The results show that women who regularly consume more than three alcoholic drinks a week for at least 10 years have about half the risk of developing rheumatoid arthritis compared with non-drinkers.

After adjusting for factors such as age, smoking and dietary habits, women who reported drinking more than three glasses of alcohol per week in both 1987 and 1997 had a 52% reduced risk of rheumatoid arthritis compared with never drinkers at both assessments.

These findings add to a growing body of evidence that long term moderate alcohol consumption is not harmful and may protect against a chronic disease like rheumatoid arthritis, say the authors. However, they stress that the effect of higher doses of alcohol on the risk of rheumatoid arthritis remains unknown.

Rheumatoid arthritis is a chronic inflammatory joint disorder that usually develops between the ages of 40 and 50. About 1% of the world's population is affected -- women three times more often than men. Some studies have shown that drinking alcohol is associated with a lower risk of rheumatoid arthritis, whereas others have found no association.

The relation between alcohol intake and rheumatoid arthritis remains controversial. So a team of researchers based in Sweden set out to analyse this association among 34,141 Swedish women born between 1914 and 1948.

Detailed information about alcohol consumption, diet, smoking history, physical activity and education level was collected in 1987 and again in 1997.

Participants were followed up for seven years (Jan 2003 to Dec 2009) when they were aged 54-89 years, during which time 197 new cases of rheumatoid arthritis were registered.

The age-standardized rate of rheumatoid arthritis was smaller among women who drank more than four glasses of alcohol a week (7 per 10,000 person years) than among women who drank less than one glass a week (9.1 per 10,000 person years) as reported in 1997.

After adjusting for factors such as age, smoking and dietary habits, women who reported drinking more than three glasses of alcohol per week in both 1987 and 1997 had a 52% reduced risk of rheumatoid arthritis compared with never drinkers at both assessments.

One standard glass of alcohol was defined as approximately 500 ml beer, 150 ml of wine or 50 ml of liquor.

The reduced risk was similar for all three types of alcoholic drink.

Further analyses made little difference to the results, supporting the theory that a moderate amount of alcohol may be a protective factor for rheumatoid arthritis. The authors suggest that this is most likely to be due to alcohol's ability to lower the body's immune response.

This is relevant because rheumatoid arthritis is an autoimmune disease -- it causes the immune system, which usually fights infection, to attack the cells that line the joints.