If someone has hormone problems from being too lean, eating only animal foods might help fix their hormones — and it could be the fat in their diet, not just calories or body fat, that makes the difference.

When you don't eat enough food and your body fat gets very low, your body slows down the hormones that make testosterone, focusing on staying alive instead of having kids.

When you have very low body fat and eat fewer calories, your body lowers testosterone levels to save energy for staying alive instead of making babies.