Obesity - a natural consequence of human evolution

Abstract

Obesity is considered a major epidemic of the 21st century. In developed countries, about 1/3 of adults are obese and another 1/3 overweight according to the oversimplified measure - the Body Mass Index. More precise indicators of adiposity: waist circumference, skinfolds, underwater weighing and absorptiometry indicate similar levels of fatness. Obesity per se does not necessarily lead to pathological states, nor to premature mortality. Recent results of large sample studies indicate that more than 1/3 of people classified as obese by fatness indices are physiologically normal. Others, however, suffer from a number of pathological conditions, common among them being the metabolic syndrome and cardiovascular disease. The classical explanation for increasing obesity is the positive energy balance - too much food intake and too little exercise. It seems, however, that this explanation is too simplistic. In societies, and in families, exposed to overeating and lazy lifestyles, about 1/3 of individuals have normal body mass and low levels of fatness, while others become obese. There is, therefore, individual variation in propensity for obesity. We have identified two specific variables differentiating fatness. People who have large lean trunk frames - large volumes of abdominal cavities and thus large gastrointestinal tracts - put on more subcutaneous fat than those with smaller trunk frames (Henneberg and Ulijaszek 2010). This may be a result of larger volumes of food required for antral extension to release ghrelin, or larger surface area of small intestines for food absorption. The second variable is concentrations of Alanine Transaminase, an enzyme responsible for conversion of an amino acid to a carbon skeleton that can be used in fat synthesis. Our study of 46000 young Swiss males (Henneberg, Rühli, Gruber and Woitek 2011) found consistent correlation between levels of Alanine Transaminase and body weight in groups of normal body mass individuals, overweight individuals and moderately obese individuals. Coupling this finding with the fact that among vegetarians, even those living in North America with overabundance of food and low levels of exercise, obesity and overweight are much less common than among non-vegetarians, we have now hypothesized that the increased obesity of modern affluent societies is a result of consumption of animal protein when energy needs are already covered by carbohydrates and fat consumed concurrently. Until the advent of agriculture, humans relied on consumption of a variety of terrestrial and aquatic animals supplemented by relatively small amounts of plant foods. In this situation our bodies became adapted to use proteins as a source of energy, and became efficient at storing occasional surpluses of amino acids by their deamination and conversion to fats. In the modern diets carbohydrates are abundant and provide, together with fats, energy required by human bodies, proteins after deamination are efficiently converted to fats. When new types of crops are introduced to mass production of cheap foods our bodies may not be able to react correctly to all their contents and some of the ingredients may cause additional fatness. An example of widespread recent introduction of industrially processed soybean products that correlates with prevalence of obesity across countries of the world is discussed.