The metabolism of fat is largely blocked, leading to fat deposits in the body. This is why you can see “skinny” alcoholics who still have a pot belly (as the belly is a priority fat deposit zone for men especially). read more
They may have low self-esteem (many alcoholics do). This results in the person hating themselves which leads to a desire to self-punishment which in turn leads to neglect including failing to eat properly. The neglect can also lead to health problems which can cause a person to be skinny. read more