I am not kosher but I am certifiably Jewish:} I actually have wondered this for years... many foods that are certified as kosher are not overtly healthy. For example, kosher meat is extremely salty. The salt technically is not supposed to penetrate the meat but rather be used to draw out blood. However, I have known many people whose doctors forbid kosher meat (my grandmother included) due to the high salt content.
Then of course is margarine which many on CH have already condemned to a lonely place after death:} Margarine has been a savior for Jews who cannot cook butter with meat, but obviously it's extremely high in trans fat.
Obviously someone following a strict kosher diet can eat tons of fresh produce, but even that has difficulties- as you're not supposed to eat bugs, organic produce probably provides some difficulties. Does anyone know how many times you have to wash your produce?