Digestion

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In the first instance the efficiency of absorption of energy from individual macronutrients is variable.

Approximately 96% of the energy from fat is absorbed, 91% from protein and a variable proportion from carbohydrate, depending on the relative content of 'available' carbohydrate and resistant starch. However, knowing the composition of the diet, the availability of energy is largely predictable (1). Moreover, except in pathological malabsorption syndromes, it is extremely consistent between individuals, which makes this an unlikely cause of individual susceptibility to weight gain.

The effect of macronutrients on diet-induced thermogenesis DIT) is also variable. When consumed individually as the only source of nourishment, short-term experiments indicate that less energy is dissipated in the processing of fat (6%) compared to carbohydrate (12.5%) or protein (21%) (2). However, measurements made over the whole day in a whole-body calorimeter indicate very little difference in overall energy expenditure on diets with radically different ratios of fat to carbohydrate. For example, in a study in which individuals were fed isoenergetic diets with constant protein content and either 9 or 79% energy as carbohydrate, with reciprocal changes in fat intake, daily energy expenditure measured on two separate occasions in a whole-body calorimeter was not significantly different (3). Similar results have been obtained in groups of lean and obese men and women (4).

Major changes in the proportion of protein in the

Figure 9.1 The oxidative hierarchy of macronutrients

diet do seem to lead to noticeable differences in diet-induced thermogenesis, but again the effect on 24-hour energy expenditure is negligible. For example, Westerterp et al. (5) showed that on a high protein/high carbohydrate/low fat diet (30: 60:10% by energy) diet-induced thermogenesis represented 14.6% of the total intake and 24-hour energy expenditure was 9.2 MJ. Whilst on the low protein/ low carbohydrate/high fat diet (10: 60:30% by energy), diet-induced thermogenesis represented only 10.5% of total energy intake (P = 0.01) and while there was a trend towards a lower energy expenditure over 24 hours (8.9 MJ), the difference was not statistically significant (P = 0.08) (5).

Macronutrient Oxidation

Once assimilated into the body a nutrient may be oxidized or stored. This process of nutrient partitioning is a key regulatory point in macronutrient metabolism and recent studies have shown a precise hierarchy in which macronutrients are recruited for oxidation. Since total energy expenditure is essentially constant, except for limited (and predictable) thermogenesis, the oxidation of any one nutrient will tend to suppress the oxidation of others. Alcohol dominates oxidative pathways, since it is a toxin and must be eliminated from the body as quickly as possible (6). Carbohydrate and protein also show a linkage between intake and oxidation. Numerous short-term studies have shown that the addition of carbohydrate to a meal will induce an increase in carbohydrate oxidation and likewise for protein (7,8). However, no such auto-regulatory process exists for fat oxidation. The addition of fat to a meal does not stimulate fat oxidation. Indeed the oxidation of fat is ultimately dependent on the intake of the other macronutrients, since fat oxidation accounts for the difference between the energy requirements of the individual and the combined energy content of the ingested alcohol, carbohydrate and protein (9). The addition or subtraction of carbohydrate from the diet causes a parallel increase or decrease in carbohydrate oxidation, with reciprocal changes in fat oxidation. However, when fat is added or subtracted from the diet the effect on substrate oxidation is negligible (10).

The impressive flexibility in carbohydrate oxidation rates in response to changes in intake is shown in Figure 9.2. Here, during profound overfeeding, subjects were receiving 150% of their baseline energy requirements including 539g/day carbohydrate. Carbohydrate oxidation increased immediately and after 4-5 days carbohydrate oxidation closely matched intake so that carbohydrate balance was re-established, although at a higher level of glycogen stores. Conversely during underfeeding, when subjects received only 3.5MJ/day, with 83 g/ day carbohydrate, the oxidation of carbohydrate was suppressed and balance was again virtually re-established. The small persistent negative carbohydrate balance probably reflects a gradual depletion of muscle glycogen stores in response to this period of profound under-nutrition. Thus there is sufficient flexibility in carbohydrate oxidation to match intake over the range of about 80-540 g/day in adult men. Throughout this period the change in fat oxidation were counter-regulatory (Figure 9.3). During overfeeding the excess energy was stored primarily as fat, with a marked positive fat balance, and during underfeeding the energy deficit was met by the oxidation of endogenous fat, leading to nega-

Figure 9.2 Carbohydrate flux during overfeeding (a) and underfeeding (b). Open circles, carbohydrate intake; closed circles, carbohydrate oxidation; hatched bars, net carbohydrate balance. Data from Jebb et al. (9)

Figure 9.3 Fat flux during overfeeding (a) and underfeeding (b). Open circles, fat intake; closed circles, fat oxidation; hatched bars, net fat balance. Data from Jebb et al. (9)

Figure 9.4 Daily and cumulative changes in fat balance during intentional overfeeding by fat or different carbohydrates. Solid bars and circles, control diet; open bars and open triangles, 50% overfeeding by fructose; stippled bars and open squares, 50% overfeeding by sucrose; heavy hatched bars and solid triangles, 50% overfeeding by glucose; light hatched bars and solid circles, 50% overfeeding by fat. Data from McDevitt et al. (11)

Figure 9.3 Fat flux during overfeeding (a) and underfeeding (b). Open circles, fat intake; closed circles, fat oxidation; hatched bars, net fat balance. Data from Jebb et al. (9)

Figure 9.4 Daily and cumulative changes in fat balance during intentional overfeeding by fat or different carbohydrates. Solid bars and circles, control diet; open bars and open triangles, 50% overfeeding by fructose; stippled bars and open squares, 50% overfeeding by sucrose; heavy hatched bars and solid triangles, 50% overfeeding by glucose; light hatched bars and solid circles, 50% overfeeding by fat. Data from McDevitt et al. (11)

The cumulative increase in fat balance did not differ significantly between treatments involving overfeeding by fat, sucrose, glucose or fructose.

If individuals eat to energy balance, the pattern of macronutrient oxidation will closely match the dietary composition (i.e. respiratory quotient = food quotient). However, in conditions of energy imbalance the oxidative hierarchy predicts that fat will be the macronutrient most likely to be mobilized or stored to balance the body's energy budget in the medium to long term.

tive fat balance. Changes in protein oxidation during both over- and underfeeding were modest, but with a trend towards auto-regulation (9).

Experiments investigating whether different primary sources of carbohydrate have variable effects on this carbohydrate-driven system of fuel selection have revealed no detectable differences between glucose, fructose and sucrose (11). When various energy sources are fed under controlled conditions (i.e. which exclude effects of appetite) fats and carbohydrates have a very similar effect on fat balance. This can be seen in Figure 9.4, which illustrates changes in fat balance when subjects are intentionally overfed in a whole-body calorimeter for 4 days.

Macronutrient Storage

The biochemical potential for the inter-conversion of macronutrients has aroused considerable interest, particularly in relation to de novo synthesis of fat from dietary carbohydrate. In energetic terms this is a very inefficient process, in which approximately 20-30% of the energy is dissipated as heat, whilst dietary fat can be stored with the loss of only 4% of its energy (12). A propensity towards de novo lipogenesis is a plausible 'energy wasting' strategy which may help some individuals to remain slim in the face of excess food. However, detailed studies using stable isotopes suggest that this process is quantitatively unimportant in humans (13), except under conditions of profound overfeeding (14). This does not preclude an effect of other macronutrients on fat storage, an effect which is achieved by their suppression of the utilization of dietary fat.

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