Genetic contributions to body mass index over adolescence and its associations with adult weight gain: a 25-year follow-up study of Finnish twins.

Introduction: High body mass index (BMI) in adolescence is a strong predictor of adult obesity. However, the nature of this association is unclear. We investigated how adolescent BMI is associated with adult weight change using longitudinal data from ages 11.5 to 37 years and examined the genetic factors behind these associations.

Data and methods: The study cohort consisted of 1400 Finnish twin individuals (40% males) with 494 complete twin pairs who reported their body mass index (BMI) at five ages: 11.5, 14, 17.5, 24, and 37 years. BMI trajectories (defined as BMI changes (i.e., slope) and BMI at baseline age (i.e., intercept)) were calculated in adulthood (from 17.5 to 37 years of age) using linear mixed-effects models. Polygenic Risk Scores of BMI (PRS_BMI) and genetic twin models were utilised to analyse the role of genetic factors underlying BMI trajectories and their associations with BMI at 11.5 and 14 years of age.

Results: Mean BMI increased in adulthood (4.06 kg/m² in men and 3.39 kg/m² in women). The BMI changes correlated with BMI at the baseline age of 17.5 years (i.e. intercept) (r = 0.24 in men and r = 0.35 in women) as well as with BMI in adolescence (11.5 and 14 years of age). Genetic factors contributed to the BMI changes during adulthood (correlation with PRS_BMI r = 0.25 in men and r = 0.27 in women; heritability estimates 0.63 and 0.64 respectively) as well as to their correlations with BMI at the baseline age (r_A = 0.5 in men and 0.54 in women) and BMI during adolescence (at 11.5 and 14 years of age) (r_A = 0.63-0.64).

Conclusion: We found that genetic factors play a role in BMI change in adulthood, and part of this genetic component overlaps with the genetics of BMI in adolescence. Genetic predisposition to high BMI in adolescence is also related to adult weight gain.