People who drink more than one serving of low-fat or skimmed milk daily are at a greater risk of developing Parkinson's disease, a new study warns.
Researchers also found that consuming at least three servings (three cups) of low-fat dairy a day is associated with an increased risk of Parkinson's compared to consuming less than one serving a day.
Parkinson's disease is a long-term degenerative disorder of the central nervous system that mainly affects the motor system.
"Our study is the largest analysis of dairy and Parkinson's to date," said Katherine C Hughes, from Harvard TH Chan School of Public Health in Boston, US.
"The results provide evidence of a modest increased risk of Parkinson's with greater consumption of low-fat dairy products. Such dairy products, which are widely consumed, could potentially be a modifiable risk factor for the disease," said Hughes.
Researchers analysed about 25 years of data on 80,736 women enrolled in the Nurses' Health Study and 48,610 men enrolled in the Health Professionals' Follow-up Study.
Participants in these studies completed health questionnaires every two years and diet questionnaires every four years. During that time, 1,036 people developed Parkinson's disease.
Researchers examined what kinds of dairy each person consumed, including milk, cream, cheese, yogurt, ice cream, butter, margarine and sherbet.
They then looked at whether full-fat dairy, as whole milk, was associated with a risk of Parkinson's disease; there was no association.
However, those who consumed at least three servings of low-fat dairy a day had a 34 per cent greater chance of developing Parkinson's than people who consumed less than one serving per day.
The researchers also found that when looking specifically at skimmed and low-fat milk consumption, there was a 39 per cent greater chance of developing Parkinson's for people who consumed more than one serving per day compared to those who consumed less than one serving per week.
Eating sherbet or frozen yogurt also was linked to a modest increased risk.
In a meta-analysis, looking at a group of studies, the researchers found that total dairy intake was associated with an increased risk of Parkinson's disease.
The overall conclusions from these studies was that frequent consumption of dairy products was associated with a modest increased risk of Parkinson's disease, researchers said.
It is important to note that the risk of developing Parkinson's was still very low, they said.
Of the 5,830 people who consumed at least three servings per day of low-fat dairy at the start of the study, only 60 people, or one per cent, developed the disease over the study period.
In comparison, of the 77,864 people who consumed less than one serving per day of low-fat dairy, 483 people, or 0.6 per cent, developed Parkinson's.
The study was published in the journal of Neurology.