Abstract: This paper draws on comparative data to explore the evolutionary origins of human nutritional needs and the diverse strategies used by human populations to meet those needs. Humans have evolved distinctive nutritional characteristics associated with the high metabolic costs of our large brains. The evolution of larger hominid brain size necessitated the development of foraging strategies that both provided high quality foods, and required larger ranges and higher levels of energy expenditure. Over time, human subsistence strategies have become ever more efficient in obtaining energy with minimal time and effort. Compared to data from traditional, subsistence-level societies, the US diet differs markedly in its fat and carbohydrate composition, but not in its absolute energy content. Energy expenditure levels of subsistence populations are significantly higher than those of the US and other industrialized societies. These data suggest that rising rates of obesity associated with lifestyle modernization is not simply the product of greater energy intakes, but rather shifts energy balance and diet composition.
Alex’s Notes: Our lifestyles have been on a dramatic change for the last 10,000 years, since the advent of agriculture. Even in just the last 30 years there has been reported obesity and chronic metabolic disease in developing worlds where such problems were previously unknown. Fortunately, many scientists and layman alike are beginning to put on their critical thinking caps and look through an evolutionary lens to address the growing problems of obesity and related health conditions.
Using this lens, the author of the review at hand, Leonard, argues that,
“In reality, what makes us human is our ability to find meal in virtually any environment. Throughout most of our past, human lifestyles were characterized by high levels of physical activity and frequent periods of marginal or negative energy balance. These conditions selected for improvements in the energetic efficiency of human foraging strategies. Today, we are in many respects victims of our own evolutionary success.”
No doubt, we may very well be living in what has been termed an “obesogenic” environment that promotes sedentariness and abundant food. But it extends beyond simple thermodynamics as well. While there is no doubt that energy shifts are important (increased intake & reduced expenditure), diet composition also plays a critical role.
But before moving forward with this thought, we must look backwards for a foundation to work from. Our brains are metabolic factories. In fact, on a per weight basis, the brain uses roughly 16 times more energy per unit of time than skeletal muscle tissue (at rest), accounting for about 400 kcal of an adult’s basal metabolic rate (BMR) daily. Thus, it is incredibly interesting to note that the resting metabolic rate (RMR) between humans and other mammals of our size is not very different, despite our significantly larger brains. What this means is that we devote a much larger share of our daily energy to brain metabolism. As Leonard points out,
“This means that brain metabolism accounts for ~ 20–25% of RMR in an adult human body; as compared to about 8–10% in other primate species, and roughly 3–5% for non-primate mammals.”
This of course raises the question of how we became able to accommodate such a greedy and metabolically costly organ. As it turns out, the diets of modern human hunter-gatherers are fat more nutrient dense than the diets of other large primates. While the human diet varies considerably across the world, it has been estimated that modern human hunter-gatherers derived an average of 45-65% of their dietary energy intake from animal foods. In comparison, other modern primates consume mainly plant foods. Gorillas eat 80% of their diet as fibrous leaves and bark, and even chimpanzees only get about 5-10% of their calories from animal products (including insects).
Our basic anatomy supports the above. Our GI tract is more similar to that of a carnivore than the enlarged colons of apes that allow for excessive fermentation of the plant foods they eat. Our small guts are an adaptation to an easily digested, nutrient-rich diet. We also have canine teeth designed for tearing flesh – a clear sign of meat consumption.
A final point that must be noted addresses the other side of the equation – energy expenditure. As if our current population of most the known world wasn’t indication enough, humans have large territory needs. Modern human hunter-gatherers roam about 13 km/day compared to less than 2 km/day in chimps and gorillas. Amusingly, energy expenditure estimates for apes are consistent with sedentary lifestyles. Put another way, if you were to plug the values of an ape into the Harris-Benedict formula for estimating BMR, their total energy expenditure would have you use the “sedentary” multiplier of 1.2. Compared to this, modern human hunter-gatherers would be “very active” (1.8x), while industrial humans would be considered “lightly” to “moderately active” (1.3-1.5x).
So from an evolutionary perspective, it appears that,
“Two of the distinctive elements of human nutritional biology compared to other primates are (1) a high quality, energetically dense diet to support our large brains, and (2) a foraging strategy that necessitated large ranges and high activity budgets.”
Since beginning our quest for world domination out of Africa, we have exploited nearly every ecosystem imaginable, which was in large part dependent our ability to develop new strategies and technologies that further increased our return on investment. These include agriculture and novel food processing methods, among others.
So what went wrong?
No one can deny our success. We are able to survive and even thrive on a remarkable diversity of diets. However, looking to the modern Westernized world, it becomes blatantly obvious that we may have been a bit too successful. The debate over the largest contributing factors to obesity is endless, but two factors that are undeniably at the root of the problem are an excessive availability of high-energy, high-reward food items, coupled with exploding portion sizes. Even so, they do not paint a full picture. In fact, US NHANES data show that there have been relatively modest increases in energy consumption over the last 40 years; when body weights and obesity rates were dramatically increasing, daily energy intakes increased by only 115 kcal in men and 260 kcal in women.
“In addition, from these data it appears that all of the increase took place between the NHANES II (1976–80) and NHANES III (1988–94) surveys. This is notable because there was a change in the methods used to assess dietary intake with NHANES III. All of the NHANES surveys rely on single 24-h dietary recalls to estimate daily energy and nutrient intakes; however, for NHANES I and II, no weekend days were surveyed, whereas, weekends were included in all of the subsequent NHANES surveys. Consequently, at least some of the apparent increase in dietary energy intakes between 1980 and 1988 may be attributable to methodological changes that allowed for weekend dietary patterns to be captured.”
The above gains significance when we consider only the data from 1988 onward where there is consistency in reporting methods, and we find that energy intakes have not kept pace with the changes in bodyweight. Ignoring that NHANES relies on dietary recalls that are prone to underreporting errors, the only rational explanation would be a simultaneous decrease in energy expenditure. Indeed, as Leonard points out,
“Among subsistence-level populations, the average daily energy expenditure is about 3000 kcal/day for men and about 2300 kcal/day for women. The energy demands of life in the industrialized world are more modest. Men of the western world sample are, on average, 12 kg (26.5 lb) heavier than their counterparts from the subsistence populations, and yet have daily caloric needs that are 150 to 200 kcal less. The pattern is similar, although somewhat less dramatic for women. Those of the industrialized world are 8 kg (17.7 lb) heavier and have daily energy demands that are about 90 kcal less than those from food-producing societies.”
Frankly, this means that living in the modernized worlds has resulted in less physical activity than living more traditionally. The difference in the predicted total energy expenditure for a 70kg male between a traditional and modern lifestyle is about 400 kcal/day. Although it would be unrealistic to demand that everyone in the Western world match the activity levels of modern hunter-gatherers, targeted lifestyle interventions that simply reduce the amount of time spent sedentary have a tremendous potential to increase energy expenditure. Notably,much of the daily work is done at a slow-to-moderate pace, implying that small changes such as standing instead of sitting, or riding a bicycle for commute, can add up into quite great changes.
Other factors may also play a role in the increasing rates of chronic disease. For instance, Americans derive 33% of their dietary energy from fat, 16% from protein, and the remaining 51% from carbohydrates. Compared to subsistence-level societies, Americans consume relatively less protein than either foraging or herding populations, who typically derive upwards of 20% of their calories from protein. Additionally, foragers and pastoralists generally consume fewer dietary carbohydrates than Americans.
Another critical factor is the type of carbohydrate consumed. Simple carbohydrates make up very small proportions of the diets of traditionally-living societies that have little or no access to market foods. In contrast, a large share of the carbohydrate calories in the American diet comes from simple sugars (19%) or refined grain products (20% of energy). Similarly, the types of fats consumed in the Western world are very pro-inflammatory dominated. Intakes of saturated fat and omega-6 fats are high, while intakes of monounsaturated fats and omega-3s are low.
So what does all this mean?
From an evolutionary perspective, it is clear that we can survive on a diverse mix of foods. However, our disproportionately large brain necessitates that the quality and nutritional density of our foods be higher than those of our primate kin. Over time, we have become ever-more efficient at extracting energy and nutrients form our environments and our current problem of “over-nutrition” can be thought of as a result of our past necessities.
Additionally, animal food consumption in the western world is not particularly high by the standards of many traditionally-living populations, and the types of carbohydrates and fats common in the modern diet appear to contribute to growing metabolic health problems. Thus, it appears that shifts in energy balance along with important changes in the composition of dietary fats and carbohydrates play central roles in explaining the rise of obesity and associated metabolic disorders in the modern world.
However, the other half of energy balance equation – reduced energy expenditure associated with increasingly sedentary lifestyles – is a critical and yet often overlooked part of the problem. When matched for body weight, the differences in daily energy expenditure between Western and subsistence-level adults are 210 kcal/day in women and over 400 kcal/day in men. Thus, the transition from subsistence to a more sedentary, modern lifestyle is associated with a 10-15% reduction in one's maintenance energy needs.
Addressing the emerging epidemic of obesity and associated metabolic diseases in the US and throughout the world will require changes in consumption patterns as well as broader lifestyle modifications.