Consumption of dairy products, especially milk, increases a man's risk of contracting Parkinson's disease, according to a recent study published in the American Journal of Epidemiology.

Previous studies have established a link between Parkinson's - a degenerative central nervous system disorder that commonly causes the impairment of motor skills, including speech - and the consumption of dairy. However, the mechanism for this effect is not yet understood.

Researchers used data from a cancer-prevention health survey of the dietary and lifestyle habits of 73,175 women and 57,689 men to compare dairy intake with Parkinson's risk. They found that the men who ate the most dairy were 60 percent more likely to contract Parkinson's disease than the men with the lowest intake. Milk accounted for most of the correlation, rather than more processed products like yogurt or cheese.

The data for the study were collected between the years of 1992 and 2001.

In agreement with prior studies, the researchers found that the link was not caused by calcium, vitamin D or fat, but by some other, as-yet-unknown characteristic of dairy products.

The average intake of the high-dairy group was 815 grams per day, approximately equivalent to three or four glasses of milk. The average intake in the lowest group was 78 grams per day.

Men's Parkinson's risk in the study increased in a manner directly proportional to their dairy intake. Women's Parkinson's risk, however, appeared to be independent of their dairy intake. It was not clear why there should be a sex difference in the effect, in large part because the mechanisms by which dairy may lead to nerve degeneration are not yet understood.

"More studies are needed to further examine these findings and to explore underlying mechanisms," the researchers wrote.

Parkinson's disease is widespread, and particularly tends to affect the elderly. There are approximately 60,000 new cases each year. The disease is much more common in men than in women.