Seal milk rivals human breast milk in nutritional complexity
Published: 26 November 2025
Atlantic grey seal milk rivals human breast milk in sugar complexity, according to new research published in Nature Communications. The findings challenge the previously held assumption that human milk is uniquely complex amongst mammals
Atlantic grey seal milk rivals human breast milk in sugar complexity, according to new research published in Nature Communications. The findings challenge the previously held assumption that human milk is uniquely complex amongst mammals.
The study, led by Professor Daniel Bojar of the University of Gothenburg with collaboration from Professor Malcolm Kennedy from the University of Glasgow, analysed milk samples from wild Atlantic grey seals to better understand its chemical make-up and benefits to pups.
The study potentially challenges our understanding of mammalian milk evolution, overturning long-held assumptions that human milk is uniquely sophisticated among mammals. The research may also open promising avenues for discovering bioactive compounds with potential applications in infant nutrition, infection control, and immune system support.
The milks of mammals, including humans, are rich in complex oligosaccharides that consist of long branched chains of sugars. Despite being carbohydrates these sugars are generally not for energy metabolism, but instead for a range of activities such as protection from pathogens, cultivating a beneficial microbiome in the gut and promoting intestinal tract development.
Despite their importance, there are enormous gaps in our knowledge of milk sugar diversity in animals other than humans. To date, most work has concentrated on the oligosaccharides of human milk.
Dr Patrick Pomeroy of the Scottish Oceans Institute, St Andrews, collected samples throughout the roughly 17-20 day nursing period of Atlantic grey seals, transferred them to Glasgow for analysis in Glasgow and Strathclyde University laboratories before they were sent to Gothenburg for the more advanced analysis reported in the paper. The research team discovered 332 different oligosaccharide molecules, and structurally characterised 240. Of these 240, approximately two-thirds (166) had not been previously documented. Some of these molecules reached a size of 28 sugar units, up to 10 units more than the largest known human milk sugars.
The researchers reveal that the sugar composition of seal milk undergoes coordinated changes throughout the lactation period to fit the changing needs of the infant, similar to the changes seen in human milk. It is also demonstrated that several of these newly discovered sugars possess antimicrobial and immunomodulatory properties, with potential applications in the biomedical sciences.
Professor Kennedy, Professor Emeritus in Natural History at the University of Glasgow, and co-author of the paper said: “The paper presages that we are now on the threshold of a dramatic improvement of our understanding of the evolution and function of the milk oligosaccharides of mammals and how they fit a given species' biology, such as how their infants are defended from various types of infections.
“This work completely changes our understanding of the diversity of oligosaccharides in the milks of mammals and has implications for the role of oligosaccharides in human and animal health, survival and development, and points to a new era in the understanding the functions and evolution of oligosaccharides in milks.”
Atlantic grey seal mothers give birth on land and nurse their pups for only about 17 to 20 days, during which the pups gain weight dramatically, while their mothers lose about a third of theirs.
After nursing, the mother seals then depart for the sea, leaving their pups to fend for themselves. Dr Pomeroy's field teams collected milk from free-ranging but well-known mother seals as part of their work into seal breeding success on the Isle of May, and initial analysis work was carried out in Glasgow. That analysis found dramatic changes in the milk with time between birth and weaning, covering a wide range of different components, including some of the simpler oligosaccharides.
Samples were then sent to Professor Bojar's team at the University of Gothenburg, to carry out advanced biochemical and mass spectrometry techniques, alongside novel computer analyses.
The study, ‘Seal milk oligosaccharides rival human milk complexity and exhibit functional dynamics during lactation’ is published in Nature Communications.
Enquiries: ali.howard@glasgow.ac.uk or elizabeth.mcmeekin@glasgow.ac.uk
First published: 26 November 2025