The researchers found that while maternal BMI influences birth weight, genetics accounted for most of the parent-child BMI similarities throughout childhood, challenging the idea that obesity risk is largely programmed before birth.
Study: Parental Body Mass Index and Body Size and Offspring’s Childhood Eating Behavior: A Structural Equation Modeling Analysis of the Norwegian Mother, Father, and Child Cohort Study. Image credit: NDAB Creativity/Shutterstock.com
A recent study published in the journal PLoS Medicine suggests that maternal obesity affects birth weight more strongly than childhood BMI, whose association with parental BMI is better explained by shared genetics.
Large family study explores the heritability of childhood obesity
Childhood obesity and overweight remain particularly prevalent in high-income countries. This is related to both biological and social factors, including family eating habits and physical activity. It is also familial, is associated with parental body mass index (BMI), and develops early in life, prompting interest in preventive interventions aimed at parents.
It is important to understand how traits associated with obesity are transmitted, as a direct biological effect of parental BMI on child BMI may accelerate the development of obesity in successive generations. The interplay of genetic and environmental factors in such transmission remains unclear.
Multiple studies suggest a strong influence of genetic characteristics on parent-child BMI associations, but this has not been precisely quantified. However, an alternative explanation is the developmental overnutrition hypothesis, which suggests that parental obesity affects offspring development before conception or during intrauterine life. Paternal obesity could affect offspring metabolism through direct or indirect genetic effects on sperm and seminal fluid.
The heritability of obesity also introduces genetic confounding into the association. Environmental confounding could also occur due to the high impact of shared family socioeconomic background, dietary habits, and physical activity behaviors.
The present study aimed to examine whether genetic factors explain the association between higher parental body mass index (BMI) at the time of conception and offspring birth weight, childhood BMI and eating behavior up to 8 years of age. It included up to 85,866 parent-child pairs depending on the specific analysis.
The researchers used an extensive Children of Twins (the Multiple Children of Twins and Siblings [MCoTS] structural equation model) to analyze associations between related family members. This model separates exposure–outcome associations into genetic confounding and a residual nongenetic component that may include causal effects and environmental confounding. This allows researchers to assess whether parent-child BMI associations were explained by genetic heritability rather than intrauterine or periconceptional effects of parental BMI on traits associated with obesity. If so, Reducing parental BMI alone, without changing the child’s postnatal environment, may be insufficient to substantially reduce the risk of childhood obesity.
Maternal BMI had a unique effect on birth weight
The researchers first compared how maternal and paternal BMI related to their children’s birth weight. They found that maternal BMI showed a much stronger relationship than paternal BMI, and structural equation modeling showed that this difference could not be explained by shared genetics alone.
Instead, the findings support an association between maternal BMI and fetal growth, suggesting that the biological environment during pregnancy plays a role in determining birth weight.
Childhood BMI was largely explained by genetics
The picture changed after birth. Although children with parents who had a higher BMI were themselves more likely to have a higher BMI, maternal and paternal BMI showed remarkably similar associations with childhood BMI beyond early infancy. This pattern suggests that shared genetic factors, rather than pregnancy-related effects, were driving much of the relationship.
Structural equation modeling supported this conclusion. From six months of age onward, most of the association between parent and offspring BMI was explained by genetic confounding, and this contribution remained consistently high throughout childhood. By age eight, shared genetic inheritance accounted for 79% of the association between maternal and offspring BMI and 94% of the association with paternal BMI.
Although the researchers note that some estimates may have been affected by statistical uncertainty, the overall pattern remained consistent: as children grew older, the association between parent and child BMI strengthened, while the proportion explained by shared genetics remained high.
Taken together, these findings suggest that much of the relationship between parent and child BMI reflects inherited genetic susceptibility rather than a major developmental effect of maternal obesity during pregnancy, although a smaller causal effect cannot be ruled out. Birth weight, in contrast, appears to have a different developmental origin and is more directly influenced by maternal BMI.
The authors also emphasize that these findings are statistical inferences and not definitive proof of causality. However, the results closely align with previous studies that used different analytical methods and genetic datasets, enhancing confidence in the overall conclusions.
Parental BMI was associated with obesity-related eating behaviors
The researchers also investigated whether parental BMI was associated with children’s eating behaviors at age 8. Higher maternal and paternal BMI were associated with both greater responsiveness to food and emotional overeating in the offspring, and lower emotional undereating. Fathers with a higher BMI were also more likely to have children with lower satiety responses and who ate faster. Additionally, children with a higher BMI tended to score higher on most obesity-related eating behaviors, with the exception of emotional binge eating.
Together, these findings suggest that inherited genetic susceptibility may influence childhood obesity risk, in part through its effects on appetite and eating behaviors. However, the researchers caution that their structural equation model was underpowered to distinguish genetic from environmental influences on these behaviors, meaning that this potential mechanism could not be confirmed.
The large family enhanced the genetic analysis
The study looked at a large prospective cohort that included non-twin siblings across generations and up to two children per parent. The model explored genetically explained relationships between parent-offspring pairs, as well as between cousins and between offspring and parental siblings. This ensured a more robust model.
The study also has some limitations. The model assumes that a shared environment is absent, a finding supported by previous studies and further analyzes in the current study. It does not take into account changes in gene expression in response to the environment, which could lead to an overestimation of the genetic contribution. However, the authors did not expect this to be a significant factor.
The study sample included a Norwegian population from a high-income country with a relatively high prevalence of obesity, and the authors noted the potential for selective recruitment and attrition, limiting its generalizability. Future studies in more diverse cohorts should replicate these findings.
Genetics appear to drive the greatest risk of childhood obesity
The results suggest that maternal BMI can significantly influence birth weight, but has a much smaller effect on childhood BMI. The paternal link is likely to be even smaller. Thus, reducing parental BMI before pregnancy may be insufficient in itself to prevent childhood obesity in the offspring through intrauterine mechanisms.
Long-term preventive strategies may also require non-stressful home environments that support healthy eating patterns and more physical activity, while recognizing genetic differences in obesity susceptibility among children.
However, strategies to reduce parental prepregnancy BMI may involve lasting changes in the family environment that contribute to reducing childhood obesity. Independent of childhood obesity risk, lower prepregnancy maternal BMI is associated with better pregnancy outcomes.
