These die remarkably young. I remember a doctor who ate only plants you can find in Finnish nature all-year-round. This implied severe calorie restriction too, while this term was not yet popular in 1970s. He died 72 years old. There was some definite component missing in this diet, and many plants were found poisonous when consumed en masse.
Most impressive to me is how he is able to track every single thing he eats, with a food scale no less.