Seal Milk Rivals Breast Milk in Complexity and Sugar Content

Human milk is recognized as containing one of the most complex and dynamic compositions of any mammalian milk. Milk sugars are key contributors to healthy infant development and play a vital role in protecting infants from pathogens.

Now, researchers from the University of Gothenburg and Chalmers University of Technology have shown that seal milk contains ~33% more sugar molecules than breast milk. Many of these sugars are unique, and the findings could provide insights into ways of improving infant formula for babies.

Seal milk complexity explained

Our current knowledge of non-human milk oligosaccharides is rather limited, due to a bottleneck in sample access. Mammalian samples that are readily available include domesticated animals such as cows, goats and sheep; however, these species have much lower oligosaccharide levels than their wild counterparts.

The researchers set out to mine the underexplored frontier of milk oligosaccharide diversity, casting their eyes on the female Atlantic grey seal. The team hypothesized that the seals' unique environment likely drives the evolution of specialized sugars with potent protective or developmental roles.

Grey seal pups suckle for 17 days, during which they need to get their digestive system up and running and build up an immune system to protect them from disease and the other dangers found in the North Atlantic. The researchers suspected that the milk from grey seal mothers must be extremely refined to accomplish this task.

“Our analysis shows that grey seal milk is extraordinary. We identified 332 different sugar molecules, or sugars, compared to about 250 in breast milk. Two-thirds were completely unknown previously,” said Dr. Daniel Bojar, senior lecturer in bioinformatics at the University of Gothenburg. “Some of these molecules had a previously unseen size of 28 sugar units, which exceeds the largest known sugar units in breast milk, which are 18 units in size.”

Bojar and colleagues collected milk samples from five wild grey seals off the coast of Scotland throughout their lactation period. Using mass spectrometry, the researchers measured and quantified 332 unique milk sugars in seal milk, of which they structurally characterized 240. Further analysis and utilization of AI models revealed changes in seal milk composition throughout the lactation period.

Support for infant nutrition and immunity

Several of the sugar molecules identified were found to have powerful properties against disease-causing bacteria.

“The sugars bind to various proteins found in, for example, the immune system. We have tested the newly discovered sugar molecules on human immune cells and found that they can regulate the cells' response to various threats,” said Bojar.

The researchers propose that the richness in grey seal milk sugars could improve our understanding of lactation and the impact of milk sugars on mammal health. Further analysis of mammalian milk sugars could lead to the discovery of new bioactive compounds with the potential to support infant nutrition, infection control and immune system development. Newly discovered sugar molecules could provide a boost to infant formula to help strengthen the immune system. These sugars could also have future applications as a supplement to help maintain a healthy gastrointestinal system.

At the very least, the researchers suggest that the potent properties of the discovered milk sugars should motivate further exploration of mammalian milk composition. “We will continue,” said Bojar. “We have milk from another 20 mammals in the freezer.”

Reference: Jin C, Lundstrøm J, Cori CR, et al. Seal milk oligosaccharides rival human milk complexity and exhibit functional dynamics during lactation. Nat Commun. 2025;16(1):10067. doi: 10.1038/s41467-025-66075-2

This article is a rework of a press release issued by the University of Gothenburg. Material has been edited for length and content.