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

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

Protein Digestion: Plant vs. Animal Sources

Essential and conditionally essential amino acids

Young and Pellett [1994, p. 1204S] provide a good summary of protein requirements:

The requirement for dietary protein consists of two components: 1) the requirement for nutritionally indispensable amino acids (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine) under all conditions and for conditionally indispensable amino acids (cysteine, tyrosine, taurine, glycine, arginine, glutamine, proline) under specific physiological and pathological conditions, and 2) the requirement for nonspecific nitrogen for the synthesis of the nutritionally dispensable amino acids (aspartic acid, asparagine, glutamic acid, alanine, serine) and other physiologically important nitrogen-containing compounds such as nucleic acids, creatine, and porphyrins.

Daily recommendations

The FAO/WHO/UNU (Food and Agriculture Organization, World Health Organization, United Nations University) recommend a daily average intake of protein for adults of 0.76 g per kg of body weight. However, on a related note, Young and Pellett [1994, p. 1205S] note that:

However, there is increasing evidence that the current international FAO/WHO/UNU (11) and national (10) requirement estimates for most indispensable amino acids in adults are far too low (9, 23, 24). Several groups (25-27) are seeking to further substantiate this evidence.


A number of procedures have been established to measure the digestibility of proteins; for an example of protein digestibility scoring methods and lists of the protein scores of a wide variety of foods, see FAO [1970]. Through the use of such scoring methods, one can compare the digestibility of the protein in various foods. Young and Pellet note that the level and quality of protein in some individual plant foods make them inadequate as sole protein sources.

They make the interesting observation that [Young and Pellett 1994, p. 1210S]:

In general, the digestibility of vegetable proteins in their natural form is lower than that of animal proteins...

Boiling in water generally improves protein quality, whereas toasting or dry heating reduces protein quality.

Note that the information above that we digest certain conservatively cooked protein foods more effectively than raw protein foods challenges the claims of some raw diet advocates that raw is always "better" than cooked.

Young and Pellett [1994] provide a table (table 10, p. 1209S) showing the digestibility of various protein sources. Meat, dairy, and eggs are 94-97% digestible, the much-maligned standard American diet is 96% digestible, but grains, beans are "only" 78-85% digestible. One is tempted to conclude that this is due to adaptation to animal foods in the diet over evolutionary time. However, that is not necessarily the case. Animal foods are in general more easily digested than plant foods, for structural reasons: plant foods come encased in cellulose cell walls, which are hard to penetrate and digest. Ruminant animals such as cows, fed animal products in their feed, have no difficulty digesting the animal protein, even though it is arguably "unnatural" for them to eat animal foods.

Protein sources in prehistoric times

Earlier sections of this paper discuss the role of animal foods in the diet during evolution. A question that arises is whether, in theory, plant foods could take the place of animal foods as a protein source in such diets. Clearly, plant foods, if available in sufficient quantity, can satisfy protein requirements. The problem, of course, is availability. Prior to the development of containers, plant foods were difficult to collect and store, in quantity. Without suitable storage technology (even crude technology), plant foods will not last long (because of spoilage and insect attack). Even "tough" plant foods like nuts and acorns are subject to insect and rodent attack unless they are kept in protective storage. The preceding suggests that plant foods were available in season, but not necessarily on a year-round basis (in temperate climates), during evolution. This in turn suggests (but does not prove) that the consumption of animal foods was necessary to satisfy protein (and calorie) requirements in pre-historic times.

Protein in raw vegan diet circles

Protein is a topic of considerable interest in raw vegan circles. One can find fruitarian extremists promoting crank theories that protein (other than the small amount found in sweet fruits) is "toxic" in the sense that the metabolic by-products of protein metabolism are toxic, and will harm you. Such theories are based on a pathological fear of "toxins" and an amazing ignorance of the reality that the human body is well-equipped to process the metabolic by-products of protein metabolism. Further, such theories are sometimes promoted in hateful and dishonest ways (in my opinion and experience). See the article, "Is Protein Toxic?" (not yet available) for a discussion of such theories.

Taurine, a conditionally essential amino acid

Introduction. Taurine is one of the most common sulfur-based amino acids found in nature. Huxtable [1992, p. 101] provides a good introduction to the topic:

2-Aminoethane sulfonic acid, or taurine, is a phylogenetically ancient compound with a disjunct distribution in the biosphere. It is present in high concentration in algae (159, 649, 748) and in the animal kingdom, including insects and arthropods, but is generally absent or present in traces in the bacterial and plant kingdoms. In many animals, including mammals, it is one of the most abundant of the low-molecular-weight organic constituents. A 70-kg human contains up to 70 g of taurine. One is not stumbling into the abyss of teleology in thinking that a compound conserved so strongly and present in such high amounts is exhibiting functions that are advantageous to the life forms containing it.

Distribution of taurine in plant/animal kingdoms

Huxtable [1992] reports that taurine is found at high concentrations in the animal kingdom, but is missing or present in trace amounts in other kingdoms. He notes (p. 105):

In the plant kingdom, taurine occurs in traces, averaging ~0.01 mc mol/g fresh wt of green tissue according to one report (424). This is <1% of the content of the most abundant free amino acids.

Taurine occurs in red algae, but not brown or green algae. Taurine has been reported at levels up to 0.046 mc mol/g, wet weight, in a few plant foods (nuts). Taurine is also present, at high levels, in insects. [Huxtable 1992].

Huxtable [1992] warns of the difficulty in assaying plant foods for taurine content. Due to the precision with which taurine must be measured, the taurine content of plant foods given by an analysis may be incorrect due to contamination of the sample with insects, insect parts, animal droppings. As Huxtable says (p. 105):

One wet thumb print contains 1 nmol of taurine (236). This amount may be compared, for example, with the analytical range of 0.1-0.3 nmol employed by the high-performance liquid chromatographic method used in one recent paper reporting [taurine] concentrations in plants (600).

Note that reference #600 above is Pasantes-Morales [1989] as cited in Huxtable [1992]. Huxtable summarizes that taurine concentrations in plants, when taurine is present at all, are measured in nmol (billionths of a mole), whereas in animal foods taurine is present in micro-moles (millionths of a mole), i.e., an order of magnitude difference of one thousand.

Taurine: selected functions and interactions

Taurine is a very important amino acid involved in a large number of metabolic processes. Huxtable provides a lengthy list of the biological functions provided by taurine [1992, Table 1, p. 102].

Taurine is important in the visual pathways, the brain and nervous system, cardiac function, and it is a conjugator of bile acids. Another important function of taurine is as a detoxifier. Gaull [1986, p. 123] notes:

Retinol [vitamin A] in excess amounts, i.e., unbound to retinol-binding protein, can act as a poison. When the long-term lymphoid cell lines are exposed to 10 mc M retinol, their viability decreases strikingly over a 90-minute period [18]. Addition of zinc improves the viability slightly. Further addition of taurine protects the cells even more. If a combination of zinc and taurine is added, there is a striking protective effect...

Note that the above suggests that taurine and zinc, both found in animal foods, provide protection from excess vitamin A--a vitamin found in full form only in animal foods. This is an interesting synergism, to say the least.

Yet another zinc/taurine interaction is mentioned by Huxtable [1992, p. 129]:

Zinc is another metal ion with which taurine interacts. Zinc deficiency leads to increased excretion of taurine (277).

Inasmuch as zinc is a mineral in relatively low supply (in terms of quantities and/or bioavailability) in raw/vegan diets, the above raises interesting questions of the possibility of yet another zinc/taurine synergism.

Is taurine essential?

Taurine is a conditionally essential amino acid in adult humans; it is an essential amino acid for infants (mother's milk contains taurine). Gaull [1982, p. 90] discusses the need for taurine:

The finding of a dietary requirement for taurine in the human infant is consistent with the negligible activity of cysteinesulfinic acid decarboxylase present both in fetal and in mature human liver (4)...

In adult man only 1% of an oral load of L-cysteine was recovered as increased urinary excretion of taurine, giving further evidence that mature human beings also have a relatively limited ability to synthesize taurine and may be largely dependant on dietary taurine (22). The rat, in striking contrast, has considerable ability to convert dietary cysteine to taurine (17).

See also Gaull [1977] as cited in Gaull [1982, 1986] for more information on taurine synthesis in humans vs. other animals.

Taurine levels in vegans

Laidlaw et al. [1988] studied the taurine levels (in urine and plasma) of a group of vegans (staff members of a Seventh Day Adventist college) compared to a control group (non-vegans, standard American diet).

Taurine levels lower in vegans. Laidlaw et al. report [1988, pp. 661-663]:

The results of the present study indicate that the plasma taurine concentrations in the vegans were significantly reduced to 78% of control values. Urinary taurine was reduced in the vegans to only 29% of control values...

These findings suggest there may be a nutritional need for taurine and that plasma levels and urinary excretion fall with chronically low taurine intakes. Possibly the diet of the vegans was low in metabolic substrates or in cofactors for taurine synthesis...

Although taurine is synthesized in humans, the current study suggests that the rate of synthesis is inadequate to maintain normal plasma taurine concentrations in the presence of chronically low taurine intakes. It is possible that a higher cysteine intake could increase taurine synthesis in the vegans...

Long-term adherence to a strict vegetarian diet may lead to clinical manifestations of taurine deficiency.

Citing Sturman et al. [1984], Laidlaw et al. [1988] report that neonatal primates developed abnormal eye function after being fed a taurine-free diet.

Section summary and synopsis

Taurine is plentiful in nature, except in the plant kingdom, and is required in the metabolism of all mammals. Cats, an obligate carnivore, have lost their ability to synthesize taurine, which implies a long evolutionary dependence on foods that contain taurine.

In humans, the ability to synthesize taurine is apparently limited. Animals that are more herbivorous (e.g., the rat) have a greater ability to synthesize taurine than humans do (they synthesize taurine with three times greater efficiency; see Gaull [1986]). The inefficient status of human synthesis of taurine (compared to more herbivorous animals) suggests a long dependence on a diet that includes taurine, i.e., a diet that includes fauna.

Vitamin A and Beta-Carotene

The primary emphasis of this section is on the low conversion rates of beta-carotene to vitamin A (i.e., low bioavailability) and its possible implications regarding humanity's natural diet.


Biesalski [1997, p. 571] provides an excellent introduction to the subject of vitamin A:

The term vitamin A is employed generically for all derivatives of beta-ionone (other than the carotenoids) that possess the biological activity of all trans retinol. Retinal [note slightly different spelling] is active in vision and is also an intermediate in the conversion of retinol to retinoic acid. Retinoic acid acts like a hormone, regulating cell differentiation, growth, and embryonic development. Animals raised on retinoic acid as their sole source of vitamin A can grow normally, but they become blind, because retinoic acid cannot be converted to retinol...

Vitamin A from animal sources is ingested as dietary retinyl esters which are hydrolyzed to retinol by the lipases of the intestinal lumen...

Human plasma contains about 2 mc mol/L retinol bound to retinol binding protein (RBP) and 5-10 nmol/L retinoic acid presumably bound to albumin (Blomhoff et al. 1990).

Vitamin A is stored in the liver, and released in the blood attached to RBP as mentioned in the above quote.

Units of measurement. Discussions of vitamin A and beta-carotene use two different measures: IU (international units) and RE (retinol equivalents). The conversion equivalents are:

1 IU = 0.3 mcg retinol = 0.6 mcg beta-carotene = 1.2 mcg other carotenoids


1 RE = 1 mcg retinol = 6 mcg beta-carotene = 12 mcg other carotenoids

We will see later that the above measures are controversial because they imply a conversion efficiency that is higher than commonly encountered.

Beta-carotene: an introduction

Wang [1994, pp. 314-315] provides an introduction to the topic of beta-carotene:

Beta-C is a symmetrical molecule with several conjugated double bonds on its polyene chain... Vitamin A activity is an important function of beta-C. Beta-C also has a variety of functions including enhanced immune response [2], antioxidant function [3,4], enhancement of gap junction communication [5,6], induction of carcinogen-metabolizing enzymes [7], and photoprotection [8]. Beta-C is one of the most important food components for which there is strong evidence for an anticancer role from epidemiological studies in human populations [9-11] and studies in animal model systems [12]...

It should be pointed out that many experimental studies on the anticancer activity of beta-C have been confounded by the poor absorption and low tissue levels of carotenoids in the rodent models used for such studies.

Beta-carotene metabolism

The metabolism of beta-carotene is very complex and is not fully understood. Wang [1994] reports that beta-carotene is converted to retinoic acid via two mechanisms: central cleavage and excentric cleavage. Wang et al. [1992] demonstrated the production of retinoic acid (RA) and retinol from beta-carotene in human intestinal homogenates (in vitro). They noted (p. 303):

The production of RA and retinol in our human intestinal incubations is much lower than that reported by Napoli and Race in rat tissues (10), and only trace amounts of retinol were formed during both beta-carotene and beta-apocarotenal incubations.

Note: Wang et al. [1992] used intestinal homogenates in their research; see Parker [1996] for concerns on the reliability of results from cell homogenates.

Note also that rats (a natural herbivore) produce more RA and retinol than humans. Meanwhile, cats, an obligate carnivore, are unable to synthesize RA or retinol from beta-carotene at all. That is, humans fall "in between" a carnivore and an herbivore in this capability. This is in line with the idea that humans are faunivores/omnivores adapted to a diet that includes some fauna (hence a diet that includes some fully formed vitamin A).

The full significance of the discovery of the cleavage of beta-carotene to retinol in the intestines is not clear, as the post-absorptive metabolism (sites, degree) is unknown [Parker 1996, abstract]. Yet another confounding factor is that some ingested beta-carotene and vitamin A may be destroyed in the gut, precluding their use (from Goodman et al. [1966], Blomstrand and Werner [1967], both as cited in de Pee and West [1996]).

Bioavailability of beta-carotene

Solomons and Bulux [1997, p. S45] discuss the relative bioavailability of vitamin A from beta-carotene:

Herbivores have lesser hepatic [liver] reserves of vitamin A than do carnivores when both meet their energy requirements. Human vegetarians show a somewhat parallel relationship in terms of retinol status when compared to omnivores.... In most of the literature's intervention studies, moreover, there was no improvement in indicators of retinol or vitamin A-nutriture status when a plant source of carotene was fed continuously over periods from months to years.

Bioavailability is highly variable. Brown et al. [1989] compared the efficiency of absorption of beta-carotene from supplements, carrots, broccoli, and tomato juice. They found that the supplements and carrots increased plasma carotenoids, but not broccoli or tomato juice. They also found wide differences in absorption rates (carotenoid utilization) among individual subjects. A similar study by de Pee et al. [1995] in Indonesia found (p. 75):

There is little evidence to support the general assumption that dietary carotenoids can improve vitamin A status.

Solomons and Bulux [1997] apparently concur; they note that (p. 45):

The findings...suggest that provitamin A compounds are converted less efficiently than the currently accepted factors of 1:6 and 1:12 would suggest.

Parker [1996] discusses the many difficulties involved in measuring beta-carotene absorption. He notes that many studies do not take into account the intestinal conversion of beta-carotene to retinoids other than vitamin A (retinol). Parker [1996] cites the study by van Vliet et al. [1995] as an exception, which found an absorption efficiency of 11% for beta-carotene supplements as measured in plasma [blood]. As most studies show higher absorption rates for supplements than for beta-carotene in food, this would suggest an actual absorption rate well below 11% in food items. (Note that the result mentioned is from only one study, and one should be cautious in making inferences from a single study).

Poor design of beta-carotene conversion studies. de Pee and West [1996], a review article, summarizes the studies on the conversion of beta-carotene to vitamin A (p. S38):

Many experimental studies indicating a positive effect of fruits and vegetables [on vitamin A status] can be criticized for their poor experimental design while recent experimental studies have found no effect of vegetables on vitamin A status. Thus, it is too early to draw firm conclusions about the role of carotene-rich fruits and vegetables in overcoming vitamin A deficiency.

Solomons and Bulux [1997] note that beta-carotene serves as a "sunscreen" for plants, it is an antioxidant, and that it may serve similar functions in those who consume the plant [Jukes 1992 and Krinsky 1992, both as cited in Solomons and Bulux 1997]. This would indicate competing uses for beta-carotene--between its potential use as a carotenoid vs. cleaving it to create vitamin A.

The bioavailability of beta-carotene is influenced by a number of factors. These are discussed in de Pee and West [1996], de Pee et al. [1995], and Parker [1996]. We will consider only 3 factors of interest here:

Vitamin A and beta-carotene: synopsis

The complexity of vitamin A/beta-carotene metabolism, coupled with the apparently inefficient conversion of beta-carotene to active vitamin A, makes it difficult to reach firm conclusions. However, given the number of vegans who do not use animal products or supplements, and who do not display symptoms of vitamin A deficiency, such observation suggests that conversion of beta-carotene to vitamin A, though inefficient and sluggish, is adequate to prevent deficiency. At the same time, however, the low efficiency of that conversion (when compared to that of the rat, an herbivore) suggests that humans show evidence of preferential use of, and some degree of evolutionary adaptation to, preformed vitamin A-containing diets, i.e., those including animal foods.


(Assimilation of Iron and Zinc in Plant and Animal Foods)

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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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