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

Key Nutrients vis-a-vis Omnivorous
Adaptation and Vegetarianism (cont.)


Preface. This section will address the issue of the low bioavailability of certain minerals in the vegan diet. The more narrow the diet, the greater the risk of deficiency, a point of real concern for some raw vegans. By way of introduction, Freeland-Graves [1988, p. 861] notes:

The fact that vegetarian populations have no gross mineral deficiencies suggests that the mineral status of most vegetarians is probably adequate.

Note that most vegetarian populations are not strictly vegan, and particularly not raw vegan; thus, as mentioned, the narrower the diet, the more likely the bioavailability issue might come into play.

Iron (Fe)


Iron is an essential nutrient. Iron occurs in two forms in foods: heme ("organic") and non-heme ("inorganic"). (Note that the terms organic and non-organic are in quotes because the usage is figurative rather than literal.) NRC [1989] suggests a daily RDA/RDI of 15 mg/day for adults. Hurrell [1997] provides a good overview for iron (p. S4):

The body requires iron (Fe) for the synthesis of the oxygen transport proteins haemoglobin and myoglobin, and for the formation of haem enzymes, and other Fe-containing enzymes, which participate in electron transfer and oxidation-reduction reactions...

The body regulates iron homeostasis by controlling absorption and not by modifying excretion as with most other metals; absorption is increased during deficiency and decreased when erythropoeisis [red blood cell production] is depressed. The body has a limited capacity to excrete Fe and that in excess of needs is stored...

All non-haem food Fe is assumed to enter a common Fe pool in the gut lumen (Cook et al, 1972), from where absorption is strongly influenced by the presence of other food components. [inhibitors]...

As little or no Fe is excreted, absorption is synonymous with bioavailability.

In general, heme iron (found in significant quantities only in animal foods) is absorbed much more readily than the non-heme iron from plant foods. In animal foods, the iron is approximately 40% heme [NRC 1989, p. 198]. However, Hurrell [1997], citing MacPhail et al. [1985], reports that in lean meat, heme iron may vary from 30-70% of total iron. The remainder of the iron in animal foods, and effectively all of the iron in plant foods, is non-heme.

The NRC reports [1989] that the absorption of non-heme iron may vary by a factor as high as ten with the presence/absence of inhibitors or enhancers of iron absorption.

Factors that inhibit iron absorption

Let's look at the factors that inhibit iron absorption, as follows.

Factors that enhance iron absorption

Iron deficiency in veg*ns

Freeland-Graves [1988, p. 861] notes:

Dwyer et al (28) reported mild Fe deficiency in 25% of preschool vegetarian children despite normal intakes of Fe. In adult lacto-ovo-vegetarians, Bindra and Gibson (30) also found a high prevalence of Fe deficiency with normal dietary Fe intakes. In new vegans (32) low Fe stores as measured by serum ferritin were found in 27% of the females, and 10% had values <10 mcg/L. Low serum ferritin has also been observed in lacto-ovo-vegetarian students (33).

The above quote indicates that at least some veg*ns are low or deficient in iron. Craig [1994] cites four studies that found vegetarians had lower serum ferritin levels, and reduced iron stores, when compared to omnivores. In contrast to the preceding, Craig [1994] cites four other studies that did not find differences in iron levels, comparing vegetarians against non-vegetarians.

Craig [1994] concludes that a balanced veg*n diet can supply adequate iron. However, Craig suggests a diet that includes grains, nuts, and seeds, as well as fresh fruits and vegetables. The idea is that the vitamin C in the fresh fruits and vegetables will counteract the iron inhibitors in the grains and seeds, hence make the iron (in the grains and seeds) available for absorption. From an evolutionary perspective, such a diet is of limited relevance: grains are available in quantity only via agriculture, and nuts and seeds were available only seasonally prior to the development of containers and food storage techniques.

From iron deficiency to iron overload:
the distribution of hereditary hemochromatosis

(Acknowledgment: This topic was introduced to me via a posting made by Gary Ditta to the Paleolithic Diet email list,, on 4/30/98.)

Introduction and incidence. Hereditary hemochromatosis (HH) is a genetic disease, a type of "iron overload," which is "the most common inherited metabolic disease among the white [European descent] population worldwide" (from Bonkovsky et al. [1996], abstract). HH is an "iron overload" disease in which the body absorbs excess iron and stores it in the major organs (heart, pancreas, liver, etc.). This can eventually cause cirrhosis of the liver, diabetes mellitus, and cardiac complications [Niederau et al. 1994]. HH is chronic and progressive, and is treated by phlebotomy--blood-letting.

The estimated incidence of HH varies from study to study. Bonkovsky et al. [1996] and Feder et al. [1996] report a gene frequency of 10%. Citing a variety of studies, Niederau et al. [1994] report a gene frequency of 2.3-10.7%, a frequency of heterozygotes (those with just one copy of the gene) of 4.3-19.1%, and a frequency of homozygotes (those having two copies of the gene) of 0.074-1.16%. The full disease occurs only in affected homozygotes, though a small number of heterozygotes may exhibit abnormal iron metabolism.

Hemochromatosis and diet: a hypothesis

The high incidence of HH has some potential implications regarding diet. The gene appears to reduce survival, yet it has become relatively common and widespread among those of European (Celtic) ancestry. Consider the following hypothetical analysis.

The above hypothesis explains the incidence of HH, and also may serve as evidence, in selected populations, of partial, limited adaptation to the "new diet"--i.e., the high-carbohydrate, grain diets provided by agriculture. Contrast the above--possible evidence of limited genetic adaptation to dietary changes in the approximately 10,000 years since agriculture began--with the unsupported claims made by certain raw/veg*n advocates that humans could not, and did not, adapt to a diet that includes animal foods, after ~2.5 million years. Reminder: the above is a hypothesis, for discussion.

Hemochromatosis: supplementary notes

Iron overload diseases in Africa. Iron overload is also common in Ethiopia and the Bantu tribe of South Africa. Hemochromatosis is not limited to hereditary transmission; it can occur any time iron is consumed in great excess. Hennigar et al. [1979] report that iron overload in the Bantu tribe is due to consuming large amounts of a local alcoholic beverage that is brewed in iron pots. Wapnir [1990, p. 101] mentions that iron overload in Ethiopia and the Bantu is due to soil type and use of iron pots and utensils.

Heme iron receptors in the human intestine

Heme and non-heme iron receptors. Both heme iron and non-heme iron are absorbed via intestinal receptors, though the absorption pathways are different. Heme iron is absorbed via specific intestinal receptors (i.e., the receptor function is limited to absorption of heme iron). The heme iron receptors are in the brush borders of the intestinal mucosa, with the duodenum having the most receptors. For confirmation of the existence of the heme receptors, see Tenhunen et al. [1980], Grasbeck et al. [1979], Conrad et al. [1967], and, for a related study on pig intestine, see Grasbeck et al. [1982]. After the initial absorption process has taken place, the iron is removed from the heme, and enters the non-heme iron pool [Wapnir 1991, p. 106].

The human intestine also has receptors for non-heme iron, but there appear to be multiple receptors, and the transfer proteins involved are general and may also absorb some metals other than iron. See Davidson and Lonnerdahl [1989] for discussion of one of the types of non-heme receptor, and its involvement in the absorption of manganese. As well, Davidson and Lonnerdahl cite other studies that propose the subject receptor is also involved in absorption of zinc. Wapnir [1991, pp. 106-107] provides an overview of the various transfer proteins involved in absorption of iron and other metals.

Heme iron receptors: an adaptation to animal foods in the diet. The information that the human intestine has receptors specifically for absorption of heme iron is significant. Heme iron is found almost exclusively in animal foods. Plant foods may contain heme iron as well, as cytochrome c, a protein found in plants, reportedly contains a heme group. However, the level of heme iron in plants is extremely low, and not nutritionally significant. (Most of the standard references, such as NRC [1989, p. 198], report that all of the iron in plants is in non-heme form).

Given that plant foods contain effectively no heme iron, and that heme iron is found only in animal foods, the presence of intestinal receptors in humans that are specific to heme iron is strong evidence of evolutionary, physiological adaptation to animal foods in the diet.

Heme iron: a quote out of context. The following quote has been used to suggest that plant foods contain significant amounts of heme iron; from Wardlaw [1999, p. 513]:

The heme iron in the meat, poultry, fish, dry beans, eggs, and nuts group is especially well absorbed.

However, examination of Wardlaw [1999] reveals the following.

Taken out of context, the above quote from Wardlaw [1999] could be used to suggest that beans and nuts are good sources of heme iron. A closer examination of the material, in context, reveals otherwise. In the quote, Wardlaw is discussing a food group, for which 4 out of 6 members are animal foods. Being animal foods, those 4 foods contain heme iron, and hence, the group, considered as a whole, provides heme iron, so long as one eats a balance of foods from the group. More precisely, eating a balance of foods from a group means that one eats, over an appropriate period of time, some of every food type in the group. Such a strategy would insure the consumption of heme iron from the animal foods in the group. Finally, note that eating a balance of foods from the food groups is "standard" advice in nutrition, and this context should be taken into account when interpreting statements based on such sources.

Iron: synopsis

The existence of receptors in the human intestine that are specific for the absorption of heme iron, which is found in nutritionally significant amounts only in animal foods, coupled with the evidence of the low bioavailability of iron in plant foods, provides evidence of evolutionary, physiological adaptation to a diet that includes animal foods.

Zinc (Zn)

Zinc is an essential nutrient involved in many enzyme pathways in the body. The human body maintains a small pool of zinc (mostly in bone and muscle), with a fast turnover rate (deficiency symptoms appear quickly in test animals). The (U.S.) RDA/RDI for zinc is 15 mg/day for adult men, 12 mg/day for adult women [NRC 1989].

Sandstrom [1997] summarizes the functions of zinc, and how homeostasis is maintained (p. S67):

Its biochemical function is an essential component of a large number of zinc dependent enzymes participating in the synthesis and degradation of carbohydrates, lipids, protein and nucleic acids...

At low and excessive intakes changes in urinary and skin losses contribute to maintain homeostasis.

The primary dietary sources for zinc are animal foods and, to a lesser extent, grains. The bioavailability of zinc in animal products is higher than in grains [Inglett 1983, as cited in NRC 1989].

Zinc bioavailability (inhibitors) and interactions

Zinc may be low in vegan diets. Freeland-Graves [1988] summarizes the issue of zinc concisely (p. 859):

Vegetarian diets have the potential to be limited in Zn because foods that are considered to be the best sources of this mineral, such as meats, poultry, and seafood, are excluded (1). In addition, these diets contain copious quantities of fiber, phytate, and oxalate, compounds that have been found to bind to minerals and reduce bioavailability.

Comments on zinc inhibitors and interactions are as follows.

Zinc deficiency in veg*ns

Low zinc levels in vegans. Freeland-Graves [1988, pp. 859-860] notes:

In a study of 79 vegetarians, our laboratory (9) observed that only female vegans had low dietary levels of Zn. These low levels were attributed to heavy reliance on fruits, salads, and vegetables, foods that are poor sources of Zn...

Levels of Zn in salivary sediment were significantly lower in vegetarians than in nonvegetarians, and vegans exhibited the lowest levels.

The first remark above, about reliance on fruits and vegetables, is relevant to those raw vegans, mostly fruitarians, who avoid grains/sprouts. It suggests that such individuals are--not surprisingly--at higher risk of a Zn deficiency. Many fruitarians report (anecdotal evidence) a "light" or "euphoric" mental feeling that actually may be a symptom of zinc deficiency (a feeling often mistaken for a "spiritual" feeling). Some advocates of fruitarianism claim the "euphoric" mental feeling is a benefit of the diet. Additional relevant anecdotal evidence is that some raw vegans and fruitarians report loss of libido, which may also be a symptom of zinc deficiency.

The possibility that one of the most hyped effects of the fruitarian diet, the allegedly "euphoric" mental state, may be a Zn deficiency is of course a hypothesis. However, it is a plausible hypothesis (based on the available scientific and anecdotal data), and deserves further research.

Side note. As a former fruitarian who has personally experienced the "light" or "euphoric" mental effect of a fruit diet, and who has also done some spiritual work, I can corroborate that, at least for me, the "light" or "euphoric" feeling of a fruit diet is not a "spiritual" feeling. Rather, the effect of the fruit diet is more like a mild drug or sugar high--an airy, apathetic feeling (lassitude).

Zinc: synopsis

That the bioavailability of Zn is higher in animal foods than plant foods may suggest that humans are better adapted to the digestion and assimilation of animal foods (i.e., in contrast, many plant foods contain zinc inhibitors). Also, specific symptoms that are commonly reported by fruitarians (anecdotal evidence), i.e., the "light" mental feeling and loss of libido, suggest the possibility that zinc deficiency may be widespread among fruitarians. This is an area where research is needed.


(Essential Fatty Acids in Omnivore vs. Vegetarian Diets)

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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?

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