Researchers recently discovered that women with larger breasts tend to produce milk with lower lactose concentration compared to women with smaller breasts. This groundbreaking finding suggests that while large breast size is not necessary for adequate milk production, it may influence the composition of the milk. The findings have been published in the American Journal of Human Biology.
Breastfeeding is widely recognized for its numerous health benefits for infants, including its role in promoting long-term health and development. Breast milk is not only a source of essential nutrients but also contains various bioactive components that can influence a child’s growth and development. While it is known that breast milk composition can vary significantly among women, the factors contributing to these variations are not fully understood.
Prior studies have explored the influence of maternal traits such as body fat, diet, and feeding frequency on milk composition, but the relationship between breast size and milk composition had not been thoroughly investigated. In their new study, the researchers aimed to fill that gap by examining whether breast size during established lactation is associated with the macronutrient content of breast milk.
The researchers recruited 162 exclusively breastfeeding women from Wroclaw, Poland, between February 2017 and July 2018. Participants were selected based on several criteria to ensure a homogeneous and healthy study sample. These criteria included being mothers of full-term, single births, not suffering from chronic diseases, and not using hormonal contraception. After exclusions due to incomplete data or unusual dietary intake, the final sample consisted of 137 women.
The study involved collecting detailed data on each mother and infant. This included anthropometric measurements such as breast and under-breast circumference, body mass index, and body fat percentage. Additionally, participants completed a general questionnaire about their socioeconomic status, reproductive history, and breastfeeding patterns.
Breast milk samples were collected using a hospital-grade breast pump. The samples were taken mid-morning, a time identified as optimal for standardizing milk composition measurements. Researchers analyzed the milk samples for energy content, fat, protein, and lactose concentration using a specialized human milk composition analyzer.
Contrary to their expectations, the researchers found a negative relationship between breast size and lactose concentration in breast milk. Women with larger breasts produced milk with lower lactose content than those with smaller breasts. Importantly, these findings held even after controlling for maternal body adiposity, dietary caloric and macronutrient intake, feeding frequency, infant size, maternal age, parity, and socioeconomic status.
This inverse relationship between breast size and lactose concentration is noteworthy because lactose is synthesized solely in the mammary gland. The researchers hypothesized that hormonal factors might explain this finding. Larger breast size is associated with higher levels of the hormones estradiol and progesterone. These hormones can decrease overall milk production and inhibit lactose synthesis by reducing the levels of a specific protein, alpha-lactalbumin, in breast milk.
“This is the first study to report a negative relationship between lactose concentration in breast milk and maternal breast size during fully established lactation. The demonstrated association indicates that women with larger breasts size may, via hormonal control, produce milk with lower lactose concentration than women with smaller breasts,” the researchers wrote.
Interestingly, no significant relationship was found between breast size and other macronutrients such as fat and protein.
Furthermore, the study found that feeding frequency and maternal caloric intake also influenced lactose concentration. More frequent breastfeeding sessions were associated with higher lactose content, likely due to the increased secretion of prolactin, a hormone that stimulates milk production. On the other hand, higher maternal caloric intake was linked to lower lactose concentration, possibly due to its impact on glucose metabolism and availability for lactose synthesis.
Lactose is a vital component of breast milk, providing a significant portion of the energy required by infants and supporting the development of the central nervous system. The variability in lactose concentration linked to breast size could have implications for infant nutrition and growth. Infants of mothers with larger breasts, who produce milk with lower lactose content, might receive different nutritional benefits compared to those of mothers with smaller breasts.
Understanding these differences can help healthcare providers offer more personalized breastfeeding advice and support. It also underscores the need for ongoing research into the factors that influence breast milk composition and how they impact infant health outcomes.
The study, “Breast size in lactating women and the content of macronutrients in human milk,” was authored by Magdalena Babiszewska-Aksamit, Agnieszka Żelaźniewicz, Anna Apanasewicz, Magdalena Piosek, Patrycja Winczowska, Olga Barbarska, and Anna Ziomkiewicz.