Any form of restricted eating is dangerous. Keto is bad for colon health. You know something is not right by the smell and consistency of your poop. The colon is vastly innervated by neurons which is why it is called the second brain and it is also important in immune system. A long time on Keto damages healthy gut microbe colony. 

Restrictive eating is also related to restrictive intake of other substances such as reading speed and variety. I once read many years ago anorexia was correlated to restrictive reading pattern. Strange as it sounds, it was convincing. 

Body weight is largely determined by genes. I eat all I want but I my BMI is 18.5. LOL.