Alcohol interferes with essential nutrients in the body.  Not only does it drain vitamins already in the body, it also slows absorption and retention of nutrients. B vitamins are easily flushed out of the body.  

Also, excessive drinking damages the gastrointestinal tract and leads to poor absorption of nutrients. Sometimes liver or pancreatic disease associated with alcoholism also contributes to nutritional deficiencies. These problems require medical assessment and intervention.

Some of the nutritional deficiencies associated with alcoholism can be caused by a poor diet. The alcoholic may consume most of their calories from alcohol, a nutritionally poor beverage in which most of the nutrients have been removed through filtering processes.