We really don't understand the relationship between diet and longevity yet. There is ample evidence to suggest that steak and eggs for breakfast at high frequency isn't a good health move for you and I. When you control for the population that also does a lot of physical labor most days, like farmers, landscapers, miners, etc than suddenly high fat/cholesterol, high animal protein diets don't seem to lead to heart disease and other conditions like diabetes nearly so often.
There is a relationship between what we should eat and how we use our bodies and its more complex than more work, means more calories or more sedentary means eat less.
We simply don't have enough astronauts and cosmonauts to do a good study and make more than a token effort to control other factors like tossing out a couple of extreme outlines or exuding an individual known to have made some unusual life style choices post mission. This is a situation where we have to make some anecdotally informed guesses. Its simply the best that can be done.