The Evolution of Obesity

5 Evolution, Adaptation, and the Perils of Modern Life

111 CHAPTER FIVE Evolution, Adaptation, and the Perils of Modern Life The preceding chapters examine some aspects of our evolutionary history as they pertain to our biology. Now we explore the ways that the modern environment, interacting with our evolved biology, might make us vulnerable to sustained weight gain leading to obesity. Modern foods and the ways we eat have changed dramatically over the last 50 to 60 years, let alone from what our prehistoric ancestors experienced . Even the meat we eat differs from that of our hunter ancestors; meat from grain-fed cattle is generally higher in fat and has a different fatty acid profile compared to wild African ruminants (Cordain et al., 2002). Of course our eating habits are only part of the puzzle. Physical activity in modern, well-off countries differs both quantitatively and qualitatively from our past. We also examine the structural form of our living habitat, the so-called built environment, to see how it contributes to decreased physical activity. All of these components, along with familial traits—genetic, cultural, and socioeconomic—contribute to our modern obesogenic environment (Figure 5.1). We examine the changes in developing countries due to modern economic and scientific progress. The obesity epidemic that is in full swing in economically advantaged countries is just beginning in the developing world (Prentice, 2005). This gives us a chance to examine the features of demographic, dietary, and cultural change that might underlie our species’ vulnerability to obesity. An ironic aspect of human obesity is that it is frequently associated with malnutrition, not only in the same population but also in the same individual! Obesity and poverty are not uncommon today; indeed obesity is becoming more prevalent among the poor compared to the rich in many 112 THE EVOLUTION OF OBESITY countries (Brown and Condit-Bentley, 1998). Unfortunately, being calorie sufficient does not necessarily translate to satisfying nutrient requirements. We examine the evidence of nutrient deficiencies linked with obesity. Finally, we turn to some intriguing ideas about how obesity may be spreading through the human race. Is obesity actually contagious in some manner? There are both social and biological methods by which obesity could spread by contact. The factors in our modern world that affect our diet, activity, body image, and eventually our motivation to behave in ways that make us vulnerable or resistant to gaining excess fat cannot be all explained by biology. They all impact our biology, and are influenced by our biology, but there are nonbiological factors to the obesity epidemic. We try, in this chapter and elsewhere in the book, to acknowledge them, and we do not discount their importance. However, we are biologists, and this book is FIGURE 5.1. A schematic of the obesogenic environment. EVOLUTION, ADAPTATION, AND THE PERILS OF MODERN LIFE 113 about human biology and its relation to human obesity. To the readers who believe we are giving sort shrift to nonbiological factors driving the increase in human obesity we can only say that we can write with authority only about that which we know. Modern Food Food is a logical place to start. Although a vulnerability to obesity comes from many factors, at base it arises from eating more food than is necessary to maintain weight. The extra food energy is stored on the body as fat. This is normal adaptive biology. The question is, why are so many people these days eating more than they expend on a consistent basis? The foods of today are generally high in one or more of the following food types: meat, starch, simple sugars, and fat. They tend to be low in fiber and other difficult to digest materials. Modern foods are easily digestible and have a high energy density. They are the kinds of foods that would have been both rare and desired in our past. The types of foods our ancestors would have been motivated to expend considerable effort and risk to obtain. Of course nowadays we can have them delivered. Unfortunately that doesn’t appear to have reduced our taste for them. Why have modern foods become so energy-dense? Some of the answer is an understandable market response. Companies are delivering foods we like, and many of our taste preferences were set long ago when the desire for high-calorie foods, and thus being motivated to exert the effort and chance the risks to obtain...