Humans are clearly omnivores. There are multiple lines of evidence to support this conclusion. And regardless of what our ancestors ate, we now have decades of research into diet and health that supports a fairly simple conclusion – eat a varied diet, eat plenty of fruits of vegetables, don’t overconsume calories, and everything in moderation. For most healthy people, if you follow this basic guidance you will be most of the way to having a reasonably optimal diet in terms of health. There are tweaks for people with medical conditions, like diabetes. If you want to squeeze out some additional optimization, the evidence generally points to a so-called Mediterranean diet as close to optimal – lots of fruits, vegetables, nuts, legumes, fish, some meat, and heart healthy fats (like olive oil).
There, in one paragraph I just summarized what most people need to know about diet. But there is an entire self-help industry worth an estimated $71 billion around giving fad diet advice and selling dieting products. That is a lot of incentive to sell nonsense.
General common sense advice is also not enough for some. Psychologically there can be a desire to focus obsessively about the details of one’s diet, or the belief that extreme diets are somehow healthier than diverse ones. In the extreme this condition is termed orthorexia nervosa, and is considered an actual disorder. This is likely a largely manufactured condition, meaning it was created by the diet industry, constantly obsessing over “good” foods and “bad” foods. This is embedded in the self-help industry general approach to life which is to find a “hack” or one “crazy trick” to solve complex problems.
The Carnivore Diet
There are countless diet fads, all worthless, and the latest iteration if perhaps the carnivore diet. As the name implies, this is a typical diet fad in that it takes the good and bad food approach. Meat is good, eat only meat. Plants are poison, avoid them at all costs. Doing this will magically cure all disease (slight exaggeration for effect).
Before we get into the theoretical problems with this diet, we should note that there is no clinical evidence that an all meat diet is not harmful, let alone that it is healthy or has any advantages. Proponents mainly cite anecdote and testimonials. There are not studies on the long term effects. Short term studies that exist are basically worthless, because almost all diet studies show short term weight loss because subjects are on a diet. Long term outcomes are all that matter anyway.
A quick PubMed search did not come up with anything, in humans, anyway. There are a lot of studies in actual carnivores (cats and dogs), but not people, because people are not carnivores.
This is a complex area of research to summarize quickly, but what decades of research has shown overall is that, while meat in the diet is healthy, there are increased health risks to high levels of meat in the diet. Further, there are health benefits to eating plants and fiber.
One good point that critics of the carnivore diet raise is that meat does not contain vitamin C. Humans cannot synthesize vitamin C and have to get it from our diet, and in fact this has been true for millions of years with our direct ancestors. A diet that is missing a critical nutritional element cannot be optimal. Also, if an all-meat diet were optimal evolution would have taken care of the vitamin C issue a long time ago. So this is both evidence that our ancestors consumed plants and that an all-meat diet is unhealthy.
One of the silliest arguments for the all-meat diet is put forward by Carnivore MD, who argues that plants are poisonous, therefore we should not eat plants. He starts with a kernel of a fact, but then completely goes off the rails. Yes, many plants evolved to produce toxic substances to keep from being eaten by animals. But you can’t stop there.
Obviously, the world is also full of herbivores, of animals that survive by eating plants. So animals have clearly evolved to tolerate those toxins, or else the carnivores would have nothing to eat. Also, not all plants use that strategy. Estimates are that about half the plants in the world are edible by humans. That doesn’t mean you would want to eat them, but they are technically edible. We can digest them and deal with any toxins.
Evolution goes both ways. It is better to think of this situation as evolutionary warfare – plants evolve defenses and animals evolve ways around those defenses.
Further still, some plants – fruiting plants – evolved a strategy of producing yummy and nutritious plant flesh to help disperse and germinate their seeds. They bribe animals with good food to provide this service. This breaks the logic that plants don’t want to be eaten. Sometimes they do.
The argument also ignores the fact that humans have cultivates almost all plants that we consume, over thousands of years, specifically to be less toxic. Plant toxins are bitter for a reason, so that we can detect potential harmful substances and avoid them. We cultivated our plants to be less bitter, which translates into them having less plant toxins. This actually makes our crop more vulnerable, because we bred their natural defenses out of them.
The evolutionary argument, therefore, does not even apply to almost all plants that we eat. It doesn’t apply to fruits, and it doesn’t apply to cultivated crops – so basically it doesn’t apply to the human diet.
Conclusion
The carnivore diet is just another diet fad focusing on eliminating allegedly bad food categories and restricting diet to only good food categories. The theoretical justification for this approach is highly flawed, and there is no empirical evidence to support it. Further, there are multiple lines of evidence that restricting one’s diet to only meat and eggs is a bad thing for health.
There is also another layer to the carnivore diet fad. If such a diet were widely adopted it would be entirely unsustainable. The feed conversion ratio for beef, the meat most recommended by proponents, is anywhere from 6x to 25x. So even if we take the most favorable number, the world would have to grow about six times as many calories as it does now in order to convert all those calories to beef for people to eat. This is far beyond the world’s capacity, especially for land and water use.
Inherent, therefore, in the carnivore diet is that this is a diet for select elites. Everyone else can eat plants. This adds an unsavory aspect to what is already a pseudoscientific and harmful fad.