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" .
|Source: Institute of Medicine |
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.
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.
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.