Organic milk has more beneficial fats than conventional milk, at least in the United Kingdom, says a new study. Whether these differences are nutritionally significant is less clear.
Surveys of U.S. milk have yielded different results, though they also show differences between organic and conventional milk.
"We had an inkling that organic milk was going to have higher levels of beneficial fats, the unsaturated fats," said Gillian Butler of Newcastle University.
The team had found such a difference in milk sampled on U.K. farms, but the new work, published in the Journal of Dairy Science,extended this finding to the milk that ends up on store shelves.
"What we found with the organic milk in the supermarket was that it was consistently higher, summer and winter," Butler said, "It showed even greater differences (than milk sampled on farms)."
By "beneficial" fats, the researchers mean the unsaturated fats, including omega-three fatty acids and conjugated linoleic acid, a type of fatty acid thought to have heart health benefits and anti-cancer properties and found almost exclusively in food coming from ruminants, like cows.
The new UK study, which was funded by a grant from the European Community and Yorkshire Agricultural Society, also found that milk made in summer had a healthier fat profile than milk from winter, and -- to the researchers' surprise -- that the fat composition varied from one year to another, an effect that the researchers chalk up to climate.
One year of the researchers' study was cooler and wetter than the other, resulting in cows receiving less pasture feeding and a making poorer quality milk, the authors suggest.
Indeed, all of the differences probably come down to what proportion of the cows' diet comes from grazing fresh grass, which, compared to other types of food, has higher levels of unsaturated fats, a portion of which are transmitted to the milk.
Warmer weather, summertime and organic farming practices all contribute to more pasturing of cattle.
"It's not organic farming that may be causing those very small differences," agreed Adam Lock of Michigan State University, who was not part of the study. "It's that those animals may be receiving more pasture."
Meanwhile, a new survey of U.S. milk, excluding organic, published in the same journal, found little seasonal difference. This may be because U.S. conventional cattle graze less year-round, resulting in all milk looking more like winter milk, said Butler.
The same U.S. authors, led by Dale Bauman of Cornell University, surveyed organic versus conventional milk in a study published last year, and found higher saturated fats in organic milk, but a higher proportion of omega three and conjugated linoleic acid in organic. The U.S. study was funded partially by Monsanto, which manufactures recombinant bovine growth hormone.
"Without exception, the magnitudes of the differences in milk fatty acid composition among milk label types were minor and of no physiological importance when considering public health or dietary recommendations," the authors wrote.
The UK study "shows the consequences, or the penalty, if we can't rely on forage." Butler said.
While conventional milk production could in principle incorporate more grazing, resulting in a different fat profile, there is no way for a consumer to know what diet the cows had, Butler said. Organic production requires a certain amount of grazing.
But, Butler cautions that omega-three levels in organic milk are still relatively low compared to other dietary sources.
"We're not really going to drink milk to get our omega-threes, but what is relevant to our diet is higher levels of conjugated linoleic acid," she said.
The nutritional differences are so small that they are "kind of meaningless" said Lock. "That's why I would personally focus more on the healthfulness of milk in general, rather than to claim that milk x is better than milk y."