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Department of Environmental Science, Policy and Management Division of Insect Biology University of California Berkeley, CA 94720-3112 E-mail: kmilton{at}socrates.berkeley.edu
Dear Sir:
In their article in the March 2000 issue of the Journal (1), and now in their letter to the Editor, Cordain et al discussed plant-animal subsistence ratios and likely macronutrient intakes (percentage of energy) in recent hunter-gatherer societies. They concluded that, worldwide, most hunter-gatherer societies derived >50% of dietary energy from animal foods and suggested that "the universally characteristic macronutrient consumption ratios of hunter-gatherers in which protein is elevated at the expense of carbohydrate" may have therapeutic health effects for modern humans.
As discussed in my March 2000 editorial on this topic (2), hunter-gatherer societies, both recent and ancestral, displayed a wide variety of plant-animal subsistence ratios, illustrating the adaptability of human metabolism to a broad range of energy substrates. Because all hunter-gatherer societies are largely free of chronic degenerative disease, there seems little justification for advocating the therapeutic merits of one type of hunter-gatherer diet over another.
What general features of hunter-gatherer diets might contribute to this lack of degenerative disease? One important feature may be that many wild foods consumed by hunter-gatherers are similar or identical to foods consumed by their prehuman ancestors. Thus, it could be said that human biology is adapted to characteristics of a wide range of wild plant and animal foods but apparently is less well adapted to characteristics of many contemporary Western foods.
Most wild foods have a low energy density compared with the refined foods of Western nations. Muscle tissue of wild prey is consistently low in fat and fat depots tend to be very small in most wild animals (3). Most wild fruit is hexose dominated (4), and wild plant foods tend to have a low glycemic index (5) and, often, considerable dietary fiber (4, 5). Such features, in combination with the slow transit of ingesta characteristic of humans (4), should make it difficult for hunter-gatherers to digest more than a limited quantity of these wild foods each day (2). In effect, then, most hunter-gatherers have a natural barrier between themselves and chronic dietary or energy excess.
In contrast, contemporary Western populations live surrounded by volumetrically concentrated foods that are high in sugar and fat and that can be ingested in enormous quantities. It is extremely easy for individuals in Western nations to consume far more energy each day than they expend. Although often stated, it bears repeating that this Western dietary pattern, in combination with a largely sedentary lifestyle, appears to contribute to many chronic degenerative diseases that affect Western nations but are largely or completely absent in hunter-gatherer and similar societies (2, 6), regardless of the macronutrient ratio or principal energy source.
To derive their conclusions on hunter-gatherer diets, Cordain et al (1) used Murdock's Ethnographic Atlas (7). Despite its general utility, the Atlas provides, at best, a "quantitative overview" (1) of the dietary behaviors of recent (largely 20th century) hunter-gatherers and "in almost all cases represents subjective approximations by Murdock of the ethnographer's or anthropologist's original observations" (1).
In his 1968 analysis of hunter-gatherer diets, Lee (8) reclassified some Atlas data and also excluded mounted hunters with guns and "casual" agriculturalists from his database. In Lee's opinion, only 24 societies from all of Africa, Asia, Australia, and South America could be classified as hunter-gatherers, whereas North America alone contained >80% (135) of the 165 "hunting" societies listed in the Atlas.
In contrast, in their analysis, Cordain et al (1) identified 229 hunter-gatherer societies in the Atlas; they also combined 2 of Lee's discrete categories (hunting and fishing) to estimate the total contribution of animal foods to energy subsistence. Given the uneven quality of most dietary data in the Atlas, the overrepresentation of hunter-gatherer societies from more temperate locales and the differences in classification and data analysis between these authors, different conclusions seem inevitable and all conclusions appear to merit closer study.
The !Kung and Hazda, dismissed by Cordain et al as "unrepresentative," differ from many hunter-gatherers listed in the Atlas precisely because they have been relatively well studied dietarily—in both cases, plant foods contributed the bulk of daily energy intake. Examination of the literature suggests that hunter-gatherers throughout the world took full advantage of any dependable sources of dietary energy in their environment (9–11), even devising complex technologies to secure energy from potentially toxic plant sources such as acorns and cycads (10, 11). Such dependable plant foods, in turn, tended to be relied on heavily for dietary energy. For this reason, Cordain et al's comments on the "low carbohydrate content of wild plant foods" seem largely beside the point—what is key is the steady availability of energy from 1 or 2 reliable wild-plant staples. To secure a dependable source of dietary carbohydrate, some hunter-gatherers, such as the Mbuti (Africa) and the Maku (South America), established symbiotic trade relationships with indigenous agriculturalists (12).
There seems little doubt that many hunter-gatherer societies had a high intake of animal protein (and animal foods) by present-day standards (1, 8, 13). However, this does not imply that such a dietary pattern is the most appropriate for human metabolism or that it should be emulated today. Past hunter-gatherers did not have unlimited dietary options but had to make the best of whatever was available in a particular habitat. The gut proportions of humans do not indicate a highly carnivorous diet; rather, they indicate adaptation to a diet made up of high-quality foods of all types and amenable to digestion primarily in the small intestine (14). Gut proportions of carnivorous mammals differ from those of humans (2). Food transit times in humans are very similar to those of apes and notably different from those of carnivores (2, 14).
To date, few genetic adaptations to diet have been identified in humans, suggesting that, in their evolution, humans tended to resolve dietary problems primarily by using technology rather than biology. The technologic abilities of humans derive from their unusually large, complex brain, a brain that, under normal conditions, is fueled by a steady supply of glucose. Consumption of digestible carbohydrate is the most efficient way for humans to obtain glucose for brain function. Potential alternatives—gluconeogenesis or the use of ketones to fuel the brain—represent alternative, more costly metabolic solutions.
Although Cordain et al noted a neutral or therapeutic effect for high protein intakes in some instances, Hu and Willard (15) recently cautioned application of their findings on heart disease and a high protein intake to public dietary advice because "a high dietary protein intake is often accompanied by high saturated fat and cholesterol intakes." Given that most Westerners do not have access to wild game, this recommendation seems prudent. Certainly the average well-nourished, inactive American might benefit from reaching for 100 g lean protein rather than a 100-g cheese danish, but foraging for a 100-g apple might prove to be the most therapeutic of all.
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