If you ate nothing but steak for breakfast, lunch and dinner, would you die? Get scurvy? Have terrible poops? To be honest, the science isn't totally clear on this, but we asked some experts anyway. (Spoiler: they think it's a bad idea.)
Do people actually eat this diet?
There are definitely people who say they have eaten only meat for years. I'm not sure if I truly believe them, though. There are also people who say they don't eat food and survive off the energy of the universe, but they kind of have to be sneaking snacks when nobody is looking.
How many supposedly strict carnivores give in and have a doughnut every now and then (or, hopefully, a fruit or vegetable?) Nobody knows.
What nutrients is meat missing?
It depends how you define "meat". A raw ribeye steak, fat trimmed off, has little to none of your vitamins A, C, E, D and K; very little folate, another vitamin; and very little calcium or manganese.
But if you allow yourself other animal products, you'll do a lot better. Eggs can make up for the missing vitamins A, E, D and folate, for example. Still, you're at serious risk of scurvy.
It seems everybody knows someone who knows someone who got scurvy during uni. So there was this guy, they say, who ate nothing but instant noodles for a month. Or pizza, according to one report from a Lifehacker staffer. Or porridge, according to one long-running Scottish legend.
Of Reddit's and Facebook's carnivore diet support groups, all encourage eating muscle meat and fat. Most allow eggs and fish as well. Dairy is controversial: in some groups it's allowed, in others you're encouraged to stick to butter and avoid milk and for the purists it's not really being a carnivore.
Some groups call themselves "zero carb", although that label is a misnomer if you eat dairy; milk contains tons of lactose, which is a sugar and therefore a carb.
When people like the Inuit live on mostly meat diets, they're not just having steaks and eggs. Traditional animal-based diets typically involve organ meats and even the stomach and intestinal contents of herbivores. That is, you'll note, a way to get the nutrients in vegetables since you are in fact eating vegetables. (The Inuit also gathered and ate plants too.)
So what happens?
We don't have any large, rigorous studies on what happens to the body when people eat a "carnivore diet". Nutrition professor Rachele Pojednic pointed us to this account from the late 1920s of two men who ate nothing but meat for a full year.
Both had been Arctic explorers and were used to living on animal products for months at a time. They lived in a metabolic ward, under observation, for a few periods of weeks at a time; the rest of the time they lived at home but promised they were keeping the same diet.
They didn't die. By the end, "the subjects were mentally alert, physically active and showed no specific physical changes in any system of the body". Both lost weight for a short period at the beginning, but weren't significantly lighter by the end of the study.
They excreted ketones in their urine, a sign that their bodies were burning fat in exactly the way you would expect if you're eating a ton of fat and no carbs. Their blood, when drawn, had a visible fatty layer in it.
Five people have recently told me they were going to "try keto" — the most recent after gushing about a mutual friend who has been doing keto, aka the popular ketogenic diet and getting awesome-looking results. You've probably heard rumblings about keto, but what the heck is it? And is it too good to be true?
Their diet was very different, though, from Mikhaila Peterson's three steaks a day. Each man ate, on a typical day, between 100 and 140g of protein, 200 to 300g of fat and almost no carbs. (Meat has a tiny amount of carbohydrate. Very tiny.) Here's what they actually ate:
The meat used included beef, lamb, veal, pork and chicken. The parts used were muscle, liver, kidney, brain, bone marrow, bacon and fat. While on lecture trips [one subject] occasionally ate eggs and a little butter when meat was not readily obtainable.
While one of the subjects was settling in to the metabolic ward, the researchers wrote, "the subject had a craving of calf brain of which he ate freely". At one point the men requested frozen raw meat for a change of pace, but this was the days before freezers were standard equipment, so none was available.
What about the poops?
This is a major subject of discussion in carnivore diet groups, with many people reporting diarrhoea as they try to get used to the diet and others describing what sounds like constipation. These groups are optimistic places, though and posters typically put a sunny spin on any obstacles they face. You're not having trouble with the diet, you're just getting used to it.
Back in 1930, the researchers were happy with the explorers' output. "The stools were smaller than usual, well formed and had an inoffensive, slightly pungent odour", they write. One man had diarrhoea, but they chalked that up to a too-high protein intake (around 40 per cent of his calories).
Ashwin Ananthakrishnan of the American Gastroenterological Association explains that a meat-only diet is likely to be bad for your gut in many ways. By avoiding plants, you're missing out on bacterial species that tend to be anti-inflammatory and you're not getting the soluble fibre from fruits and vegetables that helps to keep your gut barrier intact.
If you want to keep good intestinal health, you really need to include plants in your diet.
Are there long-term risks?
"I honestly think one of the biggest risks of the carnivore diet is colon cancer", says Pojednic, the nutrition professor, "but we won't have data on that for years to come (and this would also mean that someone needs to do a study on this diet, which I honestly don't see happening)". People who eat a lot of red meat and processed meat are at a high risk of developing colorectal cancer.
Diets high in fat have been associated with risk factors for heart attacks and other cardiovascular disease. This is still somewhat controversial, because it's been hard to separate the effects of fat from those of other foods like sugar (people who eat a lot of cheeseburgers also eat a lot of doughnuts) and because there are different types of fats and whenever you remove one from the diet, you have to replace it with another.
This is the classic question about whether butter is good for you or bad for you; the answer is, it's complicated.
The bottom line is, this may not kill you right away, but the internet's version of the carnivore diet (no relation to animal-product-heavy traditional diets) is missing essential nutrients and is probably bad for your health in the long term.