Background: Early growth and body composition may influence risk of obesity and health in adulthood. Few studies have examined how undernutrition is associated with body composition in early life.

Objective: We assessed stunting and wasting as correlates of body composition in young Kenyan children.

Methods: Nested in a randomized controlled nutrition trial, this longitudinal study assessed fat and fat-free mass (FM, FFM) using deuterium dilution technique among children at age 6 and 15 months. Cross-sectional and longitudinal associations between z-score categories of length-for-age Z (LAZ) or weight-for-length Z (WLZ) and FM, FFM, fat mass index (FMI), fat-free mass index (FFMI), triceps and subscapular skinfolds were analyzed by linear mixed models.

Results: Among 499 children enrolled, breastfeeding declined from 99% to 87%, stunting increased from 13% to 32% and wasting remained at 2-3% between 6 and 15 months. Compared to LAZ >0, stunted children had 1.12 (95% CI: 0.88;1.36, P <0.001) kg lower FFM at 6 months increasing to 1.59 (95% CI: 1.25;1.94, P <0.001) kg at 15 months, corresponding to differences of 18% and 17%, respectively. When analysing FFMI, the deficit in FFM tended to be less than proportional to children's height at 6 months (P ≤0.060), but not at 15 months (P >0.40). Stunting was associated with 0.28 (95% CI: 0.09;0.47, P =0.004) kg lower FM at 6 months. The difference lost significance at 15 months, and stunting was not associated with FMI at any time point. Lower WLZ was generally associated with lower FM, FFM, FMI and FFMI at 6 and 15 months. Differences in FFM, but not FM, increased with time, while FFMI differences did not change and FMI differences generally decreased with time.

Conclusions: Overall, low LAZ and WHZ among young Kenyan children is associated with reduced lean tissue, which may have long-term health consequences.

Trial registration: ISRCTN30012997.