Application of stable isotope dilution techniques to assess body fat and comparison with WHO BMI-for-age classification as a measure of obesity among schoolchildren in Nairobi, Kenya

Objective: WHO BMI-for-age z score (BAZ) is widely used in epidemiology, yet it does not distinguish body fat-free mass and fat mass which are better indicators of obesity and related risks. The stable isotope dilution techniques (SIDT) are gold standard methods of assessing body composition. Main objective was to assess significant differences in measurement and validity of WHO BMI-for-age classification for defining childhood obesity by comparing with body fatness using SIDT among schoolchildren. Design: A cross-sectional analytical study. A questionnaire, anthropometry and body composition data were used. SPSS was used to analyse data at P < 0 center dot 05 at 95 % CI. Setting: Primary schools in Nairobi City County, Kenya Participants: One hundred seventy-nine schoolchildren aged 8-11 years were randomly sampled. Results: Prevalence of adiposity by reference SIDT (24 center dot 0 %) was significantly higher than that of obesity by BAZ > 2 sd (2 center dot 8 %) (Wilcoxon test, P < 0 center dot 05). Concordance coefficient between SIDT and BAZ > 2 sd in diagnosing obesity was poor (kappa = 0 center dot 167). Only 11 center dot 6 % of children with excess body fat were correctly diagnosed as obese by BAZ > 2 sd. The use of BAZ > 1 sd for overweight and obesity showed fair concordance coefficient (kappa = 0 center dot 409, P < 0 center dot 001) with 32 center dot 5% of children with excess fat positively identified as overweight and obese. Conclusion: WHO BMI-for-age cut-off points severely underestimate the prevalence of overweight and obesity compared with body composition assessment by stable isotope dilution techniques. Evidence-informed interventions should be based on more accurate estimates of overweight and obesity than that can be provided by BAZ.