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Eating What Comes Naturally: An Examination of Some Differences Between the Dietary Components of Humans and Wild Primates

Katharine Milton
Department of Anthropology
University of California
Berkeley, CA 94720-3710.

Talk prepared for: The Origins and Evolution of Human Diet, 14th International Congress of Anthropological and Ethnological Sciences, July 26-August 1, 1998, Williamburg, Virginia, USA.


     Today we'll trade places with wild primates and take a look at---eating what comes naturally---an examination of some differences between the dietary components of humans and wild primates.

     Humans do not instinctively know what to eat. Rather, foodways are learned primarily through exposure to the eating habits of others. In the past, human societies ate traditional diets which represented hundreds of years of experimentation with locally available dietary resources. Today, such traditional diets are largely a thing of the past, particularly in highly technological nations such as the United States. Here most people live in urban areas totally removed from the sources of food production and largely out of touch with most or perhaps all aspects of their past dietary heritage. Recent immigrants to the United States often cannot secure familiar foods and begin to alter their traditional dietary patterns, generally for the worse.

Health and Diet:

     Dietary patterns are of interest because of the strong concensus that many health problems in highly industrialized nations relate to diet. These problems includes heart disease, around 40% of all cancers, diabetes, atopic disease and obesity, among others. The acknowledged association between diet and disease strongly suggests that many humans are out of step with their evolutionary biology in terms of diet.

     And apparently most Americans are not even following current dietary recommendations. A recent nation-wide study carried out in the United States, a study which involved over ten thousand subjects showed that less than 3% had diets that actually conformed to dietary guidelines and met National Research Council recommendations (Calloway et al. 1992).

     Of interest too is that fact that such diet-based health problems are spreading through the third world as traditional diets alter. Looking only at obesity--noteworthy in part because of its strong association with many other serious health problems--we see that it has increased dramatically in the US over the past 3 decades-- but experts note that obesity is increasing worldwide.

     Other health problems are associated even with more traditional diets. This is shown by the high prevalence of infant and childhood mortality and malnutrition in many third world populations. Childhood malnutrition in turn is associated with various health problems in later life.

     What all of these rather depressing statistics indicate is that though we may know how to put men and women on the moon, we certainly do not appear to know what to put on our own dinner table. There are many suggestions as to how some of these dietary problems might be remedied and hardly a week goes by when some nutrient or diet is not touted as the critical one for better human health.

Comparative Study of the Diet and Digestive Physiology of Humans and Non-human Primates:

     One way to approach the question of how humans should eat is through comparative study of the diets of our closest living relatives, the non-human primates, looking not only at their foods but also their digestive morphology and physiology--and then comparing these findings with similar findings for humans. In such studies, attention shuld be focused not only on species closely related to us--such as chimpanzees-- but also on more distantly related species that, like humans, eat very high quality foods (Milton 1987). Today, as time is limited, I will focus my discussion only on certain nutrients but it is important to realize that ideally both diet and digestive factors need to be considered.

     For almost 20 years now, I and my associates have been examining the nutritional components of wild plant foods, particularly those eaten by neotropical monkeys. These monkeys, like most anthropoids, are almost exclusively herbivorous. A strong focus on plant foods is characteristic of all great apes--our closest living relatives -- and there is strong concensus that the ancestral line giving rise to both humans and extant apes was likewise very strongly herbivorous (Milton 1987, 1993).

     The plant parts we have analyzed were collected for the most part on Barro Colorado Island in the Republic of Panama. Analytical protocols are described in relevant publications and I have prepared a handout of these publications for anyone at this symposium who would like one. Foods that we have analyzed are eaten by one to four different primate species in this Panamanian forest. It is important to realize that foods from many of the same tree families, genera and, occasionally, even species are also eaten by primates in the forests of Africa and southeast Asia. For this reason, results of our analyses should be characteristic of similar primate foods on a pan- tropical basis.

     We have examined these dietary items for a number of different constituents including water, protein, individual amino acids, nonstructural and structural carbohydrates, fats, vitamin C, pectic substances, minerals and secondary compounds. Today I'll discuss 2 of these with you--namely, fatty acids and vitamin C -- these selected because they tend to be overlooked in most discussions about evolution and the human diet.


     Let's begin with fats. Because of their association with cardiovascular disease, there considerable debate in medical circles about the types and amounts of different fats humans should consume as well as the percentage of one's daily calories that should come from fat. Jack Chamberlain, Gary Nelson and I thought that information on the pattern of fat consumption by wild primates might offer some useful clues here.

     The predominate fatty acids in these wild foods are palmitic (30%), linoleic (23%) alpha linolenic (16%) and oleic (15%). Fatty acids with less than 16 or more than 18 carbon chains are uncommon. (range 0 to 7%). Saturated and unsaturated fatty acids are almost equally balanced. These wild foods also contain a high percentage of both omega-3 and omega 6 fatty acids (Chamberlain, Nelson and Milton 1993).

     Much dietary fat consumed in the United States is either saturated animal fat or oil from monocot seeds Most oil seeds are high in omega 6 but low in omega 3 fatty acids. But the diet of ancestral humans, like the diets of wild monkeys and apes, was likely very rich in both omega three and omega six fatty acids.

     Comparing humans and wild howler monkeys in Panama in terms of their respective patterns of fatty acid consumption--howler monkeys eating a typical leaf and fruit diet show a polyunsaturated to saturated fat ratio (P/S ratio) of 0.85--which is very close to the P/S ratio of 1.0 recommended for humans. Unfortunately the P/S ratio for the average Americans is not around 1.0 but rather is in the range of 0.4 to 0.5 (Chamberlain, Nelson and Milton 1993). It is also recommended that humans take less than 30% of daily calories from fats. Fats contribute only around 18% of the daily calories for howler monkeys but most Americans exceed the suggested 30% recommendation. So wild monkeys are doing better than most Americans at meeting the National Research Council's recommendations re dietary fat intake!

Vitamin C:

     Now let's consider vitamin C, a vitamin of particular interest when discussing anthropoids. An insufficiency of vitamin C can cause numerous health problems in humans (e.g. scurvy) and lead to death. Though most mammals synthesize their own vitamin C internally, anthropoids (monkeys, apes and humans) are one exception. Humans, monkeys and apes lack the enzyme L-gulonolactone oxidase (GLO, EC, which catalyzes the final step in ascorbate synthesis from glucose (Milton and Jenness 1987).

     If a lineage lacks the ability to synthesize a vitamin essential for life this suggests that in the past evolutionary history of that lineage there was no need for such synthesis--thus vitamin C must have been abundant in the diet of the common ancestor of all anthropoids and must still be present in the natural diets of extant monkeys and apes today. Curious about the vitamin C content of wild plant foods eaten by primates, Robert Jenness and I analyzed various species and compared our results with similar data for domesticated fruits and vegetables.

     Results show that wild leaves and fruits are rich in vitamin C--on average, Panamanian leaves and fruits have a higher content of vitamin C than foliar cultivars (domesticated leafy vegetables); on average, wild leaves actually have a higher vitamin C content than fruit juice from a mix of cultivars (lemon, orange, mango, papaya; Milton and Jenness 1987)

     We estimate that a 7 kg howler monkey takes in 88 mg of vitamin C per kg of body weight per day for a total of 600 milligrams of vitamin C per day (7kg x 88 mg per kg). For an 8 kg spider monkeys at 106 mg/kg/day, this figure totals 744 mg of vitamin C per day. Mountain gorillas which are estimated to take in some 20-30 mg of vitamin C per kg/day therefore are estimated to take in some 2 to 4 to even 6 or more grams (not milligrams) of vitamin C per day (100 to 160 kg , weight of an adult gorilla x 20-30 mg per kg) (Milton and Jenness 1987).

     Now what about humans? Unfortunately, worldwide, most human populations take much of the daily diet from a single grain or root crop--e.g., rice, wheat, manioc-- and grains and roots are notoriously low in vitamin C. Even in the United States, where fresh fruits and vegetables are readily available, the average American diet is estimated to supply only some 1.5 mg per kg of ascorbate per day or a total of some 88 mg of vitamin C per individual per day. This is the same amount of vitamin C in total as wild howler monkeys take in per kilogram. Thus wild primates routinely consume vitamin C in amounts considerably higher than humans.

Cautionary Note:

     What these differences actually mean in terms of human health remains to be determined--but many differences between these wild plant foods and our domesticated plant foods are quite striking. In closing let me also sound a cautionary note. The metabolism and fate of these or other nutrients has not been studied in wild primates and currently is unknown. Presentation of this comparative material is not an endorsement or even a suggestion for a particular type of diet (I am not advocating a vegetarian diet, for example) or for the intake or amount of intake of any given nutrient or nutrient group.

     Human nutrition is a huge and complex field -- the field is moving so rapidly today that reviews of particular nutrients are often obsolete before they can even be published. Many nutrients come in various forms; these forms can produce quite different experimental results depending on the form used or the other nutrients present in trails (Prasad, Cole & Hovland 1997). The actual physiological or health conditions of subjects can alter experimental results (Prasad, Cole & Hovland 1997). Though I personally believe that the diets of wild primates may have much to tell us about ways in which the human diet might possibly be improved, as yet this is only conjecture and remains to be demonstrated through carefully controlled studies. Thank you.


Calloway, D.H., S. Murphy, J. Balderstom, O. Receveur, D. Lein and M. Hudes. 1992. Final report to the U.S. Agency for International Development Cooperative Agreement #DAN 1309-A-00-9090-00).

Chamberlain, J.C. G.J. Nelson and K. Milton. 1993. Fatty acid profiles of major food sources of howler monkeys (Alouatta palliata) in the Neotropics. Experientia 49(9):820-824.

Milton, K, 1987. Primate diets and gut morphology: Implications for human evolution. IN: Food and Evolution: Toward a Theory of Human Food Habits. M. Harris and E.B. Ross, editors: Philadelphia: Temple Press, pp.93-116. Milton, K. 1993. Diet and Primate Evolution. Scientific American 269:86-93.

Milton, K. and R. Jenness. 1987. Ascorbic acid content of neotropical plant parts available to wild monkeys and bats. Experientia 43: 339-342.

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