When you pick up a gallon of milk at the shop, the nutrients it contains may depend on both what is on the label and the time of year, apparently.

A new study reports that omega-3 fatty acids are highest in milk labelled as organic grass-fed and lowest in conventionally produced milk.

Other fatty acids varied by dairy production method and season. 

Emily Youngmark, a PhD student at the University of Vermont in the US presented her preliminary findings this week at NUTRITION 2026, the flagship annual meeting of the American Society for Nutrition, just outside Washington, DC.

Youngmark said: “These findings contribute to a greater understanding of the role that production systems may play in determining the nutritional composition and potential nutritional value of milk.

“The observed differences in fatty acid composition may represent one factor consumers consider when choosing milk products.” 

Milk consumers in the US

US consumers can often find a large array of options in the dairy aisle, but the differences among them are not always clear, according to the research.

Since products marketed as organic, grass-fed or pasture-based are often sold at premium prices and promoted as having nutritional advantages, the researchers sought to find out whether production systems actually influence milk composition. 

Over the course of a year, the team collected over 300 samples of milk sold by retailers across Vermont and New York.

Using information from dairy producers and product labels, they categorised each sample as:

Conventional (63 samples);
Organic (111);
Organic grass-fed (91);
Non-organic, pasture-based (40). 

On average across all seasons, organic grass-fed milk contained significantly more omega-3 fatty acids than all other milks, providing 88mg per serving compared with 25mg/serving in conventionally produced milk.

By contrast, omega-6 fatty acids were highest in conventionally produced milk, which contained 166mg/serving versus 92mg/serving in organic grass-fed milk.

While both types of fatty acids are part of a healthy diet, experts recommend focusing on increasing omega-3 intake, which is linked with reduced risk of heart disease, type 2 diabetes, and inflammation. 

The fatty acid composition of non-organic, pasture-based milk was generally comparable to organic milk, with some seasonal variation.

In general, it appears that cows with more access to pasture and fresh forage produce milk with more beneficial fatty acids.

Variation in nutritional composition throughout the year likely results from seasonal differences in forage quality, according to the research.

Health implications

Although the results revealed significant differences in fatty acid composition among milk samples, researchers noted that the health implications of these differences remain uncertain.

The researchers acknowledged that further studies would be needed to determine whether regularly consuming certain types of milk leads to measurable improvements in health outcomes. 

Youngmark said: “It is important to put these findings in perspective.

"Rather, the fatty acid composition of organic grass-fed milk may contribute as one component of an overall healthy dietary pattern and lifestyle."

Conventional milk has no formal definition but generally refers to milk produced from intensively managed dairy systems, she explained.

Certified organic milk in the US is produced under federally regulated standards that prohibit the use of pesticides, genetically modified feeds and most synthetic additives and require pasture access for at least 120 days per year.

Organic grass-fed certification imposes additional requirements, including pasture access for at least 160 days per year and the exclusion of grain products from feed.

The term 'pasture-based' can appear on both organic and non-organic milk; non-organic, pasture-based milk production systems emphasise pasture access but are not certified organic.