Units 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 is will be considered that you are filling the deficit. And to find out the total food used by your unites, click on total strength, and you can find info like (Number of units - Total marching troops - Total food use - Damaged units).
Note that you can use items to Reduce food consumption by going to enhance the base or purchase it by gold.