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(Comparative Anatomy and Physiology Brought Up to Date--continued, Part 7A)

Insights about Human Nutrition & Digestion from Comparative Physiology

Preface. The objective of this section is to investigate the topic of comparative physiology for any insights it might provide regarding the natural diet of humanity. Also of interest will be information that can be gleaned about the relative efficiencies and inefficiencies in the availability and absorption of certain key nutrients depending on their source in the diet (i.e., plant or animal foods).

To a certain degree, this topic is already discussed in the article Metabolic Evidence of Human Adaptation to Increased Carnivory on this site. This section serves to summarize the main points of that article, and to provide further information and discussion (which in some cases is extensive), as appropriate.

Due to the complexity of the issues, some of the subsections below are relatively detailed (and lengthy). This is necessary to establish a basis for discussing the implications physiology has regarding diet. If you find that a particular subsection is not of interest (or too technical, though I have tried to make the material as accessible as possible), you might want to advance to the next subsection.

Key Nutrients vis-a-vis Omnivorous
Adaptation and Vegetarianism

Vitamin B-12: Rhetoric and Reality (1 OF 5)

Introduction: standard information

Vitamin B-12 (cobalamin) is an essential nutrient required for the synthesis of blood cells and the myelin sheath of the central nervous system. Only tiny amounts are needed to sustain health, and a healthy body is efficient at recycling B-12. Additionally, the body maintains a sizable store of the vitamin. The recommended daily allowance/input, or RDA/RDI, for vitamin B-12 is as follows, from NRC [1989, p. 162]:

Adults. A dietary intake of 1 mcg daily can be expected to sustain average normal adults. To allow for biological variation and the maintenance of normal serum concentrations and substantial body stores, the vitamin B-12 RDA for adults is set at 2.0 mcg.

Vitamin B-12 is made only by bacteria; it is not synthesized by plants or animals. The very limited (usually only trace) amount of B-12 in plants comes from uptake of the vitamin from the soil, and from surface contamination with B-12 producing bacteria. (This is discussed in detail below.) Animals concentrate B-12 from the food they eat, and, in the case of folivores, biologically active B-12 may be produced by bacteria in the fermenting chambers in the digestive system. The end result of this is that plant foods provide little (if any) B-12, and animal foods are the only reliable food sources for B-12.

Because plant foods are deficient in B-12, the use of B-12 supplements (or fortified foods, e.g., nutritional yeast) is recommended for vegans by most nutrition experts. Of course, some extremists disagree, and one must seriously question whether such "experts" (or "diet gurus") are putting dietary dogma ahead of the health and welfare of those who follow their advice.

The primate connection

B-12 also an essential nutrient for non-human primates. From Hamilton and Busse [1978, p. 763]:

Many captive primate species enter into hypovitaminosis B-12 [deficiency] when maintained on vegetarian diets (Hauser and Beard 1969, Oxnard 1964, 1966, 1967, Siddons 1974, Siddons and Jacob 1975)...

Vitamin B-12 is the least readily available vitamin to omnivorous primates...

Deficiency diseases have not been identified for any wild primate population (Kerr 1972, Wolf 1972).

The fact that wild primates avoid B-12 deficiency suggests that their natural diet provides adequate B-12. Inasmuch as all primates eat insects, and some insects contain B-12 (see Wakayama et al. [1984]), this suggests insects as a possible B-12 source for some primates, along with production by fermentation bacteria as a possible source for folivorous primates (e.g., gorillas, whose consumption of insects is very small when compared to their intake of plant foods).

B-12 recycling and interactions

To a great extent, B-12 is recycled from liver bile in the digestive system. This is one reason why vitamin B-12 deficiency is rare among vegans, even those who do not use supplements or supplemented foods. The recycling is summarized by Herbert [1994, p. 1217S]:

The enterohepatic circulation of vitamin B-12 is very important in vitamin B-12 economy and homeostasis (27). Nonvegetarians normally eat ~2-6 mcg of vitamin B-12/d and excrete from their liver into the intestine via their bile 5-10 mcg of vitamin B-12/d. If they have no gastric, pancreatic, or small bowel dysfunction interfering with reabsorption, their bodies reabsorb ~3-5 mcg of bile vitamin B-12/d. Because of this, an efficient enterohepatic circulation keeps the adult vegan, who eats very little vitamin B-12, from developing B-12 deficiency disease for 20-30 y (27)...

Unlike the vegetarian whose absorption machinery is normal, the person whose absorption machinery is damaged by a defect in gastric secretion, by a defect in pancreatic secretion, or by a defect in the gut that produces intestinal malabsorption will develop vitamin B-12 deficiency in 1-3 y because these absorption defects block not only absorption of food vitamin B-12, but reabsorption of vitamin B-12 excreted into the intestinal tract in the bile (2,6).

Reduction in stomach acid promotes B-12 deficiency. A reduction in gastric (stomach) acids is associated with the development of bacterial colonies in the stomach that produce analogues of vitamin B-12, which can accelerate or promote B-12 deficiency. From Herbert et al. [1984, p. 164]:

As pernicious anemia develops, the first loss usually is of gastric acid. Figure 3 (from Drasar and Hill, 23) shows that the achlorhydric stomach [one unable to produce hydrochloric acid] is usually heavily colonized with enteric bacteria. The increased colonies of enteric bacteria in the achlorhydric stomach and small intestine of the pernicious anemia patient may produce analogue which may in three ways accelerate the development of B-12 deficiency.

The loss of gastric acids may also occur in iron deficiency. The iron in plant foods is of much lower bioavailability than in animal foods. The common grain-based vegan diet contains antinutrient factors that may inhibit iron absorption (discussed later in this section). Vegetarians, especially vegans, are at higher risk of iron deficiency. From Herbert [1994, p. 1215S]:

...iron deficiency is twice as common in vegetarians as in omnivores (3)...

Prolonged iron deficiency damages the gastric mucosa and promotes atrophic gastritis and gastric atrophy, including loss of gastric acid and I.F. [intrinsic factor] secretion, and therefore diminished vitamin B-12 absorption (3, 4, 19). This would cause vitamin B-12 deficiency in twice as many vegetarians as omnivores (3, 4, 19).

Does strict fruitarianism accelerate B-12 deficiency?

Note that loss of gastric acid, whether caused by an iron deficiency or other cause, may promote B-12 deficiency. In this regard, the anecdotal experience of some fruitarians may be relevant. (Please note that the following remarks regarding fruitarianism are based on anecdotal evidence, as that is all that is available; there are no published, peer-reviewed studies on fruitarians on this topic.)

Possible reduction in stomach acid in long-term fruitarians. Among the very few people who manage to follow the fruitarian diet with at least a limited degree of success for longer than a year (i.e., manage to stay on the diet, without extensive binges or cheating--both of which are quite common in fruitarianism), some report that when they try to eat other food again (i.e., foods other than fruits), they are unable to digest it. Food may be eaten and pass through the digestive system, coming out as stools, yet appearing almost exactly as it did when consumed. Typically this effect is observed with protein foods or cooked food, but it may occur with other foods as well. (Surprisingly, the effect may also occur with fruit, if one stays on the diet long enough.)

The above phenomenon (which this writer personally experienced after abandoning fruitarianism and returning to a more normal vegetarian diet) suggests a possible deficiency of gastric acid caused by the fruitarian diet. (In time, with a more normal diet, the digestive system appears to recover.) If so, it suggests that fruitarian diets, if they decrease gastric acid, may actively promote B-12 deficiency over and above any effect due to lack of B-12 in the food. (This is a hypothesis at present, of course, and would need research to validate it).

Other negative symptoms of fruitarianism. Many who attempt strict fruitarian diets report other negative symptoms--intermittent fatigue, lassitude, loss of libido, sugar metabolism problems (diabetes-like symptoms: excess urination, thirst, mood swings, etc.), as well as emaciation. (Note: thirst on a fruitarian diet may seem counterintuitive, since the level of fluid intake is so high. However, the diuretic effect of some fruits [such as citrus], if used as staples, can negate that, depending on the case.) The possibility of fruitarianism increasing the potential for a B-12 deficiency raises the question whether the fatigue, loss of libido, and lassitude that some fruitarians experience are possibly related to B-12 deficiency, or alternatively (or both), to the sugar metabolism/emaciation-starvation effects of the diet.

Clinical B-12 deficiency rare. Doubtless some fruitarian advocates will challenge the above and ask where are the fruitarians with clinical signs of B-12 deficiency? They are hard to find, for the following reasons:

Finally, there are a few sensible, credible fruitarians (though more loosely defined) who specifically include small amounts of animal products in the diet to satisfy B-12 requirements.


(Vitamin B-12 in Natural Hygiene, Living Foods, and Vegan Diets)

Return to beginning of article



GO TO PART 1 - Brief Overview: What is the Relevance of Comparative Anatomical and Physiological "Proofs"?

GO TO PART 2 - Looking at Ape Diets: Myths, Realities, and Rationalizations

GO TO PART 3 - The Fossil-Record Evidence about Human Diet

GO TO PART 4 - Intelligence, Evolution of the Human Brain, and Diet

GO TO PART 5 - Limitations on Comparative Dietary Proofs

GO TO PART 6 - What Comparative Anatomy Does and Doesn't Tell Us about Human Diet

GO TO PART 7 - Insights about Human Nutrition & Digestion from Comparative Physiology

GO TO PART 8 - Further Issues in the Debate over Omnivorous vs. Vegetarian Diets

GO TO PART 9 - Conclusions: The End, or The Beginning of a New Approach to Your Diet?

Back to Research-Based Appraisals of Alternative Diet Lore

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