Differences in the relationship of weight to height, and thus the meaning of BMI, according to age, sex, and birth year cohort
journal contributionposted on 13.02.2020 by Will Johnson, Tom Norris, David Bann, Noel Cameron, Jonathan K Wells, Tim J Cole, Rebecca Hardy
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Objective: Weight can be adjusted for height using the Benn parameter (kg/mB), where B is the power that minimises the correlation with height. We investigated how the Benn parameter changes across age (10-65 years) and time (1956 to 2015) and differs between sexes.
Methods: The sample comprised 49,717 individuals born in 1946, 1958, 1970, or 2001. Cross-sectional estimates of the Benn parameter were produced and cohort differences at ages 10/11 and 42/43 years were examined using linear regression. Multilevel modelling was used to develop trajectories showing how the Benn parameter changed over age from childhood to mid-adulthood in the three older cohorts.
Results: The Benn parameter was closest to 2 in childhood but consistently lower across adulthood, particularly in females and the most recent cohort. At ages 10/11 years, the Benn parameter was greater than 3 in both sexes in the 2001 cohort but between 2.2 and 2.7 in the three older cohorts. This difference was estimated to be +0.67 (0.53, 0.81) in males and +0.53 (0.38, 0.68) in females, compared to the 1946 cohort, and was driven by a much higher weight SD in the 2001 cohort. Conversely, at ages 42/43 years, the Benn parameter was lowest in the 1970 cohort due to a slightly lower weight-height correlation. This difference was estimated to be -0.12 (-0.34, 0.10) in males and -0.15 (-0.42, 0.13) in females, compared to the 1946 cohort.
Conclusions: Changes over time in the obesogenic environment appear to have firstly reduced the Benn parameter due to a lowering of the weight-height correlation but secondly and more drastically increased the Benn parameter due to increasing weight variation.
Body size trajectories and cardio-metabolic resilience to obesity in three UK birth cohorts : MR/P023347/1
National Institute for Health Research (NIHR) Leicester Biomedical Research Centre, which is a partnership between University Hospitals of Leicester NHS Trust, Loughborough University, and the University of Leicester
Economic and Social Research Council (ESRC)
UK Medical Research Council
ESRC (award reference: ES/K000357/1)
UK Medical Research Council (Programme code: MC_UU_12019/2)
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