The problem that I see with such a system is that the data entry would be extremely cumbersome. Not only would you have to enter all the foods and their associated calorie counts, you would also have to enter all the food eaten per day. There are many diet apps for counting calories, and human data entry is still a huge pain. In fact, some diet systems rely on the difficulty in data entry–they hope that the effort in entering the data is enough to make people decide not to eat that snack after all.
That said, I think you would need a table system: one for days, another for foods, and a junction table between them. Store the calorie count for each food item in the food table. In the junction table, store the day, the food item, and the number of servings. Have a system of lookups, rollups, and formula fields to do the calculations.
