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A new study suggests that infants who receive only breast milk for their first six months tend to show fewer symptoms of attention deficit hyperactivity disorder in early childhood. The research indicates that this protective effect holds up even when accounting for a child’s genetic predisposition to the condition. These findings were published in the journal Biological Psychiatry.
Attention deficit hyperactivity disorder, commonly known as ADHD, is a condition involving patterns of hyperactivity, impulsivity, and inattention that interfere with daily functioning. The behavioral traits associated with the condition exist on a broad spectrum in the general population, with clinical diagnoses given to those experiencing the most extreme symptoms. The disorder often begins in childhood and has a strong genetic component.
Research involving twins and families estimates that genetics account for the vast majority of the risk of developing the disorder. At the same time, environmental factors before and after birth also play a role in how the brain develops. This means that a person’s surroundings, early experiences, and diet can interact with their genetic blueprint over time.
Human breast milk provides a primary food source for many infants and contains components like fatty acids, immune cells, and nutrients that support neurodevelopment. The World Health Organization recommends exclusive breastfeeding for the first six months of life to maximize these health benefits. This strict definition means an infant consumes no solid foods, commercial infant formula, or even water during that initial period. Previous research has linked infant breastfeeding to a lower chance of receiving an ADHD diagnosis later in life.
It has been difficult to determine whether this relationship is a direct biological result of breast milk or if it stems from other variables in a family’s lifestyle. Mothers who breastfeed longer often have different socioeconomic backgrounds and health behaviors than those who do not. Parents with high levels of ADHD traits themselves might find breastfeeding more challenging, and their children inherit a higher genetic likelihood for the disorder. To separate the specific effects of infant feeding from these shared genetic and environmental traits, scientists needed a large dataset that tracked families over many years. A team of researchers led by Berit Skretting Solberg at the University of Bergen in Norway set out to examine this relationship.
The researchers analyzed data from a large study that tracked thousands of Norwegian families from pregnancy through childhood. The initial analysis included 37,643 children born between 1999 and 2009. When the infants were six months old, mothers filled out questionnaires detailing their feeding practices. They specifically noted how many months the child received full breastfeeding, which the researchers defined as providing breast milk without any formula or solid foods.
As the children aged, their parents completed behavioral assessments to track symptoms of hyperactivity, impulsivity, and inattention at ages three, five, and eight. The researchers also mapped the genetic profiles of the children to calculate a polygenic risk score for ADHD. A polygenic risk score is a number that estimates a person’s genetic likelihood for a trait based on thousands of tiny variations in their DNA. The researchers found that each additional month of full breastfeeding was associated with a measurable reduction in ADHD symptoms at all three ages.
To refine their analysis, the research team looked at a smaller subgroup of 18,349 mother-father-child units. In this step, the scientists included the polygenic risk scores of both parents alongside the child’s genetic data. They also adjusted the statistical models to account for the mother’s education level, the parents’ ages at the time of birth, the child’s birth weight, and the season of birth. Adding these socioeconomic and perinatal factors provided a more precise view of the relationship between early diet and behavioral development.
Even with the genetic risks of the entire family and various socioeconomic factors factored in, the association remained consistent. Infants who were fully breastfed for longer periods continued to show fewer ADHD symptoms in childhood. The researchers noted that shorter durations of full breastfeeding, or feeding regimens relying on partial breastfeeding, were consistently linked to higher symptom scores. The effect size decreased slightly as the children reached age eight, but the relationship remained present in the data.
The scientists conducted a third analysis comparing 8,209 pairs of siblings within the dataset. By looking at children born to the same mother, they could account for unmeasured environmental and genetic factors that remain constant within a specific household. They focused on over 5,000 sibling pairs where the mother breastfed one child exclusively for a different length of time than the other child. The researchers then compared the behavioral symptom scores of the siblings as they grew up.
The sibling comparison produced results that closely mirrored the main analysis. The sibling who received full breastfeeding for a longer duration tended to exhibit fewer ADHD symptoms than their brother or sister who received less breast milk. This specific finding suggests that breast milk itself, or the act of breastfeeding, might exert a direct biological or psychological influence on behavioral development. However, when the sibling data were analyzed separately by sex, the effects of breastfeeding were attenuated.
While the data points to a potential biological benefit of breast milk, the observed reductions in ADHD symptoms were small on an individual level. The researchers emphasize that these shifts might not translate into clinically observable changes for a specific child. The effect might only become apparent when looking at broad population trends, where a slight overall reduction in symptoms could mean fewer children cross the threshold into a clinical diagnosis. The study relied on parent-reported questionnaires rather than formal evaluations by a doctor, which can introduce personal bias into the symptom scores.
Because participation in the long-term study was voluntary, the sample experienced some loss to follow-up over the years. The research team used statistical techniques to adjust for this missing data, and their results were not substantially altered, but self-selection bias remains a possibility in large health registries. The study was also observational, meaning it tracked natural behaviors rather than assigning diets in a controlled laboratory setting. While the genetic and sibling analyses help rule out many external factors, researchers cannot entirely eliminate the possibility of other hidden variables influencing the outcomes.
Future research will need to explore the exact biological mechanisms that might explain how breast milk influences the developing brain. Human breast milk contains unique components like tryptophan, an amino acid that breaks down in the gut and helps regulate chemical signaling between the digestive system and the central nervous system. Milk also contains specialized complex sugars that promote healthy bacteria, which have been shown to improve working memory and attention in animal models. Scientists also plan to study how the physical skin-to-skin contact required for nursing might influence infant attachment security and long-term emotional regulation.
The study, “Breastfeeding and Development of Attention-Deficit/Hyperactivity Disorder Symptoms Across Childhood,” was authored by Berit Skretting Solberg, Anne Lise Brantsæter, Liv Grimstvedt Kvalvik, Catharina A. Hartman, Tian Xie, Kari Klungsøyr, Lin Li, Henrik Larsson, Rolf Gjestad, and Jan Haavik.