But it is too soon to switch from skimmed milk and low-fat yogurt to higher-fat versions.
The findings must be confirmed, researchers say, and it is not quite clear if trans-palmitoleic acid or a related compound was protective.
The Harvard study was funded by grants from the National Institutes of Health. It appears in the Dec. 21 issue of the Annals of Internal Medicine.
“If nothing else, I think we have confirmed that there is something in dairy that reduces the risk of diabetes,” lead researcher Dariush Mozaffarian, MD, tells WebMD. “Either it is this fatty acid or something associated with it that we have not identified. It would be highly implausible that the relationship we saw is due to something unrelated to dairy.”
The study included 3,736 participants in the National Heart, Lung, and Blood Institute (NHLBI)-run Cardiovascular Health Study.
Throughout the two-decade study, researchers measured metabolic markers of diabetes risk, including circulating blood fatty acid levels.
Higher blood levels of trans-palmitoleic acid at study entry were associated with better cholesterol, less insulin resistance, and less overall inflammation, even after researchers adjusted for other heart disease and diabetes risk factors.
Over 20 years of follow-up, participants with the highest circulating levels of the fatty acid had about a 60% lower risk of developing diabetes than people with the lowest levels.
The finding could explain earlier research suggesting that people who eat more dairy products have a lower risk of developing type 2 diabetes, Mozaffarian says.