One of the things I keep coming back to in this series is the strong natural affinity that our species has for meat. Every culture that does not prohibit meat consumption for religious reasons (e.g., Indian Hindus) seeks and eats meat avidly.
A key fact that stands out from my recent conversations with anthropologists is that hunter-gatherers and subsistence agriculturalists place a high value on meat, even if they already have regular access to it. Here's an excerpt from a paper by Kim Hill, Magdalena Hurtado, and colleagues (1):
Observations of the exchange rate between other foragers and their agricultural neighbors indicate that meat is worth much more than carbohydrate calories (e.g., Hart 1978; Peterson 1981). Hart, in his study of exchanges of meat and casava between Pygmy foragers and neighboring agriculturalists, found that approximately four and one half times as many calories of casava were exchanged for each calorie of meat given. In addition, it appears that almost everywhere in the world meat calories from domestic animals are probably expensive to produce relative to plant calories, and yet subsistence farmers continue to use at least some of their "cheap" plant calories to produce "expensive" animal calories (see Harris 1985 for discussion)Why do humans around the globe value meat so much? This strongly suggests that we've evolved an affinity for meat because eating it provides a reproductive advantage. In other words, meat may increase our "Darwinian fitness".
Susceptibility to chronic disease plays a part in Darwinian fitness, but it's not the whole story. The other factors I mentioned in the introduction, including optimal growth and development, physical and mental performance, well-being, fertility, robustness, and resilience, also play key roles. Is there any evidence that meat provides an advantage in these other facets of Darwinian fitness?
Growth and development are processes that demand a lot of energy and nutrients. If a diet provides insufficient energy or nutrients, it often results in stunted growth. Do vegetarian and vegan diets support optimal growth? Height is a simple, objective measure of growth that we can use to explore this question.
I think the macrobiotic diet is a good place to start, because in many ways it's a distillation of what many would consider to be the ultimate healthy diet. It's a vegan diet that's based primarily on brown rice, with lots of vegetables, soy, and some fruit. Macrobiotic children are often deficient in multiple important nutrients and they do not grow as tall as their omnivorous peers (2, 3, 4). The following quote from a study on macrobiotic children is instructive:
This follow-up study revealed that children from families which, since the initial study, had increased the consumption of fatty fish, dairy products, or both, had grown in height more rapidly than the remaining children (P less than 0.05). Since no indications were found for the presence of adverse social circumstances, infectious diseases or other confounding factors, our data clearly demonstrate that linear growth retardation in children on macrobiotic diets is caused by nutritional deficiencies alone.These people are not going to develop cardiovascular disease, obesity, or diabetes on the macrobiotic diet. Yet they exhibit "growth retardation" that is readily corrected by meat and/or dairy. Does this qualify as a healthy diet?
The macrobiotic diet is a more restrictive version of the vegan diet, and there are certainly more nutritious ways to be vegan. There is some evidence that non-macrobiotic vegan children tend to be a bit shorter than their omnivorous peers, but the studies have not all been consistent (5, 6, 7). Vegetarian children tend to be similar in stature to omnivorous children (8, 9, 10).
One of the most informative studies was conducted on schoolchildren in Kenya, in an area where the diet normally contains little animal food (11, 12). Supplementing the diet with meat increased cognitive development, academic performance, muscularity, and high-intensity physical activity. Milk supplementation had some of the same effects, and also increased height in younger and stunted children. Feeding additional fat also improved development somewhat, but not to the same extent as meat, suggesting that meat was not acting exclusively via added calories.
Well-planned vegetarian and vegan diets seem to support linear growth relatively well. However, vegan diets may slightly restrain linear growth, and more restrictive vegan diets such as the macrobiotic diet often lead to more serious stunting. A diet that includes animal foods probably offers a child the best chance of achieving his or her maximum physical and cognitive potential.
Fertility is a good marker of a diet's overall calorie and nutrient adequacy. This is particularly true of women, for whom reproduction is a costly endeavor.
Although the findings remain controversial, several observational studies have found a higher prevalence of menstrual problems among vegan and vegetarian women (13, 14, 15). In some studies, the difference was quite large (e.g., 5% vs 20%).
A small randomized, controlled trial published in 1986 supports a link between meat and fertility (16). Women were randomized to a vegetarian or an omnivorous diet for six weeks. By the end of the study, seven of nine vegetarian women had ceased to ovulate, while only one of nine omnivorous women had done the same.
What about men? A recent small study suggested that vegan and vegetarian men have lower sperm concentrations, and lower sperm motility than omnivorous men (17). This study was performed on California Seventh-Day Adventists-- one of the super-healthy "Blue Zone" populations. Does this mean they may be less healthy than we thought? This finding still needs to go through peer review, be formally published in more detail, and be independently replicated.
The evidence suggests that vegan and vegetarian diets may impair fertility, making it easy to speculate why natural selection has produced a meat-loving species.
I wasn't able to find much information on immune function of omnivores vs. vegetarians and vegans. One study found that vegans have lower concentrations of certain circulating immune cells, but that their functional immune capacity is similar to omnivores (18).
In a follow-up to the study in Kenyan schoolchildren mentioned above, meat supplementation reduced the probability of falling ill from infectious disease (19). However, calorie supplementation seemed to have a similar effect, so the benefit may not be attributable to meat per se.
Overall, I don't think this provides evidence that vegetarians or vegans have poorer immune function than omnivores. I'm going to have to invoke the old truism that more research is required.
Now we arrive at a topic that is much more thoroughly researched. Most of the meat we eat is muscle tissue-- precisely the tissue that is most stressed during physical activity. It stands to reason that eating muscle tissue might support the growth and maintenance of a person's own muscle tissue, and this seems to be the case.
A joint position statement titled "Nutrition and Athletic Performance" by the American Dietetic Association, the Dietitians of Canada, and the American College of Sports Medicine states that "well-planned vegetarian diets appear to effectively support parameters that influence athletic performance, although studies on this population are limited" (20). They go on to point out a number of potential nutritional problems that face vegetarian and vegan athletes. These include iron deficiency or insufficiency, inadequate intake of zinc, protein (particularly the amino acids lysine, threonine, tryptophan, and methionine), B12, riboflavin, vitamin D, calcium, and calories. They note that these nutrients are "readily available from animal proteins".
I'll add that vegetarians and vegans have lower levels of muscle creatine, a non-essential nutrient that is supplied by meat and contributes to strength and power performance (21, 22, 23). When vegetarians are supplemented with creatine during a strength training routine, their strength increases more than supplemented omnivores, suggesting that a vegetarian diet impairs strength potential.
Most studies, both observational and intervention, have not found any link between meat consumption and performance in endurance/cardiorespiratory activities, although there are a few isolated studies that found that omnivores or vegetarians perform better (24, 25, 26, 27, 28, 29, 30).
In contrast, most observational and intervention studies show that omnivores out-perform vegetarians in strength/power activities (31, 32, 33, 34). One study also suggested that omnivores recover better than vegetarians (35).
Two studies are worth highlighting.
Campbell and colleagues assigned older men to omnivorous or lacto-ovo-vegetarian diets and observed each diet's impact on the response to 12 weeks of resistance training (36). The omnivorous group experienced a significant increase in muscle mass and a decline in fat mass, while the vegetarian group experienced a decline in lean mass and an increase in fat mass. Strength gains tended to be higher in the omnivorous group, but the differences were not statistically significant.
Neumann and colleagues supplemented the diets of Kenyan children with meat, and found that it increased their muscularity and tendency to engage in high-intensity physical activity, when compared to a fat supplement or no diet supplementation (37).
My impression is that vegetarianism is not a catastrophe for the casual athlete. The differences in performance are modest, and may not be present at all for endurance activities. However, meat does seem to increase muscularity and offer a performance advantage for activities that demand strength and power. It may also be more important for high-level athletes who place major demands on their bodies. Although we have very little data on vegans, I suspect these differences would be further exaggerated.
It's telling that nearly all elite athletes eat meat. There are a few notable exceptions, however on the whole, vegetarian and particularly vegan diets are unpopular among elite athletes. This is less true among endurance athletes, and more true among strength and power athletes.
From an evolutionary perspective, a stronger, more muscular person would be more capable of defending him/herself and seeking food, providing another possible reason why natural selection has produced a meat-loving species.
I'm not going to go into great detail about what specific nutrients could be responsible for the differences I detailed above, because I don't think it's necessary. I'm simply going to focus on one simple, key point: an omnivorous diet is the easiest way to supply all essential nutrients in optimal quantities. The further one gets from an omnivorous diet, the more difficult it becomes to be optimally nourished. The less well nourished a person becomes, the less well that person's body will perform.
There is an overwhelming amount of evidence demonstrating that vegetarian, and particularly vegan diets, are associated with a higher risk of specific nutrient deficiencies:
- Vitamin B12 is only found in animal foods. Vegans must supplement B12 to avoid the debilitating long-term consequences of B12 deficiency. Vegetarians have a higher prevalence of B12 deficiency than omnivores, and vegans have a higher prevalence of B12 deficiency than vegetarians. B12 deficiency is linked to poor cognitive development (38).
- Macrobiotic vegan diets are associated with a long list of vitamin and mineral deficiencies, and this often manifests as a failure to thrive in infants and children.
- Zinc status is best in omnivores, intermediate in vegetarians, and worst in vegans (39).
- Iron deficiency is extremely common among vegetarians and vegans (40, 41, 42), even though iron intake appears adequate.
It's no coincidence that meat is a rich source of the nutrients that vegetarians and vegans tend to be deficient in. In many cases (e.g., zinc and iron), vegetarian and vegan diets appear nutritionally adequate on paper, but when blood or saliva is analyzed, vegetarians and vegans are often deficient. This is because many of the minerals in certain plant foods are poorly absorbed (due to anti-nutrients such as phytic acid, and a lack of heme iron). These differences in nutrient absorption aren't reflected on nutrition labels or typical online nutrition databases, which means that many conscientious, knowledgeable people think they're eating a nutritionally adequate diet when in fact they are not.
Another issue is calories and protein. A typical diverse, omnivorous diet is richer in both calories and high-quality protein than a typical vegetarian or vegan diet. That can be good or bad, depending on who you are. However, when it comes to reproduction, (up to a point) calories and protein are important. Body fat is the primary signal that tells the female body it's competent to reproduce, and vegetarians and vegans tend to be leaner than omnivores. When body fat drops too low, leptin levels decline, and that tells the brain to shut down ovulation. Of course, the flip side is that too many calories can lead to obesity, which can reduce fertility.
Synthesis and conclusion
Humans around the globe have a strong natural affinity for meat, and this suggests the possibility that meat plays an important role in our reproductive success or Darwinian fitness. In fact, there is good evidence that meat does support aspects of physical function that would have been critical for survival and reproduction in an ancestral environment.
These things may not be as critical today, particularly in an era of B12 supplements, and many vegetarians and vegans successfully have children and perform at a high level in life. However, an omnivorous diet does seem to offer the best shot at maximizing a person's physical and cognitive development, fertility, and physical performance throughout the life cycle. For these reasons, I would argue that meat plays an important role in health.