Can BMI Affect Breast Milk Composition?

by Katie Black

From the news, to social media, to everyday conversations, you’ll hear more and more about “the obesity epidemic.” It started cropping up in the United States in the late 1970s, spreading across the West from there.

Interestingly, the amount of calories people burn through things like exercise or labor hasn’t gone down since 1980, and we’re not eating more fat or carbs. What has changed is a massive surge in heavily processed or sugary food and drinks. Most experts agree that ultra-processed foods (UPFs) and sugar sweetened beverages (SSBs) are two of the main culprits in weight gain and high BMIs. Despite the United States’s different efforts to tackle this problem, obesity keeps on rising.

For pregnant and postpartum moms who are overweight or obese, there can be even more obstacles when giving birth or breastfeeding, and today we’re going to focus on breast milk itself.

Note: If you’re underweight, you probably already know how little focus there is on your specific health issues. Most research on BMI centers around high BMIs. While there is some data on being underweight and breastfeeding, it’s definitely lacking.

Breastfeeding has been shown to prevent obesity in children. But among those who are breastfed, higher BMIs in moms are associated with higher BMIs in children. Scientists are still piecing together the roles genetics and environment play in all this. New research is showing a mom’s BMI may affect breast milk composition. Here are some of the possible ways.

Reminder: while BMI is useful, it can’t reflect your health with 100% accuracy. BMI doesn’t take into account fat percentage, fat distribution, age, sex, ethnicity, and other factors that should be considered when assessing weight and health.

Fat

This systematic review published in 2021 in the American Journal of Clinical Nutrition included 66 studies with 4,674 breastfeeding moms. The researchers looked at whether a mom’s BMI and fat content in breast milk had anything to do with each other 1-6 months postpartum.

Among the moms:

1.4% were underweight, (BMI of less than 18.5)
66.7% were normal weight, (BMI of 18.5-24.9)
27.5% were overweight, (BMI of 25 or higher)
4.3% were obese, (BMI of 30 or higher)

They found that a higher BMI was associated with more fat in breast milk.

In more detail, a gain of around 0.00142 pounds per square inch in maternal BMI was associated with a 0.56 gram per liter boost in the fat concentration in her milk. That’s a 7.2% difference in fat content for each 0.0071 pound per square inch.

This systematic review from 2020 found similar results with mothers’ BMIs and mature milk. But interestingly, there weren’t any big differences in the fat levels in colostrum related to BMI. And moms with higher BMIs actually had lower fat concentrations in transitional milk.

Could these differences have any effect on a breastfeeding baby? Currently, while we know that the fats in breast milk are super important for a growing infant, the research on fat levels and infant growth are inconsistent or even contradictory. This doesn’t mean there’s no impact, but since fat content can change so much even during a single feeding, any differences in how studies are carried out have big effects when it’s time to compare results.

Carbohydrates and metabolites

Studies are showing that a mom’s BMI can affect the amount of small carbohydrates in her breast milk, but most likely not glucose.

This 2020 study from the American Journal of Clinical Nutrition focused on several single sugar molecules called non-glucose monosaccharides, and sugar alcohols which are a type of sweetener usually found in sugar-free or low-calorie foods.

These are examples of metabolites: small molecules made during metabolism. The researchers looked at levels of some metabolites in breast milk at two weeks, two months, and six months and compared them with infant growth scores. There were 172 mom and baby pairs, and each mom was either normal weight or obese.

The metabolites showed a pretty big relation with obesity at two weeks and six months postpartum, meaning the women who were obese tended to have more of them in their breast milk.

Concentrations at two weeks:

Mannose was 1.9% higher
Lyxitol was 1.5% higher
Shikimic acid was 1.9% higher

At two months:

Mannose was 1.8% higher
Lyxitol was 2.1% higher

And at six months:

Lyxitol was 1.7% higher

Mannose, lyxitol, and shikimic acid are linked to a mother’s body fat. Notably they were associated with higher infant fat mass and fat mass percentage during the first six months of life. In exclusively breastfed babies, the associations were even stronger.

Human milk oligosaccharides, or HMOs, are complex carbs that have recently come into the spotlight for their role in infant growth and development.

This 2021 study in the journal Nutrients measured HMO concentrations in breast milk from 194 normal weight, overweight, and obese moms at two months postpartum. Researchers also measured the body composition of the babies at two and six months old.

They found that as the moms’ BMI’s went up, so did LNnT, 3 FL, and 6’SL levels at 2 months postpartum. While LNnT didn’t seem to affect infant growth, both 3 FL and 6’SL did go up with infant fat mass during the first six months of life.

Interestingly, several HMO concentrations (DSLNT, DSLNH, and FDSLNH) were negatively associated with maternal BMI while positively associated with infant growth.

As for HMOs and being underweight, this study found that the total amount of HMOs is much lower in women with a BMI between 14 and 18 than in women with a BMI between 24 and 28. But it’s not clear if this was due to generally being malnourished or lack of a specific nutrient.

Hormones

Breast milk has a bunch of hormones for a bunch of reasons. Certain ones seem to play a role in a baby’s BMI.

For example, adiponectin helps a child’s body regulate glucose levels and fat metabolism. By boosting the body’s sensitivity to insulin, the body uses glucose more effectively, maybe even lowering how much fat is stored.
Leptin is known as the "satiety hormone" because it helps regulate appetite and energy balance. It signals to the brain that the body has enough energy stored, which can help how a person eats and prevent overeating.
Insulin can affect how infants metabolize glucose and store fat.
Ghrelin is known as the “hunger hormone”. It triggers appetite by signaling the brain to eat. It also helps balance energy homeostasis by promoting fat storage and lowering energy use, as well as aids in digestion.

The International Journal of Obesity published this study in 2018, looking at maternal BMI, adiponectin, leptin, and insulin in breast milk at four months postpartum, and its effects on the babies’ body compositions.

Hormone levels varied a lot between moms, but their BMI went up as leptin did, but not with adiponectin (which lines up with other studies). Ethnicity, past births, and exclusive breastfeeding were associated with hormone levels as well. Even when those factors were taken into account, higher leptin was associated with lower infant weight at four months and one year.

Insulin showed a U-shaped relationship, meaning both very low and very high insulin levels were associated with higher infant weight-for-length scores. Midrange levels of insulin were associated with the lowest infant weight-for-length scores.

Adiponectin didn’t have a major relationship with infant body composition, which matches other research done on this topic.

A small 2021 study from the British Journal of Nutrition got data on ghrelin plus other hormones. It found that ghrelin in colostrum was related to BMI. Since ghrelin plays such an important role in weight gain, and since really fast weight gain in infants is a big sign of obesity later in life, it’s definitely worth looking into more deeply.

MicroRNA

Finally, let’s look at microRNAs.

MicroRNAs (miRNAs) are small, non-coding RNA molecules that play a big role in how genes are expressed. Breast milk is a rich source of these miRNAs, and newborns can take them in with their immature intestinal tract, maybe even affecting a child’s development on a systemic level.

We don’t know a lot about the specific miRNA profiles in breast milk and their impact on infant obesity. Researchers from this 2009 Nutrients study hypothesized that miRNAs in breast milk, especially the ones related to the hormones we just talked about, leptin and adiponectin, could be changed in overweight or obese moms.

59 breastfeeding moms were a part of the study (38 were normal weight, while 21 were overweight or obese) and gave milk samples on day 30, 60, and 90 of lactation.

Thirteen miRNAs were found in their milk, with miR-30a, miR-146b, miR-let7b, and miR-148a being the most common.

Six of the miRNAs (miR-222, miR-103, miR-200b, miR-17, miR-let7c, and miR-146b) decreased over time, and the dip was more dramatic in overweight or obese moms.

In normal-weight moms, miR-30a expression went up over time. miR-146b went down in both normal and overweight/obese groups.

Overweight/obese mothers showed disruptions in how genes were expressed, with higher levels at first that went down a lot by the second month.

Leptin levels went down over time in normal-weight moms but were higher and more stable in overweight/obese moms. Adiponectin levels also went down in normal-weight moms over time but didn’t change a whole lot in overweight/obese moms.

Interestingly, miRNAs in normal-weight moms were negatively correlated with the BMI of babies at 24 months (miR-103, miR-17, miR-181a, miR-222, miR-let7c, and miR-146b). But no correlations were found for the overweight/obese group.

The researchers put forth the idea that this could be because a mother’s obesity is altering the supply of miRNAs in her breast milk, affecting the hormones and miRNAs her baby is getting. They couldn’t show a cause and effect relationship, but they concluded it’s plausible that the miRNA profile that works with leptin and adiponectin might help control metabolism in infants, and it could be disrupted because of higher BMIs.

Other research shows that some miRNAs are linked to infant growth metrics at one month. As miR-148a goes up, these things go down: infant weight by 1.34 pounds, fat mass by 0.64 pounds, and fat-free mass by 0.77 pounds.

Each uptick in miR-30b levels corresponds to upticks in infant weight, percentage body fat, and fat mass by 1.34 pounds, 5.62%, and 0.82 pounds.

Interestingly, miRNAs sampled at three months of lactation don’t show a huge relationship with infant growth metrics at three and six months. This highlights the nature of miRNA’s impacts and tells us that the timing of measuring miRNA is very important for understanding its impact on a developing infant.

The takeaway

All of this to say, while BMI does show some relationship in studies with changes in the makeup of breast milk, that doesn’t mean BMI causes those differences. Most of the studies mentioned can’t fully account for the role diet, exercise, genetics, and the natural changes in breast milk play. Plus they don’t have enough representation of diverse weights or ethnicities. And inconsistencies in how studies are done and samples are taken have major impacts on the conclusions.

What researchers are consistent about is that more studies need to be done on maternal BMI, breastfeeding, and a baby’s health outcomes.

By breastfeeding you’re already giving your kid so many wonderful benefits, no matter your BMI. If you get your milk tested, you might be surprised at just how packed with nutrition it is.

It’s estimated that ⅔ of US adults are currently overweight or obese, so if you’re struggling with your weight, know you’re not alone. Being overweight is not a moral, or personal failing. It’s influenced by many political, environmental, economic, social, and genetic factors, and most people need more than just willpower to reach their health goals. They need community and support.