Why babies digest milk more effectively than adults

A new study has pointed out that infants are more efficient at digesting milk than adults due to a difference in the strains of bacteria that dominate their digestive tracts.

Researchers from the University of California, Davis, and Utah State University have identified the genes that are most likely responsible for this difference.

“Human milk oligosaccharides (HMOs) are the third-largest solid component of milk. Their structural complexity renders them non-digestible to the host,” said the researchers.

“Bifidobacterium longum strains often predominate the colonic microbiota of exlusively breast-fed infants. Among the three recognized subspecies, B. longum subsp. infantis achieves high levels of cell growth on HMOs and is associated with early colonization of the infant gut,” they added.

The researchers used whole-genome microarray comparisons to associate genotypic biomarkers among 15 B. longum strains exhibiting various HMO utilization patterns.

They identified 5 distinct gene clusters on B. longum that were conserved (showed little or no variation) across all strains capable of growth on HMOs and have also diverged in strains incapable of growing on HMOs.

The results suggested that B. longum has at least 2 distinct subspecies: B. longum subsp. infantis, adapted to ultilise milk carbon and found primarily in the digestive tract of children, and B. longum subsp. longum, specialized for plant-derived carbon metabolism and associated with the adult digestive tract.

“Although early gut colonisation is likely dependent on a multitude of dietary and non-dietary factors, the delivery of complex oligosaccharides through milk creates an ideal and unique nutrient niche for the establishment of, and colonisation by, B. longum subsp. infantis strains," said the researchers.

“During weaning, a gradual transitioning from milk-based to plant-based diets generates a shift in carbon availability in the gastrintestinal tract favourable for the expansion and formtion of an adult-like gastrointestinal tract microbiota,” they added.

The results are published in the current issue of the journal Applied and Environmental Microbiology.