidation and ATP formation, resulting in increased oxidation of metabolic fuels. As body weight increases, so there is an increase in BMR, and more importantly, as body weight increases, so the energy cost of moving that body increases, so that the energy cost of physical activity increases.

Conversely, if you underfeed someone by 10 per cent, they will initially lose weight, then come back into energy balance at a lower body weight. The reasons for this are the converse of those for the re-establishment of energy balance at a higher body weight with overfeeding. If less food is eaten, there will be a lower energy cost of digesting and absorbing that food, and a lower energy cost of synthesizing fat and carbohydrate reserves. As the amount of adipose tissue in the body falls, so there is a reduction in the secretion of leptin, a reduction in non-shivering thermogenesis, and a reduction in the oxidation of metabolic fuels not linked to the formation and utilization of ATP. Finally, as body weight falls, there is a reduction in BMR, and the energy cost of moving a smaller, lighter, body is less, so the energy cost of physical activity decreases.

Physical activity and exercise

A desirable level of physical activity for cardiovascular and respiratory fitness is about 1.7 × BMR. This should be easy to achieve with normal physical activity, but fewer than a quarter of adults in most developed countries do so, and, as discussed in , a major contributor to the worldwide epidemic of obesity is a sedentary lifestyle, with relatively low physical activity.

It is obvious from the figures in that energy expenditure increases with the intensity of physical activity. What is less obvious is that the pattern of metabolic fuels used also changes. In moderate exercise, muscle is well oxygenated and uses mainly fatty acids (either from free fatty acids and lipids in the bloodstream or from its own reserves of fat laid down between muscle fibres) as its main fuel. As the intensity of exercise increases, so the need for metabolism of metabolic fuels by muscle exceeds the rate at which oxygen can enter the muscle cells. This is the so-called aerobic threshold—muscle will continue to metabolize as much fat as the available oxygen permits, and will meet the increased demand by anaerobic metabolism of glucose. This is a relatively inefficient process that leads to the release of lactic acid from muscle. This lactic acid will later be taken up by the liver and used for resynthesis of glucose. The synthesis of glucose from lactic acid requires ATP, and as a result there is an increase in metabolic rate and oxygen consumption after exercise (so-called oxygen debt) as some of the lactic acid is oxidized completely to provide the ATP needed for glucose synthesis.

This change in fuel utilization by muscle with increasing intensity of exercise means that if the main aim of exercising is to lose excess fat, a relatively prolonged period of low intensity exercise is better than a short period of intense exercise. One cynic has suggested that walking a mile each way to and from the gym, and not going in, is better than driving to the gym and exercising intensely for a short time. Certainly it is easy to increase physical activity without engaging in formal exercise, for example by walking or cycling rather than driving, or using stairs instead of lifts and escalators.

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