A Within-Sibling Pair Analysis of Lifestyle Behaviors and BMI Z-Score in the Multi-center I.Family Cohort Study

Objectives: The aim was to investigate associations between lifestyle behaviors and BMI z-scores after controlling for potential familial confounding by comparing sibling pairs.

Methods: Within the European I.Family cohort, we conducted a cross-sectional analysis of 2 subsets of children: 1) among all same-sex sibling pairs with a maximal age difference of four years (n=1404, mean age 10.7 years) and 2) among overweight-discordant sibling pairs (defined as one overweight or obese sibling with a non-overweight sibling and a BMI difference of at least 10 percentiles) (n=304, mean age 11.3 years). Physical activity was measured by accelerometry and dietary intake by 24-hour dietary recalls. Sedentary behaviors and potential covariates were assessed by questionnaires filled in by the children or their parents. Mixed linear regression was used to examine the associations between sibling differences (?) in lifestyle behaviors and ?BMI z-scores adjusting for ?height, ?age and sex, and the correlations between children within families and countries.

Results: Among all sibling pairs, the siblings with higher screen time, lower MVPA, less sleep and higher intake of energy and unhealthy foods showed higher BMI z-scores. Within the overweight-discordant pairs, ?screen time and ?MVPA were significantly related to ?BMI z-scores. Of the overweight siblings, 37% were on a weight-loss diet, while the corresponding number in non-overweight siblings was only 12%.

Conclusion: Lifestyle behaviors explain part of BMI z-score differences among siblings raised in the same household. It remains to be explored whether genetic variants distinguish overweight-discordant siblings and whether these genes influence food intake or energy expenditure behaviors. This work was done as part of the I.Family Study (http://www.ifamilystudy.eu/). We gratefully acknowledge the financial support of the European Community within the Seventh RTD Framework Programme Contract No. 266044.