Infant feeding practices may influence the timing of puberty in girls

A large study published in The Journal of Nutrition found that girls who consumed human breast milk as infants tended to start puberty slightly later than those fed exclusively with infant formula. The research suggests that early feeding practices might influence the timeline of sexual development in females, though the same pattern was not observed in boys.

Puberty is the transitional phase of physical maturation when a child develops into an adult. Over recent decades, medical professionals globally have noted a general trend of puberty starting at younger ages, particularly in girls. Early puberty is a public health concern because it is associated with a higher risk of various adult health issues, including type 2 diabetes, cardiovascular disease, and certain reproductive cancers.

Human sexual development is shaped by many elements, including genetics, childhood body weight, and socioeconomic status. Medical researchers have also questioned whether an infant’s first foods might play a role in this biological clock. Human milk provides infants with unique hormones and nutritional components that infant formula lacks, prompting investigations into its long-term effects on physical growth.

The current study was led by Marta Pinto da Costa, a researcher at the Institute of Public Health of the University of Porto in Portugal, alongside several colleagues. The team wanted to know if consuming human milk, as well as the duration and exclusivity of that feeding, was associated with the timing of puberty. They noted that previous studies on this topic had yielded conflicting results, with some finding a protective effect against early puberty and others finding no link at all.

To investigate this question, the researchers examined data from a large birth cohort study called Generation XXI. This project enrolled expectant mothers at public maternity hospitals in Porto, Portugal, between 2005 and 2006. The researchers tracked a final sample of 4,745 children from birth through adolescence.

Parents were interviewed about their infant feeding practices at multiple points when the children were six, 15, and 24 months old, and again at four years old. The researchers categorized the children into four groups based on their diets. These groups included children who never received human milk, those who received human milk along with formula or other foods, those exclusively fed human milk for less than four months, and those exclusively fed human milk for four months or longer.

The four-month cutoff was chosen because Portugal provides four months of fully paid maternity leave, heavily influencing infant feeding timelines in the region. When the children reached 10 years of age, trained nurses evaluated their physical development. The nurses looked for established markers of puberty, such as breast growth and pubic hair in girls, and genital growth and pubic hair in boys.

These characteristics were graded on a standard medical scale known as Tanner stages, which ranges from one (prepubertal) to five (fully mature). To improve the accuracy of their assessments, the researchers combined the individual Tanner stages into a total puberty score ranging from two to 10. A score of three or higher indicated that a child had officially started puberty. Later, at ages 13 and 18, female participants also reported the age at which they experienced menarche, their first menstrual cycle.

The research team used statistical models to compare the early feeding categories with the puberty scores at age 10 and the age of the first menstrual cycle. They adjusted their calculations to account for other factors that might sway the results. These adjusted variables included the mother’s age and education, the mother’s own age at her first period, and the child’s birth weight.

The researchers found that girls who received human milk had a lower likelihood of showing advanced puberty scores at age 10 compared to girls who were never fed human milk. This pattern was most pronounced among girls who were exclusively fed human milk for at least four months. Essentially, any exposure to human milk in infancy was associated with a less advanced stage of puberty by age 10.

The initial analysis also indicated that exclusive human milk feeding was linked with a later onset of the first menstrual cycle in girls. However, this specific link was no longer statistically significant once the researchers factored in the children’s body mass index at age 10. Body mass index is a common medical tool that estimates body fat based on height and weight. This attenuation suggests that childhood body weight might partly mediate the relationship between early feeding and the timing of menstruation.

Previous research indicates that longer durations of breastfeeding often protect against childhood obesity, a condition frequently linked to earlier puberty onset. Human milk contains hormones like leptin, which helps regulate infant appetite and acts as a chemical signal communicating the body’s nutritional status to the brain. While leptin is known to influence the biological pathways that trigger reproductive maturity, researchers note that the exact way leptin from breast milk might shape a child’s long-term hormone levels remains speculative. Still, breast milk could indirectly delay puberty by preventing rapid weight gain in early childhood.

In boys, the researchers did not find consistent statistical associations between human milk feeding and puberty scores. One specific group of boys, those who received some human milk but were never exclusively fed it, showed a lower likelihood of having the highest puberty scores. The researchers noted that this finding was based on a very small number of boys who had reached advanced puberty by age 10, meaning the result is less reliable. Overall, boys tend to enter puberty later than girls, which makes early signs of maturation much harder to detect at age 10.

The study relies on observational data, meaning it cannot prove that human milk directly causes a delay in puberty. Other unmeasured lifestyle variables associated with families who breastfeed might contribute to the observed timeline. For instance, mothers with more education tend to breastfeed longer, and they might also foster habits that influence child health, such as consistent sleep routines and higher diet quality.

Additionally, infant feeding details were reported by parents months or years after the fact, which introduces the possibility of memory errors. Measuring the exact stages of puberty, particularly in boys, also involves some subjective judgment by the examining nurses.

Future studies could track actual hormone levels to see if compounds found in human milk interact with a child’s endocrine system over time. Researchers may also need to evaluate boys at older ages to capture a more complete picture of their maturation timeline.

The study, “Human Milk Feeding Practices and Pubertal Timing: Insights from the Generation XXI Population-Based Birth Cohort,” was authored by Marta Pinto da Costa, Milton Severo, Carla Lopes, Augusto Anguita-Ruiz, Joana Araújo, and Sofia Vilela.