The Second Most Abundant Macronutrient in Breast Milk: Lipids
While the macronutrient content of breast milk is highly dynamic and varies across stages of lactation, time of day, within a feeding session and between individuals, the reality is that the nutritional quality of breast milk is actually highly conserved across populations around the world. When we think about breast milk in terms of meeting the nutritional needs of babies, it makes sense that the macronutrients and total energy milk provides doesn’t really vary all that much among women across the globe—all babies need enough calories to meet the needs of their growing brains and bodies.
Recently, we’ve been sharing about the macromolecular components in breastmilk. You can read about proteins here and carbohydrates here. Today, we’re exploring lipids.
Lipids, or fats, are the second most abundant component of breast milk right after carbohydrates and they provide most of the calories found in milk (40-55% of total energy)—meaning, the more fat milk contains, the more total calories it has . The amount and type of lipids in breast milk are related to maternal diet (more on that below) and how much milk a mother produces.
Lipids are the most variable component in milk; the lipid content of milk varies more between lactating women and within an individual (depending on time of day and how long a feeding session lasts) in comparison to carbohydrates and proteins. Milk expressed in the afternoon or evenings contains more fat than milk expressed in the morning or very late at night, which is partially due to circadian rhythm affecting milk synthesis . Hindmilk, the last milk of a feeding session, contains 2 to 3 times more fat than foremilk . So, in order for breast milk samples in BIOMILQ’s research study to accurately characterize milk composition, our study’s protocol asks donors to wait to collect milk until their breast is full (about 2 hours after the last feeding session) and then to empty their breasts completely when collecting milk rather than only expressing the 0.5oz (per breast) they’ll end up donating.
Triglycerides are the main type of fat in human milk, making up 98% of the total fat . Triglycerides are made of three fatty acids attached to a glycerol backbone. The order of the fatty acids on the glycerol backbone influences the digestibility of breast milk. The fatty acids in infant formulas are often arranged in a different order than what would be found in breast milk, and this can negatively affect the digestibility of formula.
The other 2% of breast milk lipids are mainly made of 5 different classes of fats called diacylglycerides, monoacylglycerides, free fatty acids, phospholipids and cholesterol . In fact, breast milk contains relatively high levels of cholesterol, at least 20 times more than in infant formula, and cholesterol is important in helping support rapid brain growth, synthesizing essential vitamins and hormones, and regulating glucose homeostasis .
While maternal diet does not seem to impact the TOTAL amount of fats present in breast milk, diet does dictate the TYPES of individual fatty acids milk contains . It’s estimated the breast milk contains at least 200 different types of fatty acids that can be used to form triglycerides or are free (not attached to a triglyceride) and support proper growth and development . Two of these fatty acids, linoleic acid (LA) and alpha-linoleic acid (ALA), are considered essential fatty acids, because we cannot synthesize them in our bodies so we must get them from our diet. While not essential fatty acids, EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) can be increased by increasing consumption of seafood like herring, mackerel, and salmon or by consuming a fish oil supplement . It is recommended that breastfeeding women eat 2-3 servings of fish per week because fish consumption during pregnancy and lactation is thought to help with a baby’s cognitive development . It makes sense then that global differences in fatty acids in breast milk have been observed as a result of different cultural dietary practices. For example, trans fatty acids in breast milk from women living in North America and Europe are higher than in milk from women living in rural Africa reflecting higher maternal intake of trans fatty acids and hydrogenated fats in resource rich nations . The health impact of trans fat ingestion on the developing infant through breast milk is not currently well understood.
Breast milk lipids are packaged into specialized 3-dimensional structures called milk fat globules where phospholipids derived from the mammary epithelial cell surface make up the outer membrane and surround a core of lipids that includes the triglycerides and other fats present in milk. In order to keep milk fat globules intact, we ask our study’s donors to gently swirl to make sure their expressed milk is well mixed instead of shaking their milk. The milk fat globule helps the fat in milk stay emulsified and researchers are only now starting to understand why the milk fat globule and the membrane that encases it, aptly called the milk fat globule membrane (MFGM for short), is so important. It is well recognized that the MFGM contains many bioactive compounds that may play a role in cognition, gut health, and protection from infections .
In our current research, we are looking to better understand the differences in the amount and types of fat in breast milk. To do this work, we are looking at how diet and different dietary supplements impact lipids in breast milk.
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