Clinical Obesity in Adults and Children. Группа авторов

nutrition. This occurs sufficiently frequently that animals have evolved specific nutritional strategies to deal with such imbalances. An important part of understanding (and for humans, managing) nutrition is learning what these strategies are, a challenge to which the Nutritional Geometry framework is well suited. To do so, experimental animals are confined to diets that systematically differ from the balanced target diet, placing them in a predicament where they cannot attain their target intake of all nutrients but are forced to under eat some and/or over‐eat others (Fig. 6.1b and c). The relative priority the animal assigns to achieving its target intake for each nutrient – i.e. avoiding excesses and deficits – is determined by measuring the ad‐libitum intakes of the experimental groups assigned to each of the foods. Such data provide a measure of the relative strength of the appetites for each nutrient – the stronger the appetite, the closer the intake of that nutrient will be to its target coordinate, with the inevitable consequence of forcing deficits or excesses of other nutrients [3].

      Several laboratory studies have examined this issue for a range of invertebrate and vertebrate species [13]. In general, the pattern of response to constrained macronutrient imbalance varies with the normal diets of species. Herbivores and omnivores tend to maintain protein at or close to the target levels, allowing fat and carbohydrate to vary more widely, a response termed “protein prioritization” (Fig. 6.1c). Carnivores tend to do the opposite, where protein intake varies more with dietary macronutrient balance.

      Animals in natural food environments

      Experiments such as those described above are a means to examine the nutrient regulatory responses of animals to simulated variation in highly simplified experimental environments, but they do not tell us whether and how these responses operate in the realistic setting of natural food environments. In recent years, several studies have examined this issue through recording the dietary intakes of individual animals in unmanipulated or minimally manipulated natural environments. Much of this work has concerned primates because they readily habituate to the presence of human observers, enabling detailed observation over entire days or even multiple consecutive days. The focus on primates is beneficial from our perspective because it helps to place the human nutritional research discussed below into a broader biological context.

      It is now clear that animals in the wild, as in the laboratory, employ nutrient‐specific appetites to compose diets with specific amounts and ratios of macronutrients. Felton et al. [28] found that spider monkeys in Bolivia have a strong preference for Ficus boliviana figs, and when these are not available, eat other food combinations to form a diet with a protein‐energy ratio very similar to that of the figs. Mountain gorillas in Bwindi and Virunga compose nutritionally similar diets from very different food combinations [29,30]. Johnson et al. [31] found that a baboon studied for 30 consecutive days composed daily diets with similar percentage energy from protein, even though she ate very different food combinations on different days. Other studies have demonstrated that wild primates change the selected diet to track specific changes in nutrient requirements. Guo et al. [32] showed that the intake of fat and carbohydrate by golden snub‐nosed monkeys living in a highly seasonal environment increased during the cold winters by an amount that closely matched the increased energy requirements for maintaining body temperature in the cold, whereas protein intake did not change. Cui et al. [33] found that rhesus macaques increased their energy intake by ~30% when lactating, but in this case, there was no difference in the ratio of macronutrients selected.

Several studies have examined the regulatory responses of wild primates when natural variation in food availability constrains them from achieving their macronutrient target. A variety of responses has been recorded for different species. Spider monkeys [34] (Fig. 6.3), chimpanzees [35], and blue monkeys [36] show protein prioritization, where protein intake is maintained at or close to the target level while fat and carbohydrate intake vary with the percentage of energy contributed by protein in the diet (Fig. 6.1c). Mountain gorillas show the opposite response, in which they over‐eat protein to maintain a constant intake of non‐protein energy [37]. Rhesus macaques show an intermediate strategy, in which the deficit of one nutrient matches the surplus ingested of the other, and consequently, total energy intake is maintained constant with variation in dietary macronutrient ratios [33].

Figure 6.3 Regulatory responses by free‐ranging Peruvian spider monkeys (Ateles chamek) to ecologically imposed variation in dietary macronutrient ratios. Each point represents the daily protein and non‐protein energy intake of an individual. The overall pattern of intakes suggests protein prioritization, in which the target intake of protein is maintained, and fat and carbohydrate intake vary with daily variation in dietary macronutrient balance (blue pattern in Fig. 6.1).

      Source: Modified from Felton et al. [34].

Human macronutrient regulation

      Across the many species studied using nutritional geometry, one thing that stands out is the power of macronutrients. Once appetite responses have been mapped to variation in dietary protein:carbohydrate:fat mixture, including intake targets and responses to constrained imbalance, an animal’s behavior, health, and life history (for example reproduction and longevity) can be predicted and manipulated with a high degree of certainty. Of course, micronutrients also play an important role in biology, and for some, notably calcium [38] and sodium [39], specific appetites have been identified. However, they seldom, if at all, have the same leverage over the animal’s interactions with its environment as do macronutrients. This suggests that evolution has converged on low‐dimensional ingestive regulatory systems, and relatively simple models can go a long way towards understanding key aspects of nutrition [19].

      Can obesity be understood within this framework? A first step towards empirically addressing this question is to determine whether the kinds of regulatory responses recorded in insects and wild primates also exist for our species.

      Do humans select an intake target?

      Several lines of evidence, spanning global patterns of macronutrient intake distributions, experimental trials, and mechanistic studies, indicate that humans have the capacity to regulate the intake of macronutrients to an intake target as do other species.

      Lieberman et al. [40] analyzed dietary macronutrient distributions from national survey data, including the US National Health and Nutrition Examination Survey (NHANES), and data from 13 countries with gross domestic products above $10,000 per capita per annum. The proportion of protein in the diets of all 14 countries was highly consistent at 16% of total energy, whereas calories from fat and carbohydrates were substantially more variable within and between populations. In the United States, protein, fat, and carbohydrate comprised 16, 33, and 48% of total energy intake, respectively.

Fat and carbohydrate varied significantly with age and race, but protein intake was not significantly related to demographic or lifestyle factors. These results support previous reports [e.g. 13,41] showing consistency across countries, populations, and time (Fig. 6.4a) of protein intake at ~15% of total energy, whereas fat and carbohydrate distributions vary more widely. Such consistency for protein is suggestive of regulation of the ratio of protein to non‐protein energy (fats and carbohydrates) in the diet.

      Experimental studies have provided evidence for balancing macronutrient