Troops cost you food. Even if you do not have enough food, they will keep consuming; if the balance reaches zero or less, the food you produce will not be counted, as it will be considered that you are filling the deficit. To find out the total food your units use, click on total strength, and you can find info like (Troop Count - Total marching troops - Total food consumption- Injury Count).
Note that you can use items to Reduce food consumption by enhancing the base or purchasing them with gold.