Despite research over several decades - some involving more than 100,000 subjects - it's still unknown whether alcohol can (in general) cause body-weight gain.

Although alcohol is caloric - at around 7 calories per gram - it also increases metabolic rate - which, in effect 'burns' energy more quickly.

Another theory is that heavy drinkers tend to eat less - some research groups have found that alcohol might even have a protective effect on obesity.

Further reading Patterns of alcohol drinking and its association with obesity: data from the third national health and nutrition examination survey, 1988–1994 BMC Public Health, volume 5, Article number: 126