The peasants did not live for long, though, did they? Hard to say if and how the diet contributed, but just because people rarely ate meat then, for example, doesn't necessarily mean it's better for your health. Even if we could agree that people these days eat too much.

People talk about how we spent generations adapting to certain aspects of diet, and so following the habits of old is surely safer, but seem to forget that evolution only really "cares" about producing offspring, not your longevity, or quality of life in old age.