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Your players gain one experience for every gram of protein they consume, to a maximum of 30 experience. This cap is refreshed every few hours, so for maximum efficiency, they should grind protein every few hours. As a bit of a cheat, I'll let you on to the fact that eggs, chicken, turkey, nuts, peanut butter, milk, cottage cheese and steel-cut oats are great sources of experience. And trust me, since you're power-leveling by grinding to the cap every few hours, you'll be happy to have small foods or snacks that are still densely-packed with XP.
You'll also need Energy (carbohydrates). Now, different carbohydrates have different rates of Energy Burn, so choose carefully. Some carbohydrates have a slow energy burn ... like from oatmeal, steel-cut oats, sweet potatoes (but no other kinds of potatoes), chicken, tuna, and whole wheat bread (no other kind of bread). These burn off slowly, giving you a stable and long-lasting source of energy replenishment. Processed carbohydrates, like from sugar, burn off very quickly. But no, quick energy isn't that good a thing here ... to keep it brief, that makes you likelier to store fat and get hungrier sooner. You can get processed carbs from most breakfast cereals (ugh), white bread, potatoes, cookies, cakes, and so on.