Trends in parent-child correlations of childhood body mass index during the development of the obesity epidemic.

BACKGROUND: The intergenerational resemblance in body mass index may have increased during the development of the obesity epidemic due to changes in environment and/or expression of genetic predisposition. OBJECTIVES: This study investigates trends in intergenerational correlations of childhood body mass index (BMI; kg/m2) during the emergence of the obesity epidemic. METHODS: The study population was derived from the Copenhagen School Health Records Register, which includes height and weight measurements since birth year 1930. Mothers and fathers with BMIs available at ages 7 (n = 25,923 and n = 20,972) or 13 years (n = 26,750 and n = 21,397), respectively, were linked through the civil registration system introduced in 1968 to their children with BMIs available at age 7 years. Age- and sex-specific BMI z-scores were calculated. Correlations were estimated across eight intervals of child birth years (1952-1989) separately by sex. Trends in these correlations were examined. Whereas the mother-child correlations reflected the biological relationship, a likely decline in the assignment of non-biological fathers through the registration system across time must be considered when interpreting the father-child correlations. RESULTS: The BMI correlations between mothers and sons ranged from 0.29-0.36 and they decreased marginally, albeit significantly across time at ages 7-7 years (-0.002/year, p = 0.006), whereas those at 13-7 years remained stable (<0.0004/year, p = 0.96). Mother-daughter correlations ranged from 0.30-0.34, and they were stable at ages 7-7 years (0.0001/year, p = 0.84) and at 13-7 years (0.0004/year, p = 0.56). In contrast, father-son correlations increased significantly during this period, both at ages 7-7 (0.002/year, p = 0.007) and at ages 13-7 years (0.003/year, p