Hormonal contraceptives and body composition by use of stable isotope dilution techniques among women of reproductive age in Nyeri County, Kenya

Background: Hormonal contraceptives are widely used by women of reproductive age (WRA) globally, yet their impact on body composition remains poorly understood, particularly in low-resource settings. This study aimed to assess the effects of hormonal contraceptives on body composition among WRA in Nyeri County, Kenya, using Stable Isotope Dilution Techniques (SIDT). Methods: A two-stage cross-sectional analytical study was conducted among 119 women of reproductive age (WRA) aged 18–49 years. However, five participants were excluded from the final analysis. Participants were purposively grouped into hormonal contraceptive users (n = 41), non-hormonal contraceptive users (n = 37), and not on contraceptive (n = 36). After accounting for loss to follow-up, 114 were analysed at baseline and 104 at endline. Contraceptive types included injectables (DMPA), oral pills, and implants (hormonal); and copper IUDs and condoms (non-hormonal). A mid-point (3-month) assessment was conducted for compliance, with the primary body composition outcome measured at six months. Demographic and socioeconomic data were collected using validated structured questionnaires. Body composition was assessed at baseline and six months later using SIDT, with saliva samples analysed to determine total body water (TBW), fat-free mass (FFM), and fat mass (FM). Physical activity (GPAQ) and dietary diversity (WDDS) were measured to control for confounding factors. Logistic regression analysis was adjusted for physical activity, and dietary diversity. Results: Hormonal contraceptive users showed a significant increase in body fat percentage from 35.3 ± 9.3% to 41.7 ± 10.6% (p = 0.039) and a reduction in %TBW (47.4 ± 6.8% to 42.3 ± 7.8%, p = 0.048). Non-hormonal and non-contraceptive users showed no significant changes. Regression analysis confirmed hormonal contraceptive use was a significant predictor of body fat percentage (Exp(β) = 0.31, p = 0.026), independent of other factors. Conclusion: Hormonal contraceptive use is associated with increased body fat and reduced total body water among WRA in Nyeri County. These changes, independent of physical activity and diet, suggest a direct metabolic effect. Regular body composition monitoring and tailored nutritional guidance are recommended for women using hormonal contraceptives.