Pesticide in Milk Linked to Lower Neuron Density

By Sue Hughes
Medscape Medical News

A pesticide used before the early 1980s and found in milk at that time may be associated with neurodegenerative findings in the brain characteristic of Parkinson's disease, according to a new study.

The study, published online in Neurology on December 9, was led by Robert Abbott, PhD, Shiga University of Medical Science in Otsu, Japan.

"Consumption of dairy products has been linked to Parkinson's disease before but a reason for this is not known," he explained to Medscape Medical News. "Organochlorine pesticides have also been linked to Parkinson's. Our results suggest that pesticides in the milk may be related to the neurodegeneration seen in Parkinson's."

This pesticide in this study — heptachlor epoxide — is not used anymore in the western world, but Dr Abbott stressed that it is a very persistent chemical that is absorbed into the soil and water and stays in the environment for many years.

"So it can still end up in fish and milk. It has recently been detected in several countries and is probably more prevalent in developing countries, where there are fewer controls," he said.

Long Window of Opportunity

He also makes the point that this study has wider implications beyond the use of this one particular pesticide.

"We know diet is important in Parkinson's. We have now shown that diet is linked to neurodegeneration in the substantia nigra, and more importantly, dietary habits many years earlier are linked to later changes in the brain," he said. "This suggests there is a long window of opportunity where it may be possible to prevent these changes from occurring."

He added: "The other message is that we need to be aware of what we are eating and drinking. This study has looked at milk but it is not just milk we should think about. The big picture is that this study suggests an environmental factor can predate brain changes typical of Parkinson's long before the condition ever develops. What we eat/drink/breathe may affect what happens in our brains years later."

For the study, 449 Japanese-American men (average age, 54 years) from Hawaii who participated in the Honolulu-Asia Aging Study were followed for more than 30 years until death, after which autopsies were performed.

Milk intake data were collected from 1965 to 1968. Postmortem examinations were conducted between 1992 and 2004. Neuron density counts in the substantia nigra were measured. The researchers also tested brain tissue from 116 men for levels of heptachlor epoxide, which was used extensively in Hawaii before the 1980s and was found at excessively high levels in the milk supply in Hawaii in the early 1980s.

Results showed that neuron density was lowest in nonsmokers who consumed high amounts of milk (>16 oz/day). Dr Abbott noted that smoking has previously been associated with a lower risk for Parkinson's.

After removing cases of Parkinson's or dementia with Lewy bodies, adjusted neuron density in all but the dorsomedial quadrant was 41.5% lower for milk intake greater than 16 oz/day vs lesser intake (P = .001). For persons who were smokers at any point, there was no association between milk intake and loss of neurons.

Among those who drank the most milk, residues of heptachlor epoxide were found in 90% of brains as compared with 63% of those who consumed no milk.

Assumptions

Dr Abbott noted that several assumptions were made in this study.

"The consumption of milk was reported in the 1960s, and we are assuming the pattern of milk consumption would have stayed the same in the years ahead," he said. "And we didn't measure levels of heptachlor epoxide in the milk, so we can't actually say for sure that the milk was contaminated with the pesticide, but brains at autopsy did contain this pesticide at higher amounts in the patients who drank the most milk."

He added: "The important messages from our study are that we can predict cell death in critical regions of the brain associated with Parkinson's before the condition is diagnosed and that dietary factors can predate the clinical symptoms of Parkinson's and can track with the neurodegeneration seen in the condition."

In an accompanying editorial, Honglei Chen, MD, National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina, and Karen Marder, MD, Columbia University, New York, New York, point out that although contamination of milk with heptachlor is a reasonable explanation for the findings, the study did not directly show that the brain heptachlor was from milk rather than from other sources.

They add that the current study makes heptachlor contamination a plausible culprit for higher Parkinson's disease risk among frequent milk drinkers in Hawaii, but it may not offer a good explanation for the association of milk or dairy consumption and Parkinson's in other cohorts where evidence of milk contamination is lacking.

But the editorialists conclude that the study "sets an excellent example of how epidemiologic studies can contribute to the search for causal mechanisms underlying PD [Parkinson's disease]," adding that "this life-course approach to unveiling the complicated process of neurodegeneration should be encouraged."

The study was supported by a contract and grants from the National Institute on Aging; by a contract from the National Heart, Lung, and Blood Institute; by a grant from the National Institute of Neurological Disorders and Stroke; by a grant from the United States Department of the Army; by the Office of Research and Development, Medical Research Service, Department of Veterans Affairs; by the Kuakini Medical Center; and in part by the Intramural Research Program of the National Institute on Aging. The authors have disclosed no relevant financial relationships.

Neurology. Published online December 9, 2015.