These days, there are any number of scientific and precisely calculated diets for athletes and other individuals who want to stay muscular and fit. Many of these eating strategies are based off of cutting edge nutrition research, and crunch the numbers in order to provide the optimal balance between necessary vitamins, nutrients, and other compounds. Diets might feature lean chicken, raw vegetables, and almost certainly a battery of supplements and powders.
But eating to encourage strength was less obvious in the medieval era. Knights and other warriors were often left to create their own meal plans out of whatever food was available in the locality at a certain time of year. Creating a good diet involved exercising the ability to balance practicality and nutrition, and needed to furnish enough calories to allow knights to move around in their heavy armor. Some of the foods knights frequently ate include:
Dairy products have always been popular in western countries because they are packed with sugars, fats and proteins. The problem is that milk is hardly portable, as it is voluminous and extremely perishable. Cheese provides a good solution to these problems, as a single wheel of cheese has the nutrition in several gallons of milk and as much longer lasting. It was a common travelling food for knights, kings, and peasants alike.
While most modern athletic diets try to minimize carbohydrate consumption in order to reduce fat development, medieval diets actually incorporated a huge amount of bread and other grains. This is because medieval foods were not nearly as calorically dense as their modern equivalents, so knights had to supplement their diet with extra calories in order to have enough energy to wear heavy armor.
Knights were often travelling, so game was frequently eaten at meals as an extra source of protein. This, in combination with bread and cheese, provided a full meal that was nutritious, portable, and tasty.