Nutrition plays an important role in so-called “diseases of affluence” such as obesity, diabetes, and cardiovascular disease – now epidemic in developed countries. Widespread prevalence of these diseases of affluence has fuelled public interest in determining the optimal human diet. However, these efforts have been hampered by the complexities of human nutrition. The observation that modern hunter-gatherer societies appear to be free from the diseases of affluence has led evolutionary biologists to hypothesize that our stone-age metabolism and modern nutritional environment are poorly matched, fostering the development of these diseases. Examining how the human diet has evolved over time could provide insight into the origins of the diseases of affluence and what an optimal diet entails, as evolution likely selects for diets that maximize nutrition. So, what does our evolutionary history tell us about how we should structure our modern-day diets?
The evolution of the human diet
Homo sapiens sapiens, the anatomically modern human, evolved from hominin ancestors over a period of approximately 3 million years during the Paleolithic Era. Anthropological evidence suggests that the majority of human evolution (approximately 2.5 of 3 million years) occurred in East Africa. Modern humans are believed to have migrated from Africa only 60,000 to 100,000 years ago, relatively recently in our evolutionary timeline, suggesting that modern humans have retained the nutritional adaptations of these early migrants. Reconstructing this Paleolithic nutritional environment can provide clues to the diet of our Homo genus ancestors, and what would have comprised the maximally healthy diet. A major source of evidence comes from comparative morphology, which infers function through form. As an example of this, a crab claw and pair of scissors share form, such that a person unacquainted with crabs but familiar with scissors could infer that a crab uses its claw to cut. Similarly, features in hominin ancestors can be compared to evolutionarily related organisms whose diets are well characterized, such as vegetarian gorillas and omnivorous chimpanzees. Our earliest hominin ancestor, Australopithecus, evolved approximately 4 million years ago, and is characterized by enormous mandibles (lower jawbones), akin to gorillas, which require their large mandibles to effectively crush plant matter. As human evolution progressed, mandibles became more slender while brain size increased, akin to chimpanzees, which eat insects and small mammals in addition to plants. Another major morphological development, which occurred alongside brain expansion, is a reduction in gut length. These two changes are seemingly paradoxical as our brains are metabolically demanding organs, yet a reduced gut length would mean less capacity for absorbing nutrients to fuel a larger brain. Scientists have argued that these morphological changes in hominins were made possible due to a transition from plant to meat-based diets. Meat is easily masticated compared to plants, removing the need for larger and more powerful mandibles, and its fat content makes it energy-dense, matching higher metabolic demands despite a reduced gut length. Isotopic nitrogen analysis has suggested that the source of this meat was largely terrestrial animals. Notably, isotopic carbon analysis suggests that certain types of plants, primarily starchy in-ground tubers (prehistoric potatoes), remained in ancestral diets following this proposed transition. Accordingly, modern humans have likely retained these nutritional adaptations to the consumption of meat and starchy plants.
The transition from plant to meat-based diets implies an increased consumption of fat and protein relative to carbohydrate. However, their precise proportions in the ancestral diet are largely unknown. One source of evidence used by anthropologists to infer the ancestral dietary composition is the Ethnographic Atlas, a database of modern hunter-gatherer societies whose diets were recorded. Analysis of 63 hunter-gatherer societies in this database—chosen because their diets were recorded extensively—has suggested that the majority of modern hunter-gatherers consume predominantly animal foods, with a dietary composition range of 28-58% fat, 22-40% carbohydrate, and 19-35% protein. These studies suggest the best approximation of the Paleolithic macronutrient composition is a high animal fat diet, with ample dietary protein, and a small-to-moderate contribution of carbohydrates.
The modern diet and its relation to diseases of affluence
Following the evolution of modern humans, another vast change in diet occurred due to manufacturing technologies developed after the Industrial Revolution. Manufacturing and automation enabled the mass production and distribution of novel foods and ingredients, reflected in the increased consumption of bread, cereal grains, refined sugars such as table sugar and fructose, vegetable oils, and salt in the 19th century. Accordingly, these relatively rapid dietary changes likely produced a nutritional environment discordant with that for which our genomes were selected over hundreds of thousands of years. For example, one of the most consequential dietary alterations following the Industrial Revolution is a dramatic increase in the glycemic load (GL) of food, largely determined by carbohydrate metabolism. The dietary carbohydrates in food typically exist as multimers such as sucrose, glycogen, or starch, and are metabolized into base molecules, commonly glucose and fructose. When a meal is consumed, carbohydrates are metabolized into glucose, which is then transported across the gut and into the blood, thereby increasing blood glucose concentrations (hyperglycemia). As a physiological response to this transient hyperglycemia, the pancreas produces insulin, which mediates the transport of glucose from the blood into fat and muscle cells. GL is a measure of the degree to which a serving of food will elevate blood glucose and insulin concentrations, compared to eating pure glucose. Higher GL foods therefore induce a greater degree of hyperglycemia and hyperinsulinemia.
In the post-Industrial modern day, high GL carbohydrate intake is at an all-time high. Carbohydrates now account for greater than 50% of total caloric intake in the US, mirrored in statistics from Canada and the UK. Developing countries are also increasing their consumption of high GL carbohydrates as they modernize. These are worrying statistics, as research has concluded that long-term consumption of high GL diets results in chronic hyperglycemia and hyperinsulinemia that initiates metabolic changes which precede insulin resistance, a major risk factor for diseases of affluence. Thus, the modern nutritional environment and diet, dominated by high GL carbohydrates that promote insulin resistance, likely contributes to the epidemic prevalence of diseases of affluence in developed countries.
A healthful modern diet: fewer carbs, more fat
If carbohydrates are so metabolically dangerous, why did our ancestors continue to consume starchy plants, and how do modern hunter-gatherer societies do so without developing diseases of affluence? One explanation is that our ancestors simply died too young to manifest these diseases. However, metabolic syndromes plague children and adolescents just as much as the elderly in the 21st century. A relatively higher amount of physical activity in our ancestors likely helped, but we know now that exercise does not negate a poor diet. Accordingly, the other half of the answer might come from the notion that our ancestors, along with modern hunter-gatherer societies, eat a fat-based diet. Dietary fat reduces the GL of meals by triggering the production of gastric peptides, which reduce gastric emptying, or the rate at which food moves into the intestine. This process slows down the digestion of carbohydrates thereby reducing their GL. In effect, a fat-based diet is a low GL diet. Similarly, the converse is true; a low-fat diet is in effect a high GL diet. Trends have shown that developed countries have been consuming less fat since the 1970s, likely in response to dietary recommendations made in the US, which suggested increasing dietary carbohydrate intake at the expense of fat. These recommendations were made at the time on the basis of a correlation between saturated fat intake and risk of cardiovascular disease. On the contrary, recent systematic reviews examining associations between total and saturated fat consumption and the risk of cardiovascular disease or diabetes have concluded that there is a lack of significant associations, and insufficient evidence to support dietary recommendations of reducing fat intake.
This does not mean all dietary fat is necessarily safe – for example, there is strong evidence linking particular types of fat such as omega-6 polyunsaturated and trans fats to adverse outcomes. Yet, saturated fat appears to be quite safe. It is the least susceptible to degradation processes such as peroxidation compared to other types of fat, and is the predominant type of fat within human cell membranes. Our ancestors also likely consumed mostly saturated fat, as it is the predominant type of fat in terrestrial animals. Accordingly, increasing consumption of saturated fat to reduce the overall GL of modern diets like our ancestors is a practical way to reduce the incidence of diseases of affluence. Practically speaking, this means eating your cereal (if you must) with whole and not skim milk, and spreading some butter on your toast. Future research critically assessing the safety profile of higher-fat diets as well as public outreach to correct prevailing inaccurate nutritional beliefs will be well poised to yield significant improvements to human health.
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