Energy for baby’s brain and nutrition for gut microbes—carbohydrates in breastmilk

While we tend to think of pasta, potato, and rice when talking about carbohydrates, there is more to carbs than the delicious side dishes. Infants get their daily dose of carbohydrates, among other macro- and micro- nutrients, from breastmilk or infant formula. Breastmilk delivers carbohydrates mainly in the form of lactose and human milk oligosaccharides (also known as HMOs) (1). Carbohydrates are essential to a baby’s growth and development.


Lactose is the most abundant component of breast milk after water. Despite lactose intolerance being common among adults (about 70% worldwide!), most babies digest lactose very well. Babies’ (and lactose tolerant adults’) small intestine produces the lactase enzyme, which breaks down lactose (2, 3).

Lactose consists of two types of sugar: glucose and galactose (1). Only mammary cells can make lactose, which means that lactose is unique to milk and can be found nowhere else in nature (4). Like adults, babies need sugar to have energy to grow and think (1). Lactose provides much needed energy, helps with absorbing calcium and other minerals, and plays a role in developing the baby’s gut microbiota, although method of delivery, gestational age, feeding method, and environment may be more important for developing a baby’s gut health (3, 5).

Breastmilk is typically 7% lactose (the highest concentration among all mammals) and 87% water (1, 6)—it’s no surprise that breastfeeding mothers are often found carrying a huge water bottle. In the mammary gland, lactose helps to draw water from the mother’s body to dilute the milk and provide babies with much needed hydration. Since cow’s milk is about 4-5% lactose (4), some cow’s milk-based infant formulas are modified to have lactose levels similar to breastmilk (7).

In comparison, grey seals’ milk only has <1% of lactose, which means their milk is extremely dense and fatty (8). Since seal mothers fast during lactation, they primarily use their body’s carbohydrates for their own energy while using stored fat to produce milk.

Image courtesy of Britannica Kids

Given that seal mothers only lactate for 16-18 days, their milk needs to be extra energy dense to help their pups double or triple their mass to become independent quicker (9). But, since human mothers typically lactate for months, not days, and do not fast like seal mothers, and human babies aren’t expected to survive on their own as soon, human milk doesn’t need to be as dense.

Although human babies grow slowly compared to seal pups, we’re still astonished by how most of them double their birth weight in 4 months!

Human milk oligosaccharides (HMOs)

Not all carbohydrates in breastmilk are used for energy. The third most abundant solid component in breast milk are human milk oligosaccharides (18). Unlike lactose, human milk oligosaccharides (HMOs) are carbohydrates that the infant does not digest (10, 11). Instead, they reach the colon intact (or are absorbed in small quantities) where they serve other important purposes in the infant’s gut (12). Given all the benefits HMOs provide, some infant formulas have started adding HMOs, produced through microbial precision fermentation, to offer these benefits to babies that may not receive them from breastmilk (13, 14).

HMOs have two main functions in supporting a baby’s gut health: helping to establish the microbiome and to block pathogens. HMOs play a role in the formation of the gut microbiome by providing nutrition to gut microbes (10). The gut microbiome is necessary for the health of all humans – it influences the baby’s immune system’s development and it helps with nutrient absorption and metabolism, and it helps protect babies from pathogens (15, 12). HMOs promote the growth of bifidobacteria, which are known to support gut health by producing short-chain fatty acids (SCFAs) that have anti-inflammatory properties and lower the pH of a baby’s intestines, improving the gut’s defense mechanisms (11, 12, 13, 16). HMOs also prevent the pathogens—harmful bacteria or viruses—from sticking to the gut surface by mimicking the features of the intestinal lining that pathogens recognize and acting as decoys on the intestinal lining (11). By fighting against viruses like rotavirus and norovirus (12), HMOs may prevent diarrhea (1). Who knew carbohydrates could be so powerful?

There are known to be more than 200 different types of HMOs (1) although the scientific community is still in the early stages of exploring what HMOs truly have to offer. The stage of lactation, maternal genetics, and potential environmental factors like seasonal and geographic variation affect the amount and composition of HMOs (12, 17). In fact, HMO profiles have been found to vary between women from different countries and continents (13). Through our current breastmilk research study at BIOMILQ, we hope to contribute to advancing the field’s understanding of HMOs and other components in human milk.


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