ORIGINAL ARTICLE Adiposity rebound is misclassified by BMI rebound S Plachta-Danielzik 1 , A Bosy-Westphal 1,2 , B Kehden 1 , MI Gehrke 1 , K Kromeyer-Hauschild 3 , M Grillenberger 4 , C Willho¨ ft 4 , SB Heymsfield 5 and MJ Mu¨ ller 1 BACKGROUND/OBJECTIVES: Adiposity rebound (AR) is defined as the nadir or the inflexion point of body mass index (BMI) percentiles between the age of 3 and 7 years. An early rebound is seen as a risk of obesity and, thus, AR is considered as a suitable time period for prevention. As BMI does not reflect body composition, we aimed to examine the rebounds of fat mass index (FMI) and fat-free mass index (FFMI) together with BMI. SUBJECTS/METHODS: Cross-sectional data of 19 264 children aged 3–11 years were pooled from three German studies (Kiel Obesity Prevention Study, the project ‘Better diet. More exercise. KINDERLEICHT-REGIONS’ and regular examinations of Jena children). Height and weight were measured. Fat mass (FM) and fat-free mass (FFM) were obtained from bioelectrical impedance analysis and analysed using a population-specific algorithm. Percentiles of BMI, FMI and FFMI were constructed by the LMS method. RESULTS: Both BMI and FMI percentiles showed a rebound, whereas FFMI percentiles steadily increased with age. On P90, FMI rebound was about 1.6–1.8 years later compared with that of BMI, that is, at ages 4.2 years (BMI) and 5.8 years (FMI) in boys and at 4.2 years (BMI) and 6.0 years (FMI) in girls. At AR, the slope of the BMI-P90 was explained by increases in FFMI rather than FMI. By contrast, at FMI rebound, the slope of BMI was strongly related to FMI. CONCLUSIONS: BMI rebound does not equal the rebound of FM. At AR, the slope in BMI is determined by the increase in FFMI. AR should be defined as FMI rebound rather than BMI rebound. European Journal of Clinical Nutrition (2013) 67, 984–989; doi:10.1038/ejcn.2013.131; published online 17 July 2013 Keywords: BMI; fat mass index; adiposity rebound; obesity INTRODUCTION Following body mass index (BMI) percentiles, BMI declines after infancy and then there is a second rise in BMI between the age of 3 and 7 years until adulthood. 1 Adiposity rebound (AR) is defined as the nadir or the inflexion point of BMI percentiles with age. AR was first described in the 1980s by Rolland-Cachera et al., 2 who found a relationship between the age at AR and later manifestation of adiposity. Children with an early AR had a higher risk to become obese adolescents 2 and/or adults. 3,4 Inter- individual differences in BMI during AR were due to weight gain rather than height velocity. 5 Thus, time at AR is seen as a critical period for higher than average weight gain and the development of obesity. It is assumed that both the size and the number of adipocytes increase and, thus, FM begins to accumulate at age of AR. 2,3 However, there are only a few detailed studies on changes in body composition at AR that investigated this idea, 5,6 and no unequivocal results were found. Contrary to the original assumption, AR was characterized by an increase in fat-free mass (FFM) rather than fat mass (FM). 6 By contrast, other studies indicated that differences in FM acquisition determine changes in BMI during the time of AR. 7,8 In addition, sex differences in AR-related changes in body composition have been proposed: BMI differentials between early and late AR were found to be due to increased deposition of FM in girls but of FFM in boys. 5 Thus, it is still unclear whether an early AR reflects an increased fat or lean mass. 4 These studies had a longitudinal design with regular assessments of body composition but were based on limited data sets of 40–458 children. In this study, we aimed to assess body composition in a large database to characterize BMI, fat mass index (FMI) and fat-free mass index (FFMI) during the time when AR occurs. SUBJECTS AND METHODS Study population A representative group of 19 264 (9777 boys and 9487 girls) children aged 3–11 years was analysed, and data were pooled from three German studies (Kiel Obesity Prevention Study, 9,10 the German-wide project ‘Better diet. More exercise. KINDERLEICHT-REGIONS’ and regular examinations of Jena children 11 ). Details of the study design and recruitment procedure have been described recently. 12 Anthropometry Anthropometric measurements were performed by trained staff following standard procedures. Body weight was measured to the nearest 0.1 kg 1 Institute of Human Nutrition and Food Science, Christian-Albrechts University, Kiel, Germany; 2 Institute of Nutritional Medicine, University of Hohenheim, Stuttgart, Germany; 3 Institute of Human Genetics, University Hospital — Friedrich Schiller University Jena, Jena, Germany; 4 Federal Research Institute for Nutrition and Food, Max Rubner-Institut (MRI), Karlsruhe, Germany and 5 Pennington Biomedical Research Center, Baton Rouge, LA, USA. Correspondence: Dr MJ Mu¨ ller, Institut fu¨ r Humanerna¨hrung und Lebensmittelkunde, Christian-Albrechts Universita¨t zu Kiel, Du¨ sternbrooker Weg 17-19, D-24105 Kiel, Germany. E-mail: mmueller@nutrfoodsc.uni-kiel.de Contributors: SPD, ABW and MJM had the original idea; SPD, BK and MIG did the statistical analyses. SPD interpreted the data and wrote the final draft of the paper. CW and MG provided data of the project ‘Better diet. More exercise. KINDERLEICHT-REGIONS’ and contributed to the final draft of the paper. KKH provided data of the examination of Jena children and contributed to the final draft of the paper. SBH discussed the data and contributed to the final draft of the paper. MJM supervised the study, interpreted the data and wrote the final draft of the paper. All authors discussed the data and approved the final version of the paper. Guarantor: MJ Mu¨ ller. Received 2 November 2012; revised 31 May 2013; accepted 14 June 2013; published online 17 July 2013 European Journal of Clinical Nutrition (2013) 67, 984–989 & 2013 Macmillan Publishers Limited All rights reserved 0954-3007/13 www.nature.com/ejcn