Studies have found a connection between the consumption of dairy products and a higher risk of developing Parkinson disease, the neurodegenerative disorder that affects motor neurons in the brain. While researchers speculated that chemicals found in cows’ milk might be responsible, there was little evidence to detail how dairy products like milk and cheese might be affecting people’s risk of the disease.

Now, scientists may have uncovered a promising clue. Reporting in the journal Neurology, Robert Abbott, from Shiga University of Medical Science in Japan, and his colleagues took advantage of an environmental scandal in Hawaii in the 1980s to investigate the connection. At the time, an organochlorine pesticide used by pineapple farmers made its way into the milk supply when cows were fed a gruel made in part from the pineapple debris. Coincidentally, there was also a study of heart disease among Japanese-American men begun then that involved more than 8,000 men who were followed from mid-life to death. All provided detailed information about what they ate, including how much milk they drank, and some agreed to donate their brains for research upon death.

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Abbott and his team studied 449 brains and recorded the density of neurons in specific areas of the brain known to be affected by Parkinson’s. They found that men who reported drinking more than two glasses of milk a day (16 oz) showed the thinnest nerve networks in these areas, suggesting compromised function of these nerves, compared to men who drank little or no milk. The milk drinkers also had residues of specific organochlorines called heptachlor epoxide.

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Interestingly, by measuring when cells in motor nerve regions died, they also learned that the accumulation of heptachlor epoxide occurred before the cells were damaged, strongly hinting that the chemical was responsible for triggering the changes associated with Parkinson’s.

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Abbott says that he and his team did not have samples of the milk the men drank, so can’t say for sure that the contaminated milk was the source of the pesticides they found in the mens’ brains, but it’s a reasonable explanation. “We don’t have all the data yet, but we are close to finding the smoking gun here,” he says. “It’s not complete, but it’s very suspicious.”

Heptachlor epoxide is no longer used as an insecticide in the U.S. According to the Environmental Protection Agency, “nearly all registered uses of heptachlor epoxide have been canceled.” But it tends to be persistent, remaining in soil and water for many years. Abbott also notes that it’s been found in goat and cow milk in Ethiopia and that other organochlorines have been detected in the milk supply in Italy.

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The data certainly don’t mean that anyone who drinks several cups of milk a day is putting themselves at risk of developing Parkinson’s. What it does mean is that diet and lifestyle risk factors should be considered more deeply. “This adds to the literature that diet may indeed play a role in Parkinson’s,” says Abbott. “But it also tells us that there is more to food than just its nutritional value. There’s contamination, and what’s on that food.”

For now, there’s no reasons to stop drinking milk. (“I drink a cup of milk every day,” says Abbott.) But he hopes his findings fuel the continued careful look at how chemicals in the environment might be affecting our health, even in indirect and not always obvious ways.

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