There's a lot of variance in what many civilizations have considered "healthy" over the years. The Romans tied the notion of spices to opulence and being unhealthy. They saw plain and simple foods as being more pure and rich and spiced foods (like the kinds being consumed by, no shockers here, the economic and military rivals of the empire) as being indicators of decadence. The Chinese for a long time ascribed (and in much of China still do) to the concepts of assigning the attributes of chi to foods, in which certain foods have a warm/energetic chi that heats your body and other foods have a calming or cooling chi that cools you. The idea being that you eat certain foods to balance your Chi in different ways for certain tasks/activities and that keeping these in balance leads to healthy eating. For example you might go out for an night of drinking with your buddies and eat a lot of grilled spicy meat on a stick along with beer (hot foods) so the next day you might need to cool yourself down with some pickled vegetables and plain rice. This extended to diseases as well, you'd eat salads or vegetables to cure a fever or eat some deep fried fish to combat having the chills.
You can probably point to the first actual "diet plan" that was backed with some kind of research as being developed by Avicenna in the Canon of Medicine where he spent a large amount of time researching what different diets could be used to help people with different ailments. Now mind you I think a lot of his research was naturally flawed and a product of his time, but he certainly was one of the first guys we know about who tried to put something close to actual scientific research into why certain foods are healthy and why we should eat them.