Metabolic Evidence of Human Adaptation to Increased Carnivory
by Loren Cordain, Ph.D.
IMPORTANT WORD DEFINITIONS: Dentition: Teeth. Hominids: Bipedal primates, including human beings and our bipedal ancestors in evolutionary prehistory. Obligate, as in "obligate carnivore" (cats are obligate carnivores in their natural environment): Able to exist or survive only in a particular environment or by assuming a particular role (definition from The American Heritage Dictionary, 3rd edition). Pongid: The class of ape mostly closely related to humans, consisting of--
Based on and edited from a posting to the Paleodiet listgroup on 1/15/98.
I am in agreement with previous posts that human dentition is adapted for a generalized diet composed of both plant and animal foods, and that human populations show amazing variability in their plant-to-animal food subsistence ratios. However, it is important to recognize that hominids have evolved important metabolic and biochemical adaptations which are indicative of an increasing physiological dependence upon animal-based foods. Further, comprehensive compilations of hunter-gatherer subsistence strategies indicate that whenever it is ecologically possible, humans will almost always consume more animal food than plant food.
Background: Hunter-gatherer plant/animal subsistence ratios.
The most frequently occurring (mode) plant/animal subsistence ratio for worldwide hunter-gatherers is 16-25% plant/75-84% animal, and the median value is 26-35% plant/65-74% animal. These values corroborate five careful modern studies of hunter-gatherers showing a mean energy (caloric) intake from animal-food sources to be 59% [Leonard et al. 1994].
Comparing the human gut with ape guts and biochemical adaptations of carnivores.
Pongids (the primates that humans are most closely related to), because their diet is largely plant-based, must maintain large and metabolically active guts to process the fibrous plant foods which compose over 93% or greater of their dietary intake. In contrast, the human gut is much smaller and less metabolically active than the ape gut. Presumably this adaptation (reduction in gut size and metabolic activity) evolved in humans because the inclusion of nutrient-dense, animal-based foods by our early hominid ancestors allowed the selective pressure for a large, metabolically active gut to be relaxed [Leonard et al. 1994; Aiello and Wheeler 1995].
In addition to the smaller gut that humans maintain relative to apes, there are other metabolic and biochemical clues which point to increased utilization of animal food by humans over our evolutionary history. By evaluating the metabolic and biochemical dietary adaptations of cats (obligate carnivores) and those in humans (omnivores), it becomes apparent that evolution has shaped both hominid and feline metabolic machinery towards a diet in which animal food was predominant.
Obligate carnivores, such as cats, must obtain all of their nutrients from the flesh of other animals and have therefore evolved certain biochemical adaptations which are indicative of their total dietary dependence upon animal-based foods. Most of these biochemical adaptations involve either the loss (or reduced activity) of certain enzymes required for the synthesis of essential nutrients. These adaptations generally occurred because the evolutionary selection pressure to maintain these metabolic pathways was relaxed as cats gradually increased the amount of animal food in their diet as they evolutionarily progressed from omnivory into obligate carnivory.
Taurine is an amino acid which is not found in any plant-based food [Laidlow et al. 1990] and which is an essential nutrient in all mammalian cells. Herbivores are able to synthesize taurine from precursor amino acids derived from plants, whereas cats have completely lost the ability to synthesize taurine [Knopf et al. 1978]. Since all animal-based foods (except cow's milk) are rich sources of taurine [Laidlow et al. 1990], cats have been able to relax the selective pressure required for taurine synthesis because they obtain all of this nutrient that they need from their exclusive meat-based diet.
Humans, unlike cats, still maintain the ability to synthesize taurine in the liver from precursor substances; however, this ability is quite limited and inefficient when compared to herbivores. Vegan vegetarians following diets devoid of animal products display low levels of both plasma and urinary taurine [Laidlow 1988]--levels which are indicative of the poor ability of humans to synthesize taurine. Similar to cats, this inability to efficiently synthesize taurine has come about because the selective pressure to produce this amino acid has been gradually reduced due to humankind's long reliance upon animal food, a food which is quite high in taurine.
20- and 22-carbon fatty acid requirements
Plant-based foods contain 18-carbon fatty acids of both the omega-3 and omega-6 families, but are virtually devoid of the 20- and 22-carbon fatty acids that are required for the normal functioning of all mammalian cells, whether the mammal is an herbivore or carnivore. Herbivores have evolved hepatic (liver) enzymes (desaturases and elongases) which allow these precursor, plant-based 18-carbon fatty acids to be chain-elongated and desaturated to their 20- and 22-carbon products.
Cats have extremely low levels of the enzymes required to make 20- and 22-carbon fatty acids from 18-carbon fatty acids [Salem et al. 1994]. Again, the selection pressure to synthesize 20- and 22-carbon lipids (fatty acids) has been almost entirely removed because cats obtain sufficient quantities of these long-chain fatty acids by eating animal tissues which are rich sources of these lipids. Humans, though not as inefficient as the cat, also have relatively inefficient elongase and desaturase enzymes [Salem et al. 1994]. Again, this metabolic change has occurred largely because the need to desaturate and chain-elongate 18-carbon plant fatty acids to their 20- and 22-carbon products has been reduced because humans, like cats, have obtained a large portion of their 20- and 22-carbon lipids directly by eating other animal tissues.
Vitamin A synthesis
All animals, whether herbivore or carnivore, require vitamin A. Vitamin A is not found in any plant-based food; consequently, herbivores must synthesize it in the liver from beta-carotene consumed from plant-based foods. Cats have lost the ability to synthesize vitamin A from beta-carotene [MacDonald et al. 1984], and must obtain all of their vitamin A from the organs (liver, kidney) of their prey. Again, cats have lost the ability to synthesize vitamin A because the selective pressure (need) to provide adaptive energy for the synthesis of proteins to catalyze the production of vitamin A was reduced as cats progressively increased the amount of animal foods in their diets.
Recently, it has been shown that humans have a limited capacity to absorb beta-carotene in plants [de Pee and West et al. 1995] (the bioavailability of beta-carotene from plants is low for humans compared to its bioavailability from other sources), presumably because humans, like cats, have consumed vitamin A-rich animal food sources for eons and are in a transitional state from omnivory to obligate carnivory.
Vitamin B-12 is an essential nutrient for both herbivorous and carnivorous mammals. Because B-12 is not found in higher plants, herbivorous mammals must solely rely upon absorption of B-12 from bacteria that synthesize it in their gut. Cats can neither synthesize B-12 nor absorb it from their gut; consequently they have become wholly dependent upon animal flesh as their source for this essential nutrient.
Humans, like cats, cannot depend on the absorption of bacterially produced vitamin B-12 from the gut, and are reliant upon animal-based sources of this essential vitamin, since it does not occur in a biologically active form in any of the plant foods which humans normally eat. While some viable B-12 is synthesized in the human colon, the site of absorption is at the ileum, which is "upstream" from the colon at the lower end of the small intestine; thus for humans, B-12 synthesized in the colon is unavailable and must come from the food eaten [Herbert 1988]. Regarding possible B-12 synthesis in the small intestine above the ileum, the consensus of scientific literature indicates any amounts that may potentially be produced are not significant or reliable enough to serve as a dependable or sole source for most individuals.
Additionally, while most cases of B-12 deficiency in omnivores are due to problems of impaired absorption rather than a deficiency of nutritional intake [Herbert 1994], the opposite situation prevails in vegetarians eating only minimal amounts of animal by-products [Chanarin et al. 1988].
Further, studies in vegans have shown that despite physiological recycling and conservation mechanisms that become increasingly efficient as B-12 intake falls below normal daily requirements--so that very little is lost from the body--the likelihood is high that B-12 deficiency will eventually develop (after 20 years or more) in pure vegans who refrain without fail from ingesting any animal-based products or do not take B-12 supplements [Herbert 1994].
Recent work has delineated four stages of inadequate B-12 levels in strict vegetarians [Herbert 1994; Herbert 1988]. Vegans in early Stage I depletion--prior to ongoing depletion of stores and the declining blood levels of Stage II, biochemical deficiency and impaired DNA synthesis of Stage III, and clinical deficiencies of Stage IV--are able to maintain normal serum B-12 levels. However, this occurs by drawing from stored reserves in the liver and elsewhere which gradually become depleted, eventually to the point where actual deficiency develops many years later in those who maintain strict habits.
It is this negative metabolic B-12 balance which occurs soon after exogenous B-12 ceases to be ingested in appreciable quantities, many years prior to actual deficiency, which points to the human requirement for animal-based B-12 sources if one is to maintain a positive B-12 balance. One need not show actual cases of deficiency and end-stage megaloblastic anemia, but only the trend of long-term negative B-12 balance, to demonstrate the human metabolic need for animal-based foods to maintain a neutral or positive homeostatic balance.
It is now possible to determine negative homeostatic B-12 balance directly by distinguishing and measuring levels of the two forms in which B-12 is carried in the blood: either bound to the transcobalamin II "delivery" molecule (called TCII, for short), or bound to the haptocorrin molecule which is a form of "circulating storage." Haptocorrin maintains equilibrium with body stores, meaning that haptocorrin levels reflect current reserve stores of B-12. TCII, however, being the "delivery" molecule for B-12, transports and gives up its B-12 to cells that are actively using B-12 in DNA synthesis, and has a half-life in the blood of only 6 minutes. Thus, when exogenous intake of B-12 falls below normal, levels of TCII-carried B-12 begin to reflect the deficit rapidly, and subnormal levels will show up within one week, demonstrating negative B-12 balance. [Herbert 1994]
It is probable, therefore, that vegans with long-term normal B-12 balance are ingesting--inadvertently or otherwise--at least small amounts of B-12, if not from supplements, then from unreported animal-based sources in their food or contaminated by such sources. (In one study of vegans for which this has been observed, the cause was due to eating unwashed vegetables that had been grown in gardens containing intentionally manured soils, from which the B-12 came [Herbert 1988]. Ironically, the manure in this case was their own excrement, which as pointed out above harbors bacteria that produce B-12 in the human colon--where B-12 cannot be absorbed. Not unless, of course, it is reingested as in the unintentional coprophagy occurring in this instance, so that it can pass back through the small intestine again to the ileum where B-12 is actually absorbed.)
An indication of the masking effect of previously stored B-12 reserves in obscuring ongoing negative B-12 balance can be seen in long-term vegan mothers and their infants. Such mothers may maintain blood levels of haptocorrin B-12 in the normal range for lengthy periods (years) due to increasingly efficient recycling of B-12 as their reserve stores become depleted, and in adult vegans with such improved B-12 reabsorption, such clinical deficiency may take 20-30 years to manifest. However, infants of such mothers are born with almost no reserve stores (little or none are available in the mother's body to pass on to them) and go into clinical deficiency much more rapidly. [Herbert 1994]
In summary, the absence of the ability of humans to absorb bacterially produced B-12 in the colon, and the evidence that strictly behaving vegans will show negative TCII-carried B-12 balance even when total serum levels are in the normal range, is indicative of the long evolutionary history of animal-based foods in our diet.
These metabolic and biochemical adaptations in humans in response to increasingly meat-based diets, as well as the anthropological evidence provided by both contemporary and historical studies of hunter-gatherer diets, provide strong evidence for the central role of meat and animal tissues in the human diet. Although it is true that human populations can survive under broad plant/animal subsistence ratios, the consensus evidence supports the notion that whenever it was ecologically possible, animal calories would have always represented the majority of the total daily energy intake.
Loren Cordain, Ph.D.
Aiello LC, Wheeler P (1995) "The expensive tissue hypothesis." Current Anthropology, vol. 36, pp. 199-221.
Chanarin I, Malkowska V, O'Hea A-M, Rinsler MG, Price AB (1985) "Megaloblastic anaemia in a vegetarian Hindu community." The Lancet, 1985(2), pp. 1168-1172.
de Pee S, West CE et al. (1995) "Lack of improvement in vitamin A status with increased consumption of dark leafy green vegetables." Lancet, vol. 346, pp. 75-81.
Herbert V (1994) "Staging vitamin B-12 (cobalamin) status in vegetarians." American Journal of Clinical Nutrition, vol. 59 (suppl.), pp. 1213S-1222S.
Herbert V (1988) "Vitamin B-12: plant sources, requirements, and assay." American Journal of Clinical Nutrition, vol. 48, pp. 852-858.
Knopf K et al. (1978) "Taurine: an essential nutrient for the cat." Journal of Nutrition, vol. 108, pp. 773-778.
Laidlow SA et al. (1990) "The taurine content of common foodstuffs." Journal of Parenteral Enteral Nutrition, vol. 14, pp. 183-188.
Laidlow SA (1988) "Plasma and urine levels in vegans." American Journal of Clinical Nutrition, vol. 47, pp. 660-663.
Leonard WR et al. (1994) "Evolutionary perspectives on human nutrition: the influence of brain and body size on diet and metabolism." American Journal of Human Biology, vol. 6, pp. 77-88.
MacDonald ML et al. (1984) "Nutrition of the domestic cat, a mammalian carnivore." Annual Review of Nutrition, vol. 4, pp. 521-562.
Murdock GP (1967) "Ethnographic atlas: a summary." Ethnology, vol. 6, pp. 109-236.
Salem N et al. (1994) "Arachidonate and docosahexaenoate biosynthesis in various species and compartments in vivo." World Review of Nutrition and Dietetics, vol. 75, pp. 114-119.
This article can be accessed at: http://web.archive.org/web/20080531035558/http://www.beyondveg.com/cordain-l/metab-carn/metabolic-carnivory-1a.shtml
More information on this subject: Books : Scientific Studies : Websites : Videos : Food Mall
Recipe of the day
Alfresco Roast Beef Salad
375g/12oz cooked, sliced cold roast beef
1 small red onion, peeled and thinly sliced
1 garlic clove, peeled and crushed
125ml/4floz extra virgin olive oil
5ml/1tsp horseradish sauce
Salt and pepper
Extra fresh thyme leaves, to garnish
1.Put the roast beef in a large bowl with the tomatoes, thyme, onions, garlic, olive oil and horseradish. Season and lightly toss.
2.Cover and leave for 30 minutes in a cool place for the flavours to develop.
3.Toss the salad again thoroughly and scatter with extra fresh thyme leaves.