A new study suggests that infants who receive only breast milk for their first six months tend to have fewer symptoms of attention deficit hyperactivity disorder during early childhood. Research indicates that this protective effect persists even when taking into account the child’s genetic predisposition to this disease. These results 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. Behavioral traits associated with the disease exist across a broad spectrum in the general population, with clinical diagnoses made to individuals with the most extreme symptoms. The disease 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 this condition. At the same time, environmental factors before and after birth also play a role in brain development. This means that a person’s environment, early experiences, and diet can interact with their genetic fingerprint over time.
Breast milk is the primary food source for many infants and contains components such as 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 that an infant does not consume any solid foods, commercial infant formula, or even water during this initial period. Previous research has linked breastfeeding to a lower chance of being diagnosed with ADHD later in life.
It has been difficult to determine whether this relationship is a direct biological result of breast milk or whether it arises from other family lifestyle variables. 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 may find breastfeeding more difficult, and their children will inherit a higher genetic risk of developing the disorder. To separate the specific effects of infant feeding from these common genetic and environmental traits, scientists needed a large data set that could follow families over many years. A team of researchers led by Berit Skretting Solberg of the University of Bergen in Norway set out to examine this relationship.
Researchers analyzed data from a large study that followed 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 completed questionnaires detailing their feeding practices. They specifically noted how many months the child received full breastfeeding, which the researchers defined as breast milk without formula or solid foods.
As the children got older, their parents conducted behavioral assessments to track symptoms of hyperactivity, impulsivity and inattention at ages three, five and eight. The researchers also mapped the children’s genetic profiles to calculate a polygenic risk score for ADHD. A polygenic risk score is a number that estimates a person’s genetic probability 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 as well as the child’s genetic data. They also adjusted the statistical models to take into account the mother’s education level, the parents’ age at the time of birth, the child’s birth weight and the season of birth. The addition of these socio-economic and perinatal factors provided a more precise view of the relationship between early feeding and behavioral development.
Even when accounting for whole-family genetic risks and various socioeconomic factors, the association remained consistent. Infants who were fully breastfed for long periods continued to have fewer ADHD symptoms during childhood. The researchers noted that shorter durations of full breastfeeding, or diets relying on partial breastfeeding, were consistently linked to higher symptom scores. The size of the effect declined slightly as children reached age eight, but the relationship remained present in the data.
The scientists conducted a third analysis comparing 8,209 sibling pairs within the dataset. By examining children born to the same mother, they could explain unmeasured environmental and genetic factors that remain constant within a specific household. They focused on more than 5,000 sibling pairs where the mother exclusively breastfed one child for a different length of time than the other child. The researchers then compared the siblings’ behavioral symptom scores as they grew up.
The sibling comparison produced results that closely mirrored the main analysis. Siblings who received full breastfeeding for a longer duration tend to have fewer ADHD symptoms than their sibling who received less breast milk. This specific finding suggests that breast milk itself, or the act of breastfeeding, may exert a direct biological or psychological influence on behavioral development. However, when sibling data were analyzed separately by sex, the effects of breastfeeding were attenuated.
Although the data indicate a potential biological benefit from breast milk, the observed reductions in ADHD symptoms were small at the individual level. The researchers point out that these changes might not translate into clinically observable changes for a specific child. The effect might only become apparent by looking at broad demographic trends, where a slight overall reduction in symptoms could mean fewer children cross the threshold for a clinical diagnosis. The study relied on parent-written questionnaires rather than formal assessments by a doctor, which may introduce personal bias into symptom scores.
As 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 account for this missing data, and their results were not significantly changed, but self-selection bias remains a possibility in large health registries. The study was also observational, meaning it followed natural behaviors rather than assigning diets in a controlled laboratory. Although genetic and sibling analyzes help rule out many external factors, researchers cannot entirely eliminate the possibility that other hidden variables influence the results.
Future research should explore the exact biological mechanisms that might explain how breast milk influences brain development. 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. The scientists also plan to study how the skin-to-skin physical contact necessary for breastfeeding might influence infant attachment security and long-term emotional regulation.
The study, “Breastfeeding and the Development of Attention-Deficit/Hyperactivity Disorder Symptoms in 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.
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