- work in progress*
Human beings need certain nutrients, not specific sources of those nutrients. Adequate nutrition deals with meeting nutritional needs in terms of essential fatty and amino acids, calories, and vitamins and minerals to avoid deficiencies that affect health in a clinically significant way. Where the question of adequate nutrition stops is at more speculative issues. Optimal nutrition picks up where adequate nutrition leaves off, with questions of ideal ratios between macro-nutrients and calorie sources, timing of meals, and other composition to achieve certain results like peak atheletic performance, disease prevention, or longevity.
The consensus among the overwhelming majority of dietetic organizations is that a properly planned vegan diet, with B-12 supplements and other considerations, is adequate for human beings during all stages of life.
It is the position of the Academy of Nutrition and Dietetics that appropriately planned vegetarian, including vegan, diets are healthful, nutritionally adequate and may provide health benefits for the prevention and treatment of certain diseases. These diets are appropriate for all stages of the life cycle, including pregnancy, lactation, infancy, childhood, adolescence, older adulthood and for athletes.
This is echoed by nutritional recommendations of major governmental bodies, including the oft criticized USDA (which has animal agriculture industry ties, and it in no conceivable sense biased in favor of veganism).
Vegetarian* diets can meet all the recommendations for nutrients. The key is to consume a variety of foods and the right amount of foods to meet your calorie needs. Follow the food group recommendations for your age, sex, and activity level to get the right amount of food and the variety of foods needed for nutrient adequacy. Nutrients that vegetarians may need to focus on include protein, iron, calcium, zinc, and vitamin B12.
- This is followed by extensive recommendations for lacto-ovo vegetarians and for vegans; the USDA considers both forms of vegetarianism.
One apparent exception among credible organizations, often cherry-picked by anti-vegan critics, is the DGE (German Nutrition Society) which released a more critically worded 2016 position paper. An important consideration is the fact that these recommendations are focused on sensitive groups within the context of German culture and food availability: for example, in Germany (due to ill-conceived EU regulations on organic labeling) the overwhelming majority of vegan substitutes like plant milks are not properly fortified with well studied vitamins and minerals, but with unusual forms of plant extracts (See discussion in comments). The position does not state that a vegan diet is inherently inadequate or unhealthy, and to the contrary outlines specifically HOW to get the necessary nutrients in table 2: Potential critical nutrients in a vegan diet and vegetable nutrient sources. The general sentiment of the position paper is that they do not believe that people are competent, and it is necessary that fortified foods and vegan specialty foods and supplements be more reliable and widely available to reduce risk of inadequate nutritional status on a population level. The tendency of German vegans to favor organic and want to avoid all supplements seems also to be a major concern.
Rejection of Consensus
Criticism of consensus tend to be either overtly conspiratorial, or vague fear-mongering claims that there isn't enough evidence which is in itself a claim without evidence-- and one that implies either irresponsibility and stupidity of professionals or a conspiracy, because obviously those professionals agree there is enough evidence to make the claims they do.
Conspiracies tend to allege those organizations defending veganism are ideologically blinded liberals or environmentalists, accusing anybody claiming veganism can be nutritionally adequate (even when those professionals are not themselves vegan) of being biased as evidenced by their acceptance of the nutritional adequacy of veganism. It's an essentially unfalsifiable position where just agreeing veganism can be healthy is proof that you are biased and unqualified to make that claim.
Beyond this, critics often appeal to pseudoscience to trump modern nutrition, commonly making faulty appeals to evolution or nature; faulty because what we evolved eating is neither the only healthy diet possible, nor even necessarily a healthy diet by modern standards since we did not historically live long enough to evolve resistance to chronic disease from diet.
Or they turn more overtly anti-science and anti-technology, categorically rejecting the claims of nutrition professionals and dietetic organizations on the basis that there are no authorities in nutrition because nobody really knows anything about nutrition. They dismiss all existing evidence as inadequate with an ever-shifting goal post or demanding unreasonable and impossible evidence we would never need for anything else. For example, some people demand randomized studies on human beings following them for 2-3 generations into old age which would take a hundred years to complete, ensuring they will never have to contend with evidence that meets their impossibly high standards.
There are good reasons professionals believe we know enough about nutrition and the ability of humans to derive nutrition from well planned vegan diets, and while some lines of data are limited, the concordance of our epidemiological and mechanistic knowledge provides a very safe level of certainty.
Studies on Vegans
A relatively common anti-vegan assertion is of some kind of survivorship bias involving special vegan genes that allow a small minority of people to eat vegan without severe health problems. The assertion is that all studies on long term vegans are only studying those people who stuck with veganism because they had those special genes, and the people without them tried veganism for only a few weeks or months and then quit due to severe health problems: thus, a bias in the studies, because if true they would only be sampling from the few people who were genetic anomalies and could tolerate a vegan diet, and so the results would not be generally applicable.
The claim is fallacious because there are samples that are not self selecting (like vegan children), however a corollary can be added that the parents are passing on those vegan genes, so the children are self selecting too. Of course, this claim is relatively easy to debunk on the basis that many vegan children are from mixed families where only one parent is vegan, and that according to genetic inheritance this would be statistically impossible for the children to ALL receive those genes... but then yet more claims can be added on that parents with the same vegan gene are attracted to each other through scent or other markers, meaning the other parent will have the gene too despite not using it. You can see how this goes: claims can be made ever more elaborate to evade falsification. The same goes for models like geocentrism. Nothing in science can be so absolutely established as to not be possible to worm around the obvious conclusions with far fetched ad hoc theories.
That is, claims like that can always be made, and you'll never be able to get around every supposed "possibility".
The important thing to understand is that such a claim is completely unsubstantiated. While the claimed bias may be "possible" in the sense that we don't know with absolute certainty that it's not, absolute certainty is not something we have with anything in nutrition, medicine, or any epistemology; it's not something that exists, which is why to be reasonable we favor things which have evidence over unfounded speculation.
When presented alone, it may seem like assuming this kind of "survivorship bias" is a safe bet if it's the only bias that seems possible. The decision theory table might look like this:
|_||Eat Meat||Go Vegan|
However, this kind of thinking employs the same false dichotomy fallacy as the classical Pascal's wager.
|_||Worship God||Don't Worship|
The point of these tables is ostensibly to prove that no matter what the odds are of the alternative scenario with the asymmetrical consequence, as long as the odds aren't absolutely zero, the better decision is to play it safe since the average outcome is better with the behavior the author wants to advocate.
However, as should be obvious, these are not the only two possibilities. In the case of Pascal's wager, it's well known that we can introduce other religions where believing in the wrong god could have a worse outcome than being a non-believer, or even an atheist-friendly deity that rewards people for independence and dislikes worship.
|_||Worship God||Don't Worship|
There are no empirical reasons to favor one over the other or get any sense of the probability of one "possibility" being true vs the other, making any speculation or evaluation highly problematic. Treating them all equally, if you want to presume that to be the null hypothesis, results in a wash.
In the case of the "survivorship bias" argument against veganism, there's a similar false dichotomy employed. Consider the speculative assertion that veganism is actually much healthier despite health metrics seeming similar or only slightly better than average, because the sample is biased with people who would naturally be in bad health and are only sticking to veganism because it is such a great diet that it is the only diet to keep them normal and alive, while other people who can tolerate omnivory so do despite the health consequences because the benefits of veganism are more modest for them in the short term and the social pressures are enough to sway them away from it.
|_||Eat Meat||Go Vegan|
As you can see, here too where other opposing claims are added, they cancel out or even outweigh (depending on how many claims you arbitrarily add) the anti-vagan claim. In principle, adding various discrete assertions could go on forever, piling up an infinite number on each side. Decision tables like these are only rarely useful, and when they are the utility usually derived from some actual evidence letting us add weights to the "possibilities".
The important thing to understand is that speculation can go either way, and so can these speculative biases. Anybody trying to make the case that current results are biased in favor of veganism and not against veganism is doing so without any evidence, and favoring that particular speculative bias is employing a false dichotomy fallacy. The burden of proof is on those who make the claims that there is only one kind of expected bias, and to provide evidence that suggests this bias actually exists. Until then the appropriate conclusion is agnosticism. The appropriate practice is following existing evidence (even if it has limits, as ALL evidence does for anything) and more importantly the interpretations of professionals who are qualified to draw reasoned conclusions and advice from the available evidence.
Here the advice is very clear: vegan is a healthy option, and may have some health benefits. The benefits are probably modest for most people, but in no sense can this evidence based advice be dismissed with unfalsifiable assertions like the "survivorship bias".
Animal products have many drawbacks, introducing unwanted substances and disease risk factors to the body, and optimal nutrition status may be more difficult to achieve on a diet including them. However, they are rich in a number of nutrients, particularly minerals and amino acids, which make adequate nutritional status somewhat easier (in much the same way that taking a multivitamin does). I.O.W. You might have a higher risk of a heart attack, but less risk of ever being low in B-12.
A properly planned vegan diet (with B-12 supplements) is perfectly adequate, but the nutritional knowledge to plan a vegan diet is neither intuitive nor instinctual, and where cultural eating patterns can help pass on that knowledge through the brute force of surviving traditions that didn't make people sick (at least while they were young), there have never been any large and long-standing vegan cultures to adopt dishes from (likely due to the B-12 issue).
While the developing industry of meat replacements and other animal product replacements will in time solve this issue through fortification, unfortunately not all animal product replacements are properly fortified, and some are offensively inadequate alternatives.
In the mean-time, vegans need to familiarize themselves with the basics of nutrition and the nutrients of concern so that they can overcome the cultural shortcomings by properly planning their diets. There is a learning-curve to veganism, both in understanding the basics of nutrition and learning to prepare nutritious foods that you like and will reliably eat, and particularly for groups that are high-risk (or lazy eaters) it is probably better to go slowly and get accustomed to e.g. lacto-vegetarianism or pescetarianism before moving on and excluding all animal foods.
There are a number of aspects of genetic variation that are incredibly important to the topic of adequate nutrition, but not in the way critics want them to be.
A properly planned vegan diet, actually followed (easier said than done, for some), is as suitable to anybody as an omnivorous diet would be; with millions of vegans there has never been a credible case study of anybody who could not be vegan because they needed some nutrient not available from a balanced vegan diet with B-12 supplementation as recommended.
Recommended daily intakes are based in a broad array of information for most nutrients, both human and non-human animal data (like closely related rodents and other primates), and include information we gained from developing TPN (Total Parenternal Nutrition, IV-nutrition based on a mix of fatty and amino acids, vitamins and minerals), along with studies of deficiencies which have in the past manifested in populations without access to adequate nutrition, and in vegans in the early days (who manifested with B-12 deficiency due to lack of supplementation).
While it remains possible that there is somebody out there who can not synthesize arachidonic acid or taurine (as in cats) due to spontaneous mutation, the risks are unmeasurable and astronomically low (and must be compared to the risks of consuming animal products). Given the evidence, a more probable explanation for cases of failure on a vegan diet is nutritional ignorance or lax practice: however, that does not mean condemnation is in order, because both of these are typical. The likely difference is genetic:
- Super Heroes: some people thrive with otherwise inadequate diets (ones that fall below the RDI and would not be suitable for most people), one well known historical example (which is no longer relevant in modern vegan diets which include more oil) is a genetic mutation common among vegetarian populations in India that allows extremely efficient conversion of ALA to arachidonic acid in the context of very low fat carbohydrate rich diets.
- Food preference: While this can be significantly modified by environment and anybody can find a vegan diet they can enjoy, a significant part of this is genetic --like the ability to taste bitterness in vegetables, or tasting cilantro as soap. Due to innate food preferences, particularly a preference for nutrient-dense vegetables and savory vegan foods like tofu rather than sweets, many successful vegans likely stumble on adequacy by mere accident of nutrient density of the foods they naturally prefer.
- Allergies and intolerances: Some people have allergies or problems with certain foods, gluten for celiacs being the most notable. These people can of course be vegan, but this can limit food selection more and make a varied diet and accidental adequacy just a little more difficult. For example, with gluten intolerances selenium can be an issue, because one of the richest sources for vegans is wheat (likewise, protein because wheat is richer than likely alternatives such as rice). A celiac who does not know to eat Brazil nuts or other specific foods to compensate for the lack of it could become deficient.
All of these are things that can make it very difficult to accidentally fall into a vegan diet that is adequate without planning. Food preferences and intolerances are all things that a registered dietitian could help with, but even so not all people follow dietary advice: a significant number of people persistently fail at diets for weight-loss or to reduce cholesterol even when it's a matter of life and death, and this is much more complex. However, appropriately planned vegan diets are probably also something that are easier to follow once establishing the habit (whereas other diets tend to result in a yo-yo) because other dietary changes typically involve necessary weight-loss and veganism does not by necessity (thus no need to fight "set-point" and trigger the cascade of starvation hormones that make reasoned behavior very difficult).
There ARE also practical issues that make it legitimately difficult to follow a balanced vegan diet like certain food allergies (particularly to nuts and legumes) and intolerances to fiber, but these are immediately manifest and would be known by anybody prior to going vegan: none make it impossible given professional help and the will to succeed. However, there may be some cases where the effort is not worth the result, and it may be better just to eat oysters, focus on reduction, and put efforts into doing other good in the world: people with bizarre intolerances and allergies who would have to spend every waking hour preparing food or the majority of their incomes to be vegan could easily be advised to volunteer and donate that money to an effective animal charity instead when there's no practicable alternative. Those who can go vegan easily with a little transitional period and a learning curve should do just that; it has the largest impact for the least long-term effort, and we should not fall victim to the mentality of buying and selling of indulgences except as a last resort.