Milk and cheese can be great sources of protein and calcium.
But as it turns out, there may be one type of milk that's nutritionally superior: the organic kind that comes from grass-fed animals.
Michael Tunick, a research chemist at the U.S. Department of Agriculture and author of "The Science of Cheese," told Business Insider that the difference has to do with what the cows eat.
"When the cows eat grass and things they want, it raises levels of healthy fats in the milk," Tunick said.
To meet the USDA's "organic" criteria, farmers have to maintain or improve the land they're raising animals on, and can't use synthetic fertilizers or genetic engineering. There are also strict rules about what the milk cows can and can't eat.
To figure out how those factors impacted the nutrition of the cows' milk, Tunick and other researchers at the USDA looked at two neighboring farms — one that's organic and one conventional — and compared the nutritional content in their milk over the course of three years.
In a report published in 2015, the team showed that when cows eat grass and are given the freedom to graze, their milk has higher levels of healthy fats than milk from cows raised in more restricted, non-organic conditions. Specifically, there was 36% more omega-3 fatty acids and 25-30% more conjugated linoleic fatty acids than the milk from the conventionally raised cows.
Everything else —including protein, lactose, minerals, acidity — stayed the same regardless of how the cows were fed.
Other research into the fatty acids in milk has not yielded such conclusive results, however. A 2013 study funded in part by the organic milk industry suggested that because organic milk has more fatty acids, it could eliminate "probable risk factors for a wide range of developmental and chronic health problems." But shortly after, the Washington Post reported that the paper might have gone too far, since there aren't actually enough omega-3 fatty acids in organic milk to lead to those health benefits. (You'd have to drink 5.5 gallons of full-fat milk to get the same amount of omega-3s as an eight-ounce piece of salmon.)
The 2015 results were strong enough, however, that the USDA researchers concluded that "the importance of pasture grazing must be considered in supplying milk and dairy products that address consumers’ demand for foods that support human health and wellness."