In moderation, alcohol is metabolized using the enzyme alcohol dehydrogenase. The alcohol combines with NAD to produce acetaldehyde and NADH. The NADH goes into your respiratory system and produces ATP, which is what provides the calories in the alcohol.

When binge drinking alcohol, the body treats the substance as a drug and metabolizes it with a different pathway. Rather than using a reaction that creates NADH, the body uses a microsonal ethanol oxidizing system (MEOS), which uses up energy (takes NADPH and produces NADP).

And because energy in versus energy out is what determines weight loss and weight gain, an energy deficit would cause weight loss in a person. Since alcoholics are frequently metabolizing through MEOS, they have a net loss of energy.