Friday, March 24, 2017

Dental Microwear Study: Neandertals and Early Modern Humans Had Different Dietary Strategies.

In "Neandertal versus Modern Human Dietary Responses to Climatic Fluctuations" Zaatari et al. discuss dental microwear evidence that early modern humans differed in dietary strategy from contemporaneous Neandertal humans.  Whereas Neandertals exploited plants only when in habitats rich in edible plant life (e.g. woodlands) and defaulted to almost strictly carnivorous diets when in open rangeland habitats, early modern humans appear to have opted for a more plant-rich diet regardless of whether they were in wooded or open habitats:


"Thus, the microwear data suggest that whereas Neandertals relied solely on animal meat in open habitats and only exploited plants as they became more available and diverse, modern humans seem to have indulged in plant exploitation more extensively and to have used plants to supplement their diets even in open habitats where they would have been less abundant in comparison to wooded habitats."
".....The microwear data hint that while Neandertals seem to have followed a more opportunistic dietary strategy, exploring resources only when they were most abundant and easily accessible in their local habitat (i.e., almost exclusively animal protein in open conditions but substantial amounts of plants in wooded ones), modern humans seem to have been willing to invest more effort in extracting resources from their environment (e.g., more plant foods in open conditions compared to Neandertals)."
Zaatari et al. hypothesize that a dedication to broad omnivory may have given early modern European humans an economic edge over Neandertals:
"Thus, the results of this study do not support the view that the Neandertals’ disappearance was primarily due to their inability to adapt to the severe climatic fluctuations of MIS 3. But, starting at around 42 ka cal BP, modern humans came into Western Europe, having likely entered Eastern Europe a couple of millennia earlier [3]. This could have potentially brought about competition with the Neandertals making them face an extra survival pressure [65]. If, indeed, there was any competition, and if behavioral differences like the ones suggested in this study were already established at the time of first contact, these differences might have given modern humans an advantage over the Neandertals by enabling more efficient exploitation of dietary resources in their environment and more flexibility in changing the percentages of contributions of these different resources in their diets."

Thursday, March 16, 2017

Meta-analysis Concludes Vegans Virtually Destined for B12 Deficiency Disorders Despite Supplementation

Obersby et al. reviewed  17 studies including 3230 subjects and found that only 2 studies reported that vegans had concentrations of plasma homocysteine and serum vitamin B12 similar to omnivores.  The other 15 studies found B12 deficiency and elevated plasma homocysteine to be the "normal" state for vegans.

Most defenders of veganism will argue that this problem can easily be solved by taking supplemental vitamin B12.  However, the authors refer to some interesting research on vitamin B12 supplements:

"The present review reveals that there is only poor evidence available of vegetarians consuming vitamin B12 supplements and/or vitamin B12-fortified food and beverages. However, supplements, fortified food and beverages normally contain the less efficient cyanocobalamin form of vitamin B12, which when it enters the bloodstream must be converted to methylcobalamin(62), the only form of vitamin B12 that has a methyl donor that is required to neutralise homocysteine(63). It takes 4–9 weeks for this conversion to take place(64), assuming there are no disruptions by genetic factors, age-related problems and metabolic obstacles that may be present. Furthermore, research suggests that vitamin B12 that is not dissolved in the mouth will not (up to 88 %) be absorbed(65), due to the lack of R-binder mostly obtained from saliva, which is required to start the absorption process. The aforementioned study indicates that supplementation with cyanocobalamin can be poorly absorbed, which will have little or no effect on raising vitamin B12 levels."
Indeed, Crane et al. (reference 65 in above passage) found that oral supplementation of B12 did not raise serum B12 unless the tablet could be readily dissolved.  Reference 64 is a paper entitled "The coenzyme forms of vitamin B12: towards an understanding of their therapeutic potential" by Kelly [here].

"The compound most commonly referred to as vitamin B12 is CN-Cbl; however, this molecule does not occur naturally in plants, micro-organisms, or animal tissues. CN-Cbl has a cyanide molecule at the metal-carbon position and its cobalt atom exists at an oxidative state of +3, not the biologically active +1 state. In order to be utilized in the body, the cyanide molecule must removed and eliminated through phase II detoxification. It is thought that glutathione (GSH) might be the compound performing the function of decyanation in vivo, since glutathionylcobalamin (GS-Cbl) has been isolated from mammalian tissue. If, in fact, GSH is needed as a cofactor to activate CN-Cbl to the coenzyme forms of vitamin B12, clinical situations characterized by decreased tissue levels of GSH might be expected to result in functional deficiency of vitamin B12, even in the presence of adequate plasma or tissue levels of the cobalamin moiety (typically labs are looking only for a cobalamin moiety and do not differentiate between CN-Cbl and the active forms of vitamin B12)."

Apparently, one can take CN-Cbl, a.k.a. cyanocobalamin through supplements and fortified foods, and thereby raise your blood B12 level, yet remain functionally B12 deficient because CN-Cbl is inactive.  Further, we do not use the basic cobalamin molecule produced by micro-organisms, but the methylcobalamin (MetCbl) and adenosylcobalamin (AdeCbl) found only in animal tissues:

"Although the basic cobalamin molecule is only synthesized by micro-organisms, all mammalian cells can convert it into the coenzymes AdeCbl and MetCbl. OH-Cbl, MetCbl, and AdeCbl are the three forms of cobalamin most frequently isolated from mammalian tissue. However, only MetCbl and AdeCbl actually function as cofactors in human enzymes. AdeCbl is the major form in cellular tissues, where it is retained in the mitochondria. MetCbl predominates in blood plasma and certain other body fluids, such as cerebral spinal fluid, and, in cells is found in the cytosol."

As already stated, apparently the body can convert CN-Cbl to the active forms, but the process takes 1-2 months, and may be disrupted by genetic factors, age-related problems, and metabolic issues.  Kelly lists a number of problems that may develop in the absence of sufficient active B12, including liver disease, sleep disturbances, elevated homocysteine and methylmalonic acid (common in vegans), anemia, male infertility (low sperm concentrations, counts and motility, which have been reported common in Adventist vegetarians),  eye disorders, anorexia, and cancer. 
"While information is very limited, both AdeCbl and MetCbl might eventually be shown to have a supportive role in the prevention or treatment of cancer. A significant body of experimental evidence suggests a deficiency of vitamin B12 can enhance the activity of various carcinogens [17].  Experimental results also indicate a link between alterations in the intracellular metabolism of cobalamin and the increased growth of human melanoma cells [18]."
 Who is investigating the role of B12 deficiency in the genesis and promotion of cancer?

In view of all of this data,  Obersby et al. conclude that in the absence of improved delivery systems for "vegan" B12,

"The present study confirmed that an inverse relationship exists between plasma tHcy and serum vitamin B12, from which it can be concluded that the usual dietary source of vitamin B12 is animal products and those who choose to omit or restrict these products are destined to become vitamin B12 deficient."
In other words, in the absence of ideology and technology, it appears that humans have an evolved nutritional requirement for animal products to obtain the metabolically active forms of vitamin B12.  If you eat a vegan diet, you should supplement with methylcobalamin, which is better retained and utilized than cyanocobalamin, although it is unknown at this time whether this will adequately address all vitamin B12 requirements in the absence of dietary animal products which also contain adenosylcobalamin. 



Sunday, March 12, 2017

Human Ancestors: Vegetarians or Obligate Omnivores?

On July 23, 2012 Scientific American ran an editorial by biologist Rob Dunn with the title:

https://blogs.scientificamerican.com/guest-blog/human-ancestors-were-nearly-all-vegetarians/
Although at the end of this opinion piece, he admits that his article doesn't really support the claim that human ancestors were nearly all vegetarians:


He supports his very misleading title choice by the claim that he used "the definition of vegetarian that most humans use" which according to him refers to someone who eats no meat in public but does eat meat in private.  Aside from the fact that he fails to provide any evidence that this is "the definition of vegetarian that most humans use," using the word "vegetarian" in the title with the assumption that this is the meaning most readers will assign to it is misleading click bait at best.  Many people who read his title want to believe that he is asserting that most human ancestors did not eat meat.

Nevertheless he retains the claim that our ancestors "tended to mostly eat vegetable matter."  He supported this claim by enlarging the pool of "our ancestors" to include pre-human primates.

In 2002 Hladik and Pasquet published, in the journal Human Evolution, their article "The human adaptations to meat-eating: a reappraisal," in which they argued that humans are unspecialized omnivorous frugivores:


In this paper, Hladik and Pasquet argued that starch-rich cooked tubers were the most likely source of increased caloric intake needed for evolution of the human brain:
In January of 2014, I published Powered by Plants (PBP) in which I attempted to provide further support for classifying humans as unspecialized, omnivorous frugivores with no specific adaptations to meat-eating.  Like Hladik and Pasquet, I also referred to Wrangham's team's 1999 paper "The Raw and the Stolen" to support the same conclusion that human brain evolution was more likely primarily supported by inclusion of cooked starch-rich plants in the diet, than by meat-eating.

However, as Hladik and Pasquet note, one main weakness of this argument is a paucity of strong evidence for controlled use of fire previous to 250,000 years ago.  Hladik and Pasquet think that this lack of evidence is not a strong argument against the use of cooking that long ago because the cooking methods used by contemporary Pygmy hunter-gatherers leave no trace within a matter of months:

Did Meat-Eating Make Ancestral Humans More Fecund?

On April 18 of 2012, Live Science published:

This article reports on a study comparing weaning ages of humans to other species.  The study found that humans wean their young at a proportionally much younger age than other primates.



Humans' natural weaning age is similar to that of carnivores that obtain at least 20 percent of their calories from meat:


To my fault, I did not take notice of this research in 2012-2014, when I was working on PBP.  This finding provides some support for the idea that regular consumption of fairly large amounts of meat (≥20 percent of calories) by prehistoric humans resulted in a natural selection favoring the reproduction of individuals having a lactation period proportionally shorter than that of other great apes and similar to that of carnivores. If so, this would be a specific adaptation to a carnivorous diet.

However, I do not see how we can rule out the possibility that this change in human weaning time was facilitated simply by increased caloric intake, which could have been supplied by either cooked starches, or a combination of cooked starches and meat.  It does not immediately strike as a characteristic that can only be supported by inclusion of meat in the diet because there is no reference to any nutrient uniquely or even predominantly provided by meat. 

As an aside: Is it really any wonder that people can get very confused about the natural human diet, when it looks like Scientific American endorses the claim that human ancestors were "nearly all vegetarians," yet other LiveScience headlines assert that prehistoric meat-eating made human ancestors more "fruitful"?

Revisiting the "Meat-Eating Fueled Brain Evolution" Hypothesis

On November 19 of 2012 LiveScience reported that meat-eating was necessary for the evolution of the human brain:
This report focuses on a study that calculated that early humans would not have been able to acquire enough dietary energy from a raw plant food diet to support the evolution of the human brain through the enormous increase in number of brain neurons that separates humans from other apes:

Human ancestors would have had to spend more than nine hours a day eating, in addition to the time required to acquire the foods.  Either eating more calorie-dense meat, or cooking the plant foods, would have been necessary to supply sufficient dietary energy to support addition of neurons to the brain over time.  Once again we're talking about energy (calories) which is not uniquely supplied by meat.  As Wrangham's team showed in their 1999 paper "The Raw and the Stolen" early humans would have gained substantially more calories by simply cooking their starchy underground storage organs and seeds than by switching from a wholly plant-based diet to one providing 60% of its energy in the form of raw meat.

However, this report also refers to an archaeological find providing evidence that "meat must have been an integral, and not sporadic, element of the prehuman diet more than 1 million years ago."


Presumably if raw wild plants provided plenty of iron, folate and B12, this child would not have had this malnutrition condition.  While wild plants may provide plenty of folate, they lack B12 and have non-heme iron that is poorly absorbed.  Relevant here is the fact that childhood iron-deficiency is world-wide one of the most common mineral deficiencies and it is most common in nations having cereal and legume based diets where meat intake is low.

Source:  Zimmermann et al.

This find  was separately reported on by LiveScience on October 3, 2012 [orginal article in PLO One].  As stated, the remains consist of a toddler's skull showing that the toddler suffered from porotic hyperostosis, a bone condition associated with low iron, folate, and vitamin B12 status.


Now we are in the realm of neural nutrients that are for practical purposes and particularly in the context of the paleolithic era uniquely supplied by meat.  Wild plants may have traces of B12, but they simply are not sufficient sources of B12 for lactating women or growing children; for practical purposes B12 was a nutrient exclusively provided by meat in human prehistory.  While plants provide iron, they don't provide heme iron and as documented above, we know that even contemporary children in populations subsisting on cooked plant-based diets with very low meat intakes are highly prone to iron deficiency. Cooking underground storage organs just won't add to their B12 or iron value.

Are Humans Obligate Carnivores?

Thus, this finding of porotic hyperostosis in a child 1.5 million years ago provides evidence that before 1 million years ago the human lineage had already been molded by natural selection to require during childhood intakes of iron and B12 that only regular consumption of meat could provide.

It is very significant that both the weaning data and this case of ancient porotic hyperostosis pertain to reproductive efficiency.  The nutritional needs of pregnant and lactating women and their children are increased in comparison to adults, particularly adult men, who are not pregnant or lactating.  During pregnancy, lactation, and childhood growth, the faults of a diet become apparent.  This evidence indicates that human ancestors 1.5 million years ago could not successfully reproduce without eating meat.

This interpretation is further supported by the finding by Beasley et al that "humans, uniquely among the primates so far considered, appear to have stomach pH values more akin to those of carrion feeders than to those of most carnivores and omnivores." Most likely, an need to eat microbe-laden meat imposed on our ancestors a naturally selection that favored the survival and reproduction of those who produced a gastric pH of 1.5, sufficient to protect them from deadly meat-borne pathogens.

What would you call an animal that in its natural environment eats both plants and animals, but has a specific biological adaptation to meat-eating (gastric pH of 1.5) because its ancestors were in their natural habitat compelled to eat animal flesh in order to get adequate nutrition, survive and reproduce?

I would like to call such an animal an obligate carnivore, because this term literally means "compelled meat-eater."  If the animal must eat meat to meet nutrient needs, survive and reproduce then it is certainly compelled to eat meat.

However, biologists use this term to refer only to animals like the felines that in their natural habitats are restricted to meat-eating and virtually never eat plants. 

Zoologists classify animals that are capable of but not restricted to meat-eating facultative carnivores.  This is essentially another term for omnivores: animals that in their natural habitats can and do eat both plants and animals

These terms fail to capture the nutritional adaptations of some omnivorous animals, such as wolves, because neither of these terms connotes nutritional dependence on meat-eating.  Although wolves are omnivores, they can't survive in the wild unless they eat meat.

I suggest the term obligate omnivore to refer to animals that in their natural environment, without the aid of modern agriculture or food processing, need to eat both plants and animals in order to survive and reproduce.  Since an obligate omnivore has a dietary requirement for animal flesh, every obligate omnivore is also an obligate carnivore in the literal sense discussed above: a "compelled meat-eater," compelled to eat meat by dietary requirements, even if it can also eat plants.


To me, the archaeological evidence produced by Dominguez-Rodrigo along with the finding that humans have stomach acid stronger than many other carnivores and similar to scavengers indicates that humans evolved as omnivorous obligate carnivores: a species that was in its natural habitat and evolution compelled to eat meat in order to obtain nutrients required to successfully reproduce.

In Powered by Plants I (in retrospect, foolishly) argued against the idea that humans are obligate omnivores.  If this research is valid – and it seems to be – I stand both embarrassed and corrected.

I would say that I made the very common mistake of rejecting the idea that humans are obligate omnivores because some modern humans can live for extended periods of time without eating meat.
The problem with this is that even if it is in modern circumstances possible for some people to avoid eating meat to some extent by eating plant foods that weren't available to our preagricultural ancestors, or by extensive processing of plants (fermentation, refining, etc.), or by taking iron or B12 supplements, etc., this does not change the fact that without these technological innovations, in nature, human ancestors could not successfully reproduce without eating meat.

Just as the fact that some people succeed in feeding dogs completely plant-based diets does not prove that the dogs are not obligate carnivores, the fact that some people seem to get along without eating animal foods does not prove that humans did not evolve as obligate omnivores. Just as a dog is an obligate carnivore without human technological intervention, human ancestors lacking modern agriculture, food processing technologies and dietary supplements were obligate carnivores (in my preferred sense of the term).

If humans evolved as obligate omnivores, this would certainly explain why people self-identifying as vegetarians or vegans comprise no more than 3% and 0.8% of the population, respectively.  Some researchers estimate that no more than 0.1% of the population is strictly vegan or vegetarian (i.e. never eating meat).  The rarity of strict vegetarians makes perfect sense if humans have an evolved drive and genetic requirement for meat-eating.

Research shows that while 3% of people self-identify as vegetarians, 66% of these people report eating red meat, poultry, and fish on follow-up challenges; only 0.9% of the total study population both self-defined as vegetarian and provided dietary recalls that included no animal flesh. Why would they misrepresent themselves?  This reminds me of how priests trying to suppress their biological drive for procreation often get caught engaging in sexual abuse behind closed doors. If humans have a strong innate biological drive to eat meat then it is really no surprise to find that most people who attempt to eat meat-free diets are unable to consistently maintain a meat-free diet, or that deception of self or others is common among self-identified meat-avoiders. 

The fact that at least 84% of people who attempt meat-free diets eventually return to eating meat (see also Hal Herzog's article on this topic) also supports the conclusion that humans are not adapted to meat-free diets and have an innate biological drive to eat meat.

None of these facts makes sense if humans have no biological drive or requirement for meat-eating.

Nature always has the last word.  Biology will always trump ideology.

The Natural Selection and Survivor Bias of Successful Vegetarians and Vegans 

A vegetarian or vegan diet itself exerts a natural selection on populations.  Among people who attempt meat-free diets, only the small fraction who have greater genetic, biological capacity to survive or thrive on such diets will stick to them for long.  The fact that some individuals are able to do so doesn't prove that all individuals can do so, any more that the fact that some individuals can thrive in professional basketball proves that all individuals can do so.

People vary in their nutritional requirements.  Even well-planned vegetarian diets tend to be marginal in choline and zinc.  The Linus Pauling Institute notes that "Strict vegetarians, who consume no meat, milk, or eggs, may be at risk for inadequate choline intake" and "The requirement for dietary zinc may be as much as 50% greater for strict vegetarians whose major food staples are grains and legumes, because high levels of phytic acid in these foods reduce zinc absorption."

I tracked my intake of both of these nutrients for several years on a calorie adequate animal-free whole foods plant based diet, and I never met the full requirement for either nutrient on any day I analyzed.  My choline intake almost always fell below 50% of the recommended intake of 550 mg per day for men.    When you look at the top sources of choline it is obvious why this is the case:

Source: Linus Pauling Institute
The plant foods richest in choline are very poor sources.  I found it difficult to consistently obtain the 17 mg zinc recommended daily for vegan men.  Again a look at food sources of zinc indicates why this is the case: the zinc content of even the best plant-food sources is very low and variable on top of having low bioavailability due to phytates and other inhibitors.

Source: Linus Pauling Institute
Plant-based diets do not provide any long-chain omega-6 or omega-3 fats, which are likely required dietary nutrients for people of European and Asian ancestry.  Vegan and vegetarian diets will naturally select people who have lower than average dietary requirements for arachidonic acid, long-chain omega-3 fats (EPA and especially DHA), choline, zinc, and iron.

Just as basketball leagues select for people who are genetically suited to excel in basketball, with the result that people who don't excel at basketball defect to other sports, vegetarian diets select for people who are more suited to such diets, while those who don't do well on such diets just quit them.   This in turn means that vegetarian and vegan communities are always dominated and led by survivors of the selection process imposed by the restriction of animal products, and these people will tend to believe that since they survived and perhaps even thrive on the diet, everyone else should be able to as well.  This is known as survivor bias.

Unfortunately, this bias enables and emboldens a tiny minority population of more or less strict vegans and vegetarians to believe that they both have proven that everyone can thrive without meat-eating and have the "moral authority" to demand that everyone else give up eating meat.

But if as the evidence indicates evolution gave humans a biological drive, specific adaptations and nutritional requirements for meat, no moral question exists.  In this case, telling people they must never eat meat is like telling them they must never have sex.  No one can be required to behave in a way contrary to genuine biological drives and needs. Any attempt to enforce restrictions on biological drives will only result in physical and mental dis-ease and dysfunction in most people; very few will get away with it.

Eurasians' Neanderthal Ancestors Were Top-Level Carnivores

On March 14, 2016, Science Daily posted this report:


According to this report, not only Neanderthals, but also more recent Stone Age humans subsisted on predominantly meat-based diets:


In fact this is not new information.  Ten years earlier, in 2002 MP Richards reviewed the evidence for Paleolithic and Neolithic subsistence.  Neanderthals were found by stable collagen isotope studies to have been "all top-level carnivores who derived the vast majority of their protein from animal sources, likely to be large herbivores."  For upper Paleolithic Europeans "the main source of dietary protein was animal-based, and most likely herbivore flesh."  The evidence clearly indicated high animal protein diets for Stone Age Europeans.  Unlike Africans, Europeans and Asians are descended from "top level carnivores" who occupied cold northern habitats that had long winters. 

Neanderthal Descendants Have Top-Level IQs

While mulling over these findings, I recalled the evidence that Europeans and Asians have a very limited activity of the enzymes required to produce long-chain neural fatty acids arachidonic acid,  and DHA from plant-derived precursors linoleic acid and alpha-linolenic acid, compared to people of East Indian or African descent.  To adapt to northern climates, due to the relative paucity and seasonality of edible Eurasian plants during the ice ages, Eurasians were required to eat meat-based diets, which would have naturally selected for the survival of individuals having either mere tolerance for or outright dependence on nutrients exclusively or primarily supplied by meat.  It would have also selected against individuals who were genetically highly dependent on highly plant-based diets. 

What I find very interesting is that Europeans and Asians rank highest in the world for average IQ.  Ethnic Indians (who appear to have a specific adaptation to vegetarian diets) have a mean IQ of 82; most African nations have mean IQs below 80, and Africans also possess the SNP that facilitates conversion of LA and LNA to AA and DHA respectively.  Meanwhile, all European and Asian nations have mean IQs above 90, with many having mean IQs 98 or above. 


If you think these IQ disparities are only a result of technological development, note that Mongolians have only very recently adopted some technological modernization but have a mean IQ of 101, equal to Iceland and Switzerland.  Meanwhile, the mean IQ in Kenya, home of the pastoral Masai, is only 80.  These IQ differences have a biological and genetic basis.

European and Asian nations are composed of people who, unlike Africans, have ~3-5% exclusively Neanderthal genes, which control genetic expression in many important ways.  They are descended from people who had to have the wits to adapt to very cold ice age habitats and who hunted very large game animals to survive.

In other words, Eurasian ancestors had to outwit animals, understand laws of nature, and develop technologies in order to thrive in a habitat that is much more demanding and unforgiving than Africa and other tropical and subtropical regions.  It appears that this may have selected not only for dependence on dietary AA and DHA, but also for a significantly higher mean IQ in European and Asian populations.

So the higher mean IQs found in Europe and Asia are associated with Neanderthal ancestry, ancestral diets very rich in meat, and a probable biological dependence on animal sourced dietary long-chain fatty acids (and perhaps other neurologically important nutrients).  Think about that.

No doubt there will continue to be conflicting information about optimum human nutrition.  We will also see new information emerge as research continues into the future.  Each individual will need to make his or her own decisions about what to eat.  One thing seems certain to me: humans are not the creators of the universe or of human nature and do not have the knowledge or competence required to decide that it should be remade to fit their preferences. Very often acting on our desire to "improve" the world has resulted in unintended consequences and disasters producing suffering far greater than that which we sought to avoid by "improving" things.  For a simple example, agricultural and medical technology have been applied to reduce childhood infectious disease mortality, resulting in overpopulation and consequent suffering from starvation and ecological devastation.  I am reminded of Chapters 29 and 19 of the Tao Te Ching:

29:
Do you think you can take over the universe and improve it?
I do not think it can be done.

The universe is sacred.
You cannot improve it.
If you try to change it, you will ruin it.
If you try to hold it, you will lose it.


19:
Give up sainthood, renounce wisdom,
And it will be a hundred times better for everyone.

Give up kindness, renounce morality,
And men will rediscover filial piety and love.

Give up ingenuity, renounce profit,
And bandits and thieves will disappear.

These three are outward forms alone; they are not sufficient in themselves.
It is more important
To see the simplicity,
To realize one's true nature,
To cast off selfishness
And temper desire.






Sunday, March 5, 2017

Study: Adventist Vegetarian Women 5 Times More LIkely to Have Menstrual Irregularities

Pedersen et al studied 41 nonvegetarian and 34 vegetarian premenopausal Caucasian women, some of whom were Seventh Day Adventists, who reported maintaining their respective diets for at least 5 years:

They found that menstrual irregularities were about 5 times more common among vegetarian women compared to non-vegetarian women:

Increased intake of magnesium and fiber were associated with increased risk of menstrual irregularity.  Increasing intake of protein and cholesterol were associated with reduced risk.
The vegetarians had ostensibly adequate diets and were on average consuming 133 mg of cholesterol and 26 g of fiber daily, compared to 198 mg and 15 mg respectively for non-vegetarians.



Pedersen et al hese findings are "consistent with the notion that premenopausal vegetarian women as a group have decreased circulating estrogen concentrations"  and suggest "that vegetarian women may have decreased reproductive capability."

Pedersen et al comment that "The positive association between cholesterol/kJ and menstrual irregularity may be secondary to cholesterol's role as a biological precursor for estrogen synthesis" and also that protein intake influences estrogen signaling systems.  On the other hand, dietary fiber binds cholesterol in the gut, so high fiber intake reduces cholesterol reabsorption from the intestines and increases cholesterol excretion in the stools.  Vegetarian diets are also rich in lignans and phytoestrogens that have been found to be negatively correlated with plasma estrone and estradiol concentrations.

The Institute of Medicine Food and Nutrition Board of the National Academy of Sciences (FNB) asserts that humans have no dietary requirement for cholesterol, and that any amount of dietary cholesterol increases the risk for cardiovascular disease.  However the FNB does not recommend eliminating cholesterol from the diet because doing so "may introduce undesireabl effects (e.g. inadequate intakes of protein and certain micronutrients) and unknown and unquantifiable health risks" [1].
Source: Institute of Medicine [1]

 Some advocates of plant-based diets suggest that high fiber diets are desirable for all populations and there is little danger of obtaining excess fiber in one's diet.

However, this study done by a team composed largely of women, and focusing on the effect of vegetarian diets on women's reproductive health and capacity, provides evidence suggesting that it is possible that menstruating women have a dietary requirement for a certain amount of cholesterol or animal protein to maintain adequate estrogen levels, or that relatively small amounts of fiber from cereal fiber-rich vegetarian diets adversely affect estrogen levels, menstrual cycling and fertility in women.

It has been found that prehistoric Europeans consumed diets consisting of 80% meat and 20% plants.  
This means that the biology of these Caucasian women evolved in adaptation to a diet with a high cholesterol content. 

A high cholesterol diet environment naturally selects for higher reproductive rates for individuals who invest less energy in endogenous cholesterol synthesis, i.e. it positively selects for individuals who have a dietary requirement for cholesterol.   In the presence of abundant dietary cholesterol, those women who invested less genetic material and metabolic energy in cholesterol synthesis would have had more energy available for reproduction (pregnancy, lactation) itself.  They would have left more descendants than individuals who had metabolisms that invested dietary energy in (unnecessary) endogenous cholesterol synthesis.

Overall a high cholesterol i.e. high meat diet selects for both women and men who have a nutritional dependence upon meat-eating for health and fertility.

Thus it is also interesting to note that Orzylowska et al from the Seventh Day Adventist Loma Linda University School of Medicine have reported in 2014 and 2016 that vegetarian and especially vegan men in the Adventist "Blue Zone" have reduced sperm concentration and motility compared to non-vegetarians, which may be related either to estrogenic or toxic compounds present at high concentrations in vegetarian and vegan diets, or to nutrient deficiencies due to meat avoidance, or both.


Although the vegetarian and vegan diets did not reduce sperm concentrations and motility into the infertile range, it is certain that vegetarian or vegan men with reduced sperm concentrations and motility will perforce be less fertile than meat-eating men who have greater sperm concentrations and motility.

This data supports the inference that the habitual meat-eating of our prehistoric and pre-industrial ancestors positively selected for individuals (both male and female) who were more fertile when regularly consuming meat.   The result would be a population in which the typical woman has more estrogen and regular fertility cycles, and the typical male has more sperm with greater motility, when eating meat than when avoiding meat.

This being the case would partially explains why many historical religious orders demanding celibacy (and infertility) of their cloistered members or restricted reproduction among lay members practiced various degrees of meat-avoidance, up to and including veganism (e.g. Pythagoreans, Jains).  Perhaps they discovered by trial and error that meat-avoidance was somewhat effective as a method of nutritional castration.  I am very familiar with historical philosophical and religious literature, and it is very common to find texts, particularly monastic Buddhist sutras, recommending avoidance of meat to reduce or eliminate the sexual drive.  I doubt people intent on celibacy and infertility would have adopted vegetarian diets if they had the obvious opposite effect of enhancing sexual drive and fertility.

In light of these findings it is also interesting to note that some studies find vegetarians have a lower rate of marriage and child-bearing than non-vegetarians.  This would be consistent with the idea that vegetarian diets reduce sexual desire and reproductive capacity. 

The idea that humans have no evolved dietary requirement for animal products can not account for these findings and is inconsistent with evidence from research in nutrition, archaeology, anthropology, genetics, and evolutionary biology.

However, the principles of evolutionary biology adequately account for the findings of both Pedersen et al and Orzylowska et al as well as the vegetarianism of celibate cloistered religious orders.

A theory which accounts simply for the larger set of data in the realm to which it pertains is the better than one which is either more complex or fails to account for some of the data.  "The human is an obligate meat-eater" accounts for more data, and more simply, than "the human is a natural vegetarian." 

The impulse to avoid eating animals arises from emotion, not reason.  The arguments for vegetarianism have historically come from advocates of religious or moral ideology, not scientific biology.

In Powered by Plants I discussed a study that I interpreted as evidence that the human female reproductive system is adapted to very low animal flesh intake:
"Barr et al. found that vegans who had avoided eating flesh for at least 2 years had a higher proportion of normal ovulatory cycles than did nonvegetarians (75% vs. 62%).364  Barr et al. found that the vegan women they studied had no anovulatory cycles over a six month period, while among the lactovegetarians 7 of every 100 cycles produced no ovum, and among those with the highest variety and intake of animal protein (nonvegetarians), 15 of every 100 cycles produced no ovum.  In this study, as the dose of animal protein increased, so did the impairment of ovulation in direct proportion, and only women eating no animal products had ovulation on every cycle.  This study provides more evidence that consumption of animal flesh impairs human fertility in a dose-response fashion."  
However, upon reflection, this study also could be interpreted as indicating that the consumption of dairy products has an adverse effect on ovulation, since only the vegan subjects in this study were on dairy-free diets, and, it is likely that the non-vegetarians consumed more dairy products than the lactovegetarians.  Also, the vegans were the leanest group (23.7% body fat, vs. 24.1 and 27.4 for lacto vegetarians and nonvegetarians, respectively).  This suggests another possible interpretation, namely that increased adiposity impairs ovulation regularity independent of dietary style.  Also, this study was limited by the fact that it included only 8 vegans, but 15 lactovegetarians and 22 nonvegetarians.  The fact that there were only half as many vegans as vegetarians and only about one-third as many vegans as non-vegetarians results in a statistical imbalance.  Additionally, the more reliable ovulation of the plant-based dieters may have been due to their increased consumption of plant-based nutrients, particularly carotenoids, compared to meat-eaters, as there is a growing body of evidence that carotenoids concentrate in the ovaries and are very important for ovarian function in not just humans but probably all mammals [2, 3, 4]. In that case the impaired ovarian function of the meat-eaters may have been due to deficiencies of fruits and vegetables, not the ingestion of animal products.  Finally, this study did not report the races of study subjects, which from an evolutionary nutrition standpoint is unacceptable, because we know that people of different races have different genetic adaptations to diet.


Notes:

1. Food and Nutrition Board, Institute of Medicine of the National Academies, Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids, National Academies Press 2005: 542. 
2. Czeczuga-Semeniuk E, Wolczynski S. Identification of carotenoids in ovarian tissue in women. Oncol Rep. 2005 Nov;14(5):1385-92. PubMed PMID: 16211314.
3. Kawashima C, Matsui M, Shimizu T, Kida K, Miyamoto A. Nutritional factors that regulate ovulation of the dominant follicle during the first follicular wave postpartum in high-producing dairy cows. J Reprod Dev. 2012;58(1):10-6. Review. PubMed PMID: 22450279.
4.  Ikeda S, Kitagawa M, Imai H, Yamada M. The roles of vitamin A for cytoplasmic maturation of bovine oocytes. J Reprod Dev. 2005 Feb;51(1):23-35. Review. PubMed PMID: 15750294.