I spend a lot of pixels trying to describe food addiction, sometimes succeeding and sometimes not. I just found this on the website of Promis, a treatment center in Britian, and found it clear, direct, and brief:
Food addiction is the use of food substances and processes for mood-altering effect rather than primarily for nutrition and culinary pleasure. Some foods (sugar and refined, white, flour) have a mood-altering effect so that they act as a drug in some people although not in others. Some processes (bingeing, starving, vomiting or purging) also have a mood-altering effect in some people. Remarkably, they feel temporarily relieved of stress when they indulge in these behaviours. The relief may not last long but the memory of the relief does last and therefore the behaviour is repeated.