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.
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.