Consumption of low-fat dairy products, such as skim and low-fat milk and frozen yogurt, is associated with an increased risk for Parkinson’s disease (PD), new research suggests.
Compared with people who consumed less than 1 serving of low-fat dairy per day, those who ate at least 3 servings of low-fat dairy per day were about 30% more likely to develop Parkinson’s disease, the researchers report in a paper published online June 8 in Neurology.
The finding supports previous epidemiologic investigation into the link between dairy products and PD, said lead author, Katherine C. Hughes, ScD, from the Harvard T.H. Chan School of Public Health, Boston, Massachusetts.
“There had been a number of previous epidemiologic studies regarding this general topic of dairy intake and risk of Parkinson’s disease, including an earlier analysis in our cohorts from the Nurses’ Health Study [NHS] and the Health Professionals Follow-up Study [HPFS],” Dr Hughes told Medscape Medical News.
“However, we had many more cases in our cohorts that had been diagnosed with Parkinson’s disease since this earlier analysis, which allowed us to have much more statistical power to study this association. Since dairy products are widely consumed, we thought it was important to use this large data set to try to better understand how dairy consumption may relate to PD risk,” she said.
Participants in the NHS and HPFS have been filling out biennial questionnaires for many years, detailing their lifestyles and diets as well as diseases they have been diagnosed with.
For the current analysis, the researchers used data collected since 1984 for the NHS and 1986 for the HPFS. A questionnaire of particular interest was the food frequency questionnaire, where participants detailed how frequently over the past year they consumed a commonly used portion of different foods. They were then grouped into categories according to their intake of dairy foods.
The analysis included about 80,000 women in the NHS and about 48,000 men in HPFS who completed the food frequency questionnaires.
Over about 25 years of follow-up, the researchers documented more than 1000 cases of PD that were first self-reported and later confirmed by medical record review. They were then able to use these data to estimate the risk for PD according to how much dairy participants consumed.
Total dairy intake was not significantly associated with PD risk. However, intake of low-fat dairy foods was associated with an increased risk for the disease.
After adjustment for smoking, physical activity, coffee intake, body mass index, alcohol intake, and total caloric intake, the hazard ratio (HR) comparing people who consumed at least 3 servings of low-fat dairy per day with those who consumed none was 1.34 (95% confidence interval [CI], 1.01 – 1.79; P trend = .04).
This association appeared to be driven by an increased risk for PD associated with consumption of skim and low-fat milk (HR, 1.39; 95% CI, 1.12 – 1.73; P trend < .01).
Dr Hughes and colleagues then combined their results from the NHS and HPFS cohorts with those of other papers published in separate cohorts. The results of that meta-analysis showed the relative risk for people consuming the highest levels of total milk was 1.56 (95% CI, 1.30 – 1.88) and the association between total diary and PD became significant (HR, 1.27; 95% CI, 1.04 – 1.55).
“While it appears that certain dairy foods are associated with an increased risk of PD, we don’t yet understand very well what biological mechanisms may lead to this association,” Dr Hughes said.
“It is important to note that the risk of PD is still low, even among people who consume higher amounts of dairy. We are talking about relative risks in the paper. The differences in absolute risk are modest,” she said. “In addition, as far as for patients who already have PD, unfortunately our results cannot speak to whether dairy may or may not affect the progression of the disease. This is not a question that we are able to answer using the data from our cohorts.”
Compass to Future Research
“This is a modest change in the risk for someone developing Parkinson’s disease,” James Beck, PhD, chief scientific officer at the Parkinson’s Foundation, New York City, told Medscape Medical News.
“This is not something that I would say people need to hold the horses and stop eating yogurt or drinking milk. The benefits of having adequate calcium and vitamin D and protein in one’s diet are probably greater than any potential harms or increased risks for developing Parkinson’s disease, and this is what people should really take home from this study,” Dr Beck said.
The fact that the study differentiated low-fat milk from high-fat or regular milk as a potential risk factor for PD is “interesting,” he noted.
“It points to the fact that casein, the milk protein, may have an effect by altering the serum urate levels. Basic science has shown and epidemiology studies have suggested that increased levels of urate, not to where people get gout, but to the high-normal range, may offer a protective effect against developing Parkinson’s disease, and high-normal levels of urate may lead to a slower disease progression of Parkinson’s disease,” Dr Beck said. “There is actually a clinical trial called the SURE-PD III, currently ongoing, to determine the answer to that question.
“There is a lot of biological evidence that suggests that urate and Parkinson’s disease may be related, and the nice thing about this study is that it adds further support to the SURE trial to suggest that potentially modulating urate levels might alter the course of someone’s Parkinson’s disease,” he added.
Asked for her comments, Kathleen Shannon, MD, Detling Professor and chair, Department of Neurology, University of Wisconsin School of Medicine and Public Health, Madison, said, “I think the study is very interesting but it is unclear how important it is.
“It’s a fairly modest effect. It’s not like doubling the risk — it’s a 30% increase in risk. Also, it is never clear with epidemiology why some people are drinking more low-fat products. Is there a weight problem?”
Dr Shannon, who is a spokesperson for the American Academy of Neurology, also noted that the mechanistic link between dairy and PD is not clear.
“Some people have suggested it might have to do with uric acid levels, but even the relationship of uric acid levels and Parkinson risk is not entirely worked out yet,” she said.
The power of the study lies in the size of its population and the fact that data on dairy intake were collected prospectively, Dr Shannon said.
“Still, I don’t think the results should be taken to change the way people approach their diet,” she said.
The study was supported by grants from the National Institutes of Health and the US Department of Defense. Dr Hughes, Dr Beck, and Dr Shannon have disclosed no relevant financial relationships.
Neurology. Published online June 8, 2017. Abstract