Assortative weight gain in mother-daughter and father-son pairs: an emerging source of childhood obesity. Longitudinal study of trios (EarlyBird 43).

OBJECTIVE: To look for same-sex (gender assortative) association of body mass index (BMI) in healthy trios (mother, father and child) from a contemporary birth cohort, which might imply shared environment rather than shared genes because selective mother-daughter and father-son gene transmission is not a common Mendelian trait.

DESIGN: Prospective (longitudinal) cohort study with four annual time points, from 5 to 8 years.

SUBJECTS: 226 healthy trios from a 1995 to 1996 birth cohort randomly selected in the city of Plymouth, UK.

MEASUREMENTS: Average BMI of the two parents and maternal/paternal BMI separately related to the BMI-SDS (standard deviation score) of all offspring and to the BMI-SDS of the sons and the daughters separately.

RESULTS: There were big differences in BMI-SDS among the daughters grouped according to mothers' category of BMI (effect size 1.37 SDS), but not their sons (effect size 0.16 SDS, gender interaction P<0.004), and among the sons grouped according to their fathers' BMI (effect size 1.28 SDS), but not their daughters (effect size 0.17, gender interaction P=0.02). Children whose same-sex parents were of normal weight, weighed either close to (girls+0.20 BMI-SDS) or less than (boys,-0.34 BMI-SDS) children of 20 years ago, and did not change from 5 to 8 years. In contrast, the risks of obesity at 8 years were 10-fold greater (girls 41%, P<0.001) or sixfold greater (boys 18%, P<0.05) if the same-sex parent was obese. Longitudinal linear mixed effects (multilevel) modelling showed a marked influence of maternal and paternal BMI on the rate of weight gain, which was unaffected by birth weight of the child. We report perhaps the largest effect sizes so far recorded in childhood obesity.

CONCLUSIONS: Childhood obesity today seems to be largely confined to those whose same-sex parents are obese, and the link does not seem to be genetic. Parental obesity, like smoking, might be targeted in the interests of the child.