Normative values for body composition in 22,191 healthy Norwegian adults 20–99 years: The HUNT4 study

Background: Body mass, body mass index (BMI), and body composition components are essential for health and longevity. Considering the influence of demographic factors on body composition, there is a need for tailored reference values based on age-, sex-, and geography. We aimed to construct a comprehensive reference material on body composition in healthy Norwegian adults. Methods: In this cross-sectional study, we estimated age- and sex-specific reference values for body-, fat-, and muscle mass variables using multi-frequency bioelectrial impedance analysis (such as body fat percentage, skeletal muscle mass and visceral fat area) in 22,191 healthy adults aged 20–99 years participating in the Trøndelag Health Study 4 (HUNT4). We calculated the fat mass and skeletal muscle mass index as the total fat and muscle mass relative to height squared and used general linear models to explore the associations between physical activity (PA), BMI, and age. Results: With a BMI (kg/m²) of 25.4 (SD 5.1) and 26.0 (4.5) for women and men, respectively, the youngest age group (20–39 yrs) had a lower BMI compared to their counterparts aged 40–59 years (26.3 [4.5] and 27.5 [3.8]) and ≥ 60 years (25.7 [4.1] and 26.5 [3.4]), respectively. Those aged 20–39 years also had the lowest values for the different body fat variables measured. Fat mass index (kg/m²) was 8.41 (4.00) and 5.81 (3.29) for women and men aged 20–39 years, respectively, compared to 9.25 (3.21) and 6.86 (2.46) for those aged ≥60 years. The oldest age group had the lowest values for the various muscle mass variables; women and men aged 60+ years had a skeletal muscle mass index (kg/m²) of 8.91 (0.85) and 10.96 (1.00), respectively. Corresponding values for those aged 20–39 years were 9.33 (0.97) and 11.49 (1.15). For all age groups and both sexes, regular physical activity was associated with lower levels of fat mass, whereas the association between muscle mass and PAwas less conclusive. When using body fat percentage as an obesity measure, we observed a much higher obesity prevalence (41.2%) in the study population compared to BMI (17.3%). Conclusions: Our study offers a comprehensive reference for body composition among healthy adults in Norway, aiding the identification of abnormal fat and muscle mass values across age groups. We also highlight that BMI often misclassifies individuals with adiposity levels in the overweight or obese category as lean. Therefore, incorporating body composition when defining obesity could enable early intervention to prevent cardiometabolic diseases.