Did you know that the complexity of wild seal milk might rival, or even surpass, that of human breast milk? It’s a game-changer in the world of biochemistry, and it’s challenging everything we thought we knew about mammalian milk. A groundbreaking study published in Nature Communications (DOI: 10.1038/s41467-025-66075-2) has revealed that wild seal milk contains an astonishing array of sugar molecules, some of which could hold the key to new health benefits for humans. But here’s where it gets controversial: could this discovery mean that human milk isn’t as biochemically unique as we’ve been led to believe?
Led by glycoscientist Daniel Bojar of the University of Gothenburg, the research team analyzed the sugar chains in wild seal milk and found hundreds of unique sugar molecules, many of which have never been seen before in any species. Bojar describes this as ‘the most diverse milk, biochemically speaking, that we have investigated so far.’ These sugar chains aren’t just for nutrition—they play a crucial role in helping infant mammals develop a healthy microbiome and fend off harmful bacteria, much like the well-studied complexities of human milk. But what makes this study stand out is its focus on wild animals, as opposed to domesticated or zoo-dwelling species, providing a more accurate picture of how environment shapes biochemistry.
And this is the part most people miss: the researchers focused on Atlantic gray seals because, unlike humans or cows, seals rely on fats rather than lactose for energy in their milk. This means any sugars present are specifically tailored for microbiome development. The team collected milk samples from five female seals on Scotland’s Isle of May at four different points during their 17-day lactation period, using advanced mass spectrometry techniques to identify the sugars and their timing. The results? A staggering 332 unique oligosaccharides, 166 of which were entirely new to science. This places gray seals as the species with the second-highest number of characterized milk oligosaccharides, right behind humans. One particularly impressive find was an oligosaccharide containing 28 sugar units—10 more than the largest known human milk oligosaccharide—in a complex branching structure.
Despite seals’ shorter lactation period, the study also revealed that their milk oligosaccharides undergo dynamic changes over time, similar to what’s seen in humans. For instance, α-gal is more prevalent earlier in lactation, while later samples contain more sulfated structures. One standout discovery was LacdiNAc, a disaccharide that prevented pathogenic bacteria from forming biofilms and modulated immune activity in human white blood cells. This raises a thought-provoking question: Could these compounds lead to new therapeutic breakthroughs?
Sabrina Spicer of Vanderbilt University, whose PhD research focused on human milk oligosaccharides, calls the study ‘completely upending’ the idea that human milk is the pinnacle of complexity. While five seals might seem like a small sample size, the depth of information extracted is nothing short of remarkable. Bojar and his team plan to continue exploring milk glycans in understudied species, but there’s still much to learn about seals too, such as how oligosaccharides vary between populations or closely related species.
So, here’s the big question: If wild seal milk is as complex, or even more so, than human milk, what does this mean for our understanding of mammalian evolution and biochemistry? Could this discovery reshape how we approach infant nutrition or even medical treatments? Share your thoughts in the comments—this is a conversation that’s just getting started!
