Mother’s milk is nutritious stuff. Human breast milk contains hormones, fats, antibodies, and more than 200 sugars that promote immune system development and reduce the risk of infections. These sugars (“oligosaccharides”) also help establish the newborn’s developing gut microbiome, which is crucial to health. As such, physicians consider breastfeeding the optimal approach for nourishing infants.
A new study published in Nature Communications shows that Atlantic grey seal milk makes human breast milk look like weak tea.
When a human baby is born, its main task for the first few weeks is to lie around and grow. In contrast, the 17 days of nursing by grey seal pups must prepare them to dive right into the North Atlantic to start earning their living. These seal pups may triple their birth weight during those 17 days, laying on the blubber they’ll need for plying the frigid waters in search of food.
Researchers from the University of Gothenburg in Sweden studied the composition of milk from five wild grey seals (Halichoerus grypus) on the Isle of May off Scotland. Knowing that the chemical makeup of human milk changes as a nursing baby grows, they collected and stored five samples from each seal mother at intervals throughout their 17-day lactation.
Read more: “When Stress Comes with Your Mother’s Milk”
Collecting milk from lactating wild seals was no mean feat; it required anesthetizing the more than 500-pound animals, administering oxytocin to promote milk release, and following up with a preventive antibiotic.
But the time-series characterization of the seal milk via mass spectrometry, a method that precisely determines the compounds present in a sample, was unprecedented. “For the first time, we have been able to analyze the milk sugars in the milk of a wild mammal throughout the entire lactation period,” said study co-author and University of Gothenburg bioinformatician Daniel Bojar, in a statement.
Bojar and his colleagues found 332 distinct sugar molecules, called oligosaccharides, in the seal milk, compared to approximately 250 in human breast milk. Two-thirds of the seal oligosaccharides were previously unknown molecules. Whereas the largest known sugars in human milk are 18 units in size, the seal milk boasted molecules of 28 units. As in humans, the seal milk composition changed as the offspring grew, becoming more diverse in later lactation stages to rival the complexity of human milk.
During the development of nursing humans and seals, the sugars bind to proteins in the immune system to regulate cellular responses to stressors such as pathogens. The seal milk molecules had powerful anti-bacterial properties, a useful trait for a marine animal.
The authors suggest that their work points to possible anti-pathogenic molecules that could be mined for human medical uses, perhaps even for human baby formulas.
And finding new sugar molecules in the milk of these seals could be just the tip of the mammalian lactation iceberg. “We have done this for 10 different mammals, and we find unique sugar molecules every time,” Bojar added. “We will continue. We have milk from another 20 mammals in the freezer.” 
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Lead image: Waitblock / Wikimedia Commons
