We expend energy continuously over each 24-hour period. Some of this energy expenditure represents the basic requirements for staying alive: at the cellular level, pumping of ions across membranes to maintain normal gradients, turnover of proteins and other cellular constituents; at the organ level, pumping of blood around the body, respiration, etc. This 'basal' level of metabolic activity is known as the basal metabolic rate. Basal metabolic rate is measured after an overnight fast, in a room at a comfortable temperature, with the subject awake but resting; these conditions have been found to give very reproducible answers. When we sleep, the metabolic rate (rate of energy expenditure) is lower than the basal metabolic rate, but at all other times during normal daily life it is higher. Energy expenditure is increased by physical activity. It is also increased after meals. The increase in the rate of energy expenditure after meals used to be called the specific dynamic action of food; more usually now it is referred to as diet-induced thermogenesis or DIT, thermogenesis meaning the generation of heat. DIT represents the energy cost of gastrointestinal tract activity, digestion, absorption and the metabolic cost of storing the fuels (e.g. the formation of glycogen by the direct pathway from glucose involves the hydrolysis of two high-energy phosphates, one ATP and one UTP, per molecule of glucose).
The total expenditure of energy over a 24-hour period can be broken down into the basal metabolic rate, the energy cost of physical activity and diet-induced thermogenesis (Fig. 11.3). Physical activity varies considerably from person to person. However, the largest component of the 24-hour energy expenditure is, for most people, the basal component. The basal metabolic rate is very closely related to the amount of non-fat tissue in the body, the fat-free mass or lean body mass.2 The larger someone's fat free mass, the larger (in general) their basal metabolic rate (this will be illustrated later, in Fig. 11.5). The basal metabolic rate is also regulated by hormones, primarily by the thyroid hormone, triiodothyronine. During starvation or food deprivation thyroid hormone concentrations fall and basal metabolic rate decreases (see Section 126.96.36.199). The significance of this for weight reduction programmes will be discussed again later. Leptin does not seem directly to regulate energy expenditure signi ficantly in humans as it does in rodents; this is based on evidence from measurements of energy expenditure in the children with leptin de ficiency (Fig. 11.2).
The human body takes in the macronutrients carbohydrate, fat and protein. They eventually leave the body as CO2, H2O and urea. There is almost no loss of other products (e.g. partial oxidation products such as pyruvic acid or ketone bodies); in other words, the macronutrients are virtually completely oxidised (with the exception of urea formation from protein). The body produces heat and external work from the oxidation of these substances. It is irrelevant that the process of oxidation within the body may not be direct - e.g. glucose may form glycogen, then lactate, then be recycled as glucose before oxidation; or even that glucose may be converted to fat before oxidation. The net heat production will be the same as if the oxidation occurred directly.
The equations for oxidation of the individual fuels are given below:
(the quantities are shown for one mole of glucose):
180 g 6 X 22.4 litres 6 X 22.4 litres 6 X 18 g 2.80 MJ
(AH is the enthalpy change - i.e. heat produced; the negative sign is the convention when heat is liberated.)
Note that oxidation of 1 g of glucose liberates 2.80/180 MJ or 15.6 kJ. The ratio of CO2 production to O2 consumption, the respiratory quotient (RQ) for this reaction, is 6/6 or 1.00.
(the quantities are shown for one mole of a typical triacylglycerol, palmitoyl, stearoyl, oleoyl-glycerol, C55H106O6):
2 X 862 g 157 X 22.4 litres 110 X 22.4 litres 106 X 18 g 68.0 MJ
Note that oxidation of 1 g of triacylglycerol liberates 68.0/1724 MJ or 39.4 kJ.
The RQ for this reaction is 110/157, or 0.70.
(the quantities are shown for one mole of a standard protein):
C100H159N32O32S0.7 + 104 O2 ^
2257 g 104 X 22.4 litres
The other products are assumed to be urea (11.7 mol), ammonia (1.3 mol), creatinine (0.43 mol) and sulphuric acid (0.7 mol).
Note that oxidation of 1 g of standard protein liberates 45.4/2257 MJ or 20.1 kJ.
The RQ for this reaction is 86.6/104, or 0.83.
We may look at this another way, by calculating the heat liberated for each litre of O2 used:
Energy equivalent of 1 litre O2 Respiratory quotient
Glucose* 20.8 kJ 1.00
Protein (forming urea) 19.4 kJ 0.83
*Slightly different values will be obtained depending upon whether the substrate is assumed to be pure glucose, or a glucose polymer such as glycogen. The same also applies to fat and protein: different fats and proteins give slightly different values.
Note that the heat produced per litre of O2 consumed is almost constant. Thus, measurement of O2 consumption alone allows the calculation of energy expenditure (heat production) to a reasonable accuracy. However, the estimate can be improved by also measuring CO2 production and urinary urea (or total nitrogen) excretion, to allow the appropriate energy values to be used. These figures may be combined into a formula such as:
Energy expenditure (kJ) = 15.9 VO2 + 5.2 VCO2 - 4.65 N
where VO2 represents the volume of O2 consumed (litres), VCO2 the volume of CO2 produced (litres) and N the amount of urinary nitrogen excretion (g), over whatever measurement period is used.
Data taken, in part, from Elia & Livesey (1992).
Box 11.3 Measurement of energy expenditure using doublelabelled water
The subject is given water (2H218O) in which both the oxygen and hydrogen atoms are isotopically labelled with a stable isotope (i.e. it is not radioactive), so that these atoms can be 'traced'. The oxygen atoms equilibrate with CO2 through the action of the enzyme carbonic anhydrase in blood. Then the loss of 18O atoms from the body is related to the rate of expiration of CO2. However, 18O is also lost in water (in sweat, breath, urine, etc.). This is allowed for by following the loss of 2H. Thus, 18O is lost somewhat faster than 2H, and the difference (averaged over 2-3 weeks) gives a measure of the rate of CO2 production. As described in the text, this can be used to derive an estimate of energy expenditure. A typical experimental result is shown below.
Some recent work has suggested that people may vary considerably in a component of energy expenditure that reflects involuntary physical activity or 'fidgeting'. This has been called non-exercise activity thermogenesis (NEAT). People with a low degree of fidgeting have been shown to have an increased risk of weight gain.
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