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Association between trajectories of body mass index and changes in fat free mass index and fat mass index in a cohort of European children. The IDEFICS/I.Family studies

European Journal of Clinical Nutrition (2026)Cite this article

Subjects

Abstract

Objective

To examine the associations between BMI trajectories and changes in fat-free mass index (FFMI) and fat mass index (FMI), assessed via bioelectrical impedance analysis (BIA) and skinfold thickness (ST).

Methods

The study included 4708 European children with BIA data and 3627 with ST data, aged 2–9.9 years at baseline, participating in two waves of the IDEFICS (IDEFICS (Identification and Prevention of Dietary- and Lifestyle-Induced Health Effects in Children and Infants))/I.Family (I. Family (Determinants of eating behavior in European children, adolescents, and their parents)) studies (w0: 2007/08; w2: 2013/14). Children were classified into three BMI trajectory groups: retained normal weight (RNW), remained overweight/obese (ROO), and excessive weight gain (EWG). Analyses of covariance evaluated differences in BMI z-score changes, FFMI, and FMI across groups. Mixed-effects linear regression assessed associations between BMI z-score changes and FFMI/FMI over time.

Results

BMI z-score changes were more strongly associated with FMI (β = 1.16–1.70 in boys; 1.17–1.62 in girls, p < 0.05) than with FFMI, for both BIA and ST. In ROO and EWG groups, associations with FMI were consistently stronger, except among girls <6 years in the ROO group (BIA). In the RNW group, boys >6 years showed stronger associations with FFMI.

Conclusions

Associations between BMI changes and FFMI/FMI vary by BMI trajectory and body composition method. BIA showed closer alignment with BMI changes than ST. These findings suggest that BMI alone may not adequately capture changes in specific body compartments. While convenient, BMI should be interpreted with caution, especially when fat or fat-free mass plays a differential role in disease risk or health outcomes.

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Fig. 1: Flow chart of the population included in the study.
Fig. 2: Mean change of FFMI and FMI during six years (time span between W0 toW2 follow-up surveys) in boys and girls estimated by bioelectrical impedance.
Fig. 3: Mean change of FFMI and FMI during six years (time span between W0 toW2 follow-up surveys) in boys and girls estimated by skinfold thickness.

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Data availability

Due to the sensitive nature of data collected from children and adolescents, ethical restrictions prohibit the authors from making the minimal data set publicly available. Each cohort center received approval of the corresponding local Ethical Commission. Data are available on request, and all requests need approval by the study’s Steering Committee. Interested researchers can contact the study coordinator (ahrens@leibniz-bips.de or i.family@leibnizbips.de) to request data access. All requests for accessing data of the DEFIES/I. Family cohort are discussed on a case-by-case basis by the Steering Committee.

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Acknowledgements

This work was done as part of the IDEFICS (http://www.idefics.eu) and I.Family studies (http://www.ifamilystudy.eu/.) We gratefully acknowledge the financial support of the European Commission within the Sixth RTD Framework Programme Contract No. 016181 (FOOD), and the Seventh RTD Framework Programme Contract No. 266044. This analysis was also supported by the Spanish Ministry of Science and Innovation (JCI-2010-07055) with the contribution of the European Regional Development Fund (FEDER). The second author (IIA) was funded by a RICORS from Carlos III Institute “Primary Care Interventions to prevent maternal and child chronic diseases of perinatal and developmental origin” Ref: RD21/0012. The first author (LCE), was supported by CONACYT (Consejo Nacional de Ciencia y Tecnología) with a scholarship for a research stay at the University of Zaragoza.

Author information

Author notes

  1. These authors contributed equally: Elizabeth Lizárraga, Iris Iglesia-Altaba.

Authors and Affiliations

  1. Medical Faculty. University of Colima. Doctorate in Medical Sciences, Colima, Mexico

    Elizabeth Lizárraga

  2. GENUD (Growth, Exercise, Nutrition and Development) Research Group, Faculty of Health Sciences, Agri-food Institute of Aragón (IA2), University of Zaragoza, Zaragoza, Spain

    Iris Iglesia-Altaba & Luis A. Moreno

  3. Instituto de Investigación Sanitaria Aragón (IIS Aragón), Zaragoza, Spain

    Iris Iglesia-Altaba & Luis A. Moreno

  4. “Primary Care Interventions to prevent maternal and child chronic diseases of perinatal and developmental origin” Ref: RD21/0012/0012 and RD24/0013/0022 founded by Carlos III Health Institute (ISCIII), Madrid, Spain

    Iris Iglesia-Altaba

  5. Faculty of Health Sciences, Universidad Internacional de La Rioja, Logroño, La Rioja, Spain

    Iris Iglesia-Altaba

  6. Department of Preventive Medicine and Public Health, University of Zaragoza, Zaragoza, Spain

    Javier Santabárbara

  7. Leibniz Institute for Prevention Research and Epidemiology - BIPS, Bremen, Germany

    Wolfgang Ahrens & Claudia Börnhorst

  8. University Bremen. Faculty of Mathematics and Computer Science, Institute of Statistics, Bremen, Germany

    Wolfgang Ahrens

  9. Institute of Food Sciences, Unit of Epidemiology and Population Genetics, National Research Council (ISA-CNR), Avellino, Italy

    Annarita Formisano

  10. National Institute for Health Development, Tervise Arengu Instituut Tallinn, Tallinn, Estonia

    Toomas Veidebaum

  11. Research and Education Institute of Child Health, Strovolos, Cyprus

    Michael Tornaritis

  12. Department of Public Health/ Department of Movement and Sport Sciences, Faculty of Medicine and Health Sciences, University Gent, Gent, Belgium

    Stefaan De Henauw

  13. School of Health Sciences, University of Skövde, Skövde, Sweden

    Gabriele Eiben

  14. School of Public Health & Community Medicine, Institute of Medicine. The Sahlgrenska Academy at University of Gothenburg, Gothenburg, Sweden

    Monica Hunsberger

  15. Medical Faculty, Department of Pediatrics, University Pécs, Pécs, Hungary

    Dénes Molnár

  16. Centro de Investigación Biomédica en Red de Fisiopatología de la Obesidad y la Nutrición (CIBEROBN), Instituto Carlos III, Madrid, Spain

    Luis A. Moreno

Authors
  1. Elizabeth Lizárraga
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  2. Iris Iglesia-Altaba
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Consortia

on behalf of IDEFICS and I.Family consortia

  • Iris Iglesia-Altaba
  • , Wolfgang Ahrens
  • , Toomas Veidebaum
  • , Michael Tornaritis
  • , Stefaan De Henauw
  • , Dénes Molnár
  •  & Luis A. Moreno

Contributions

ELC, IIA, and LAM were responsible for the conceptualization. ELC and IIA wrote the original draft. The formal analysis was conducted by ELC. JS and CB gave advise on the statistical methods. IIA, WA, TV, MT, SDH, GE, MH, DM, AF and LAM were responsible for the acquisition of data. All authors critically revised the work for important intellectual content and approved the version to be published and have agreed to be accountable for the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

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Correspondence to Luis A. Moreno.

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Lizárraga, E., Iglesia-Altaba, I., Santabárbara, J. et al. Association between trajectories of body mass index and changes in fat free mass index and fat mass index in a cohort of European children. The IDEFICS/I.Family studies. Eur J Clin Nutr (2026). https://doi.org/10.1038/s41430-026-01706-5

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