For many years, studies have shown an association between eating dairy products and lower diabetes risk. But how dairy protects against diabetes remained a mystery. The answer may lie in dairy’s fat.

Researchers at the Harvard School of Public Health have identified a naturally occurring trans fat in dairy that may substantially reduce the risk of type 2 diabetes. Trans-palmitoleic acid is a fatty acid found in milk, cheese, yogurt, and butter. It’s not produced by the body. It only comes from your diet.

In a study published in The American Journal of Clinical Nutrition Harvard researchers analyzed data from 6,814 adults. They measured circulating blood levels of trans-palmitoleate. At the end of 5 years participants with the highest levels of this fat had higher levels of good cholesterol, and had lower fasting insulin levels  and systolic blood pressure . Compared to those with the lowest level of the fat, those with the highest levels had half the risk of developing diabetes. The results confirmed an earlier Harvard study reported in the Annals of Internal Medicine.

When the researchers combined the data from the two studies they found that each .05% increase in trans-palmitoleate in the blood levels was associated with a 34% lower risk of diabetes. They noted that this trans fat seems to have an extremely strong protective effect, stronger than other things known to be beneficial against diabetes.

In the meantime, enjoy full-fat milk, cheese, and yogurt, preferably from grassfed animals. And you can add reducing diabetes risk to the other healthy reasons to eat real butter.

But continue to avoid artificial trans fats from hydrogenated and partially hydrogenated oils.