the most abrupt morphological transition in the history of our species, if not any species. For researchers accustomed to studying the ecology and mechanisms of diet selection in simpler animals, such as insects, the scale, rapidity, and tenacity of the transition stood out as a biological enigma [1].
Many insect species studied to that point showed a remarkable ability to select food combinations and regulate the amounts of each eaten to compose a diet balanced in macronutrients. Some experiments simulated the essential properties of human obesogenic environments, involving unlimited access over prolonged periods to combinations of synthetic foods manufactured from refined ingredients, including at least one food high in palatable carbohydrate [2]. Yet, there was no obesity produced. This could not be explained by high levels of energy expenditure because even though the insects could move freely within the experimental boxes, they moved little. Rather, it was due to their ability to self‐compose a diet that defended a healthy body composition. Indeed, when the insects were experimentally induced into increased levels of physical activity, they changed their diet selection to compensate for the additional energetic costs [3]. More complex non‐human animals, such as rodents and chickens, likewise balance their macronutrient intake in this way, and change the selected diet to match changing nutritional requirements [3,4].
Why is it that our own species, with unprecedented privilege in terms of science and technology, fails in something so fundamental as eating a healthy diet, whereas species as simple as insects succeed using raw biology alone, even when surrounded by abundant palatable, calorie‐dense refined foods with no need for exercise? Were the psychological and biological capacities for diet selection by humans a casualty of 10,000 years of self‐domestication? Or was the problem in the environment? Or some combination of the two?
These questions are more than academic curiosities; they are central to framing obesity science and deeply entangled in the political issue of individual vs. societal responsibility [5–7]. They are pivotal for the management and prevention of obesity determining, inter‐alia, who bears the cost, the extent to which public resources are focussed on changing the behavior of individuals versus implementing public health policy [8], and indeed the severity and time course of the epidemic [9]. They also influence the quality of life for people with obesity through their role in generating blame and stigma [10] or normalizing a condition that can have serious health implications [11].
With the rise of obesity having continued unabated since it was first declared a global epidemic, to the point where the prevalence in several countries now exceeds 50% and others are fast approaching these levels [12], it is more urgent than ever to find fresh approaches to understand, prevent and manage the problem. In this chapter, we show how research, initially on insects and subsequently many other species (from single cellular slime molds to apes in the wild), has suggested a new ecologically inspired approach for understanding the roles of biology, environment, and their interactions in driving the obesity epidemic, and potentially identifying solutions [13]. We explain the theoretical foundations for the approach, illustrate its application to addressing relevant questions in some non‐human species, and show how it has been applied in studies of humans. The research suggests that humans are no different from other species in fundamental respects and that these similarities can provide a powerful guide for identifying relevant biological and environmental factors and examining how they interact to generate obesity.
An ecological view of nutrition
The simple and the complex
Among the most important challenges for biological and biomedical scientists is to identify the appropriate level of complexity at which to address a problem. In obesity research, approaches range from the simple energy balance frameworks (“energy in/energy out” and “eat less/exercise more”), to complex systems models that integrate many biological and environmental factors across multiple scales [9,14–16]. There is no general answer as to which is the optimal level of complexity required because this depends on the system under consideration, the question being asked, and the goal of the study – e.g. whether it is to understand the mechanisms, predict the behavior of the system, or manage it for particular outcomes. An important general principle, however, is that attributed to Albert Einstein: “Things should be as simple as possible … but no simpler.”
But how for a given problem does one decide on what is “as simple as possible,” but not too simple? In the early 1990s, biologists were grappling with this problem, not in obesity research but in the ecological sciences. Ecologists study the interactions among organisms and their environments, and the most ubiquitous and important of such interactions involves diet: eating and being eaten. Understanding the drivers of food and diet selection is thus fundamentally important in ecology, but it is also potentially very complex. To begin with, animals need, and foods contain many nutrients and other components such as indigestible fiber and toxins, and their requirements and tolerances for each vary – for example with age, health, reproductive state, and activity levels. Further, nutrients do not act independently but interact in complex ways, such that the levels in the diet of some influence the effects of others, and therefore their ratios (i.e. balance) can be as, or more important than the individual amounts and concentrations in the diet. Such interactions can involve several nutrients. For example, calcium balance is influenced by dietary phosphorus, vitamin D, and some carbohydrates, such as lactose [17]. To further complicate things, for almost all species, very few, if any, foods contain all the required nutrients in the right ratios to satisfy nutrient requirements and even if they did, no food (with the exception perhaps of mammalian milk) changes its composition to oblige the changing nutrient needs of its consumer. Animals therefore need to combine several, often many, different foods in their diets in the right proportions to satisfy their complex and dynamic nutrient needs. And these are just a few of the complexities that nutrition entails.
In the 1990s, there were three dominant frameworks for understanding the role of diet in ecology, all of which were founded on highly simplified models of nutrition [18]. Two of these, Optimal Foraging Theory and Classical Nutritional Ecology, assumed that animals in the wild are disproportionately influenced by a single dietary component, energy, and protein, respectively, without regard for nuances such as those discussed above. The third framework, ecological stoichiometry, does emphasize the importance of dietary ratios, but makes the simplifying assumption that all nutrients can be represented by chemical elements – principally carbon, nitrogen, and phosphorus. By avoiding the complexities of nutrition, these frameworks enabled diet to be integrated into ecological theory and, in this respect, have made important contributions. However, there was no telling what had been lost to ecological theory by omitting essential features of nutrition not captured by these simplified frameworks. At the other end of the complexity spectrum, the applied and mechanistic nutritional sciences were amassing substantial detail about the chemistry and physiology of nutrition, with little attempt to integrate this detail into theory or draw on theory to synthesize and apply the information [13].
Against this background, the question arose what is “enough but not too much” nutritional complexity for ecological models? The most direct way to answer this is through discovering how animals themselves deal with the complexity of nutrition. There are, in theory, benefits from regulating the dietary intake of all nutrients with great precision, but there are also constraints and costs [19]. For example, the computational machinery required to solve such a high‐dimensional optimization problem is immense, and brains cannot be dedicated to nutrition alone. Even if they could perform the required integration beyond a certain level of dietary perfectionism, the gains from foraging will run into diminishing returns, and time would be better spent on other activities, such as predator avoidance, mating, and sheltering. On the other hand, animals that are too cavalier in their nutritional regulation will be driven to extinction. For these reasons, we might expect animal nutrition to achieve a balance between complexity and simplicity, and identifying that balance will inform the appropriate level of nutritional complexity to incorporate into ecological models.
Laboratory studies of animals
A strategic research approach for addressing this question is to start simple and see how