Does BMI become useless as we age?

(Press-News.org) Body mass index (BMI) is key method for measuring a person’s weight status, and defining if they have normal weight, overweight, or obesity. However, new research to be presented at this year’s European Congress on Obesity (ECO 2025, Malaga, Spain, 11-14 May) shows that people with obesity at similar BMIs display significant differences in body composition in different body compartments according to their age group, such as higher body fat especially in central regions and lower muscle mass in arms and legs.

The authors suggest that people with obesity might experience a ‘redistribution’ of their body compartments (i.e. fat and muscle) as they age, in the absence of any meaningful changes in their BMI, making BMI a less useful tool. The study was conducted by researchers from the University of Rome “Tor Vergata” and University of Modena and Reggio Emilia in Italy and Beirut University in Lebanon, including Professor Marwan El Ghoch, University of Modena and Reggio Emilia, Modena, Italy.

Obesity is a major health problem characterised by an excessive accumulation of body fat (BF) that can also co-exist with a reduction in lean mass (LM). In people with obesity, little is still known about the changes in fat and lean masses across the lifespan. Thus the authors, in this new study, aimed to analyse the differences in the total and segmental body composition between age groups.

A total of 2,844 adults of both sex and a Body Mass Index (BMI) of 25 kg/m² and above referred to the Division of Clinical Nutrition at the Department of Biomedicine and Prevention at the University of Rome “Tor Vergata” underwent body composition assessments by means of Dual-energy X-ray Absorptiometry (DXA). The sample was categorised into three different age groups: ‘Young-age’ (20–39 years), ‘Middle-age’ (40–59 years), and ‘Older-age’ (60–79 years) adults that were compared with each other after being matched by body weight and BMI.

The males showed an increasing trend in total BF percentage and a decreasing total LM, from the younger to older groups, while females maintained similar values for these total compartments (total BF% and LM) across the three age groups.

However more interestingly, participants in the middle- and older-age groups of both sexes, showed higher trunk fat percentage by +1.23% to +4.21%, and lower appendicular lean mass (ALM) by –0.81 kg to –2.63 kg with respect to the young-age group, revealing higher abdominal fat and lower muscle mass in arms and legs, despite the fact that all age groups in this study were of similar BMIs (Figure 1).

Professor El Ghoch explains: “These results clearly indicate that we cannot rely only on BMI without considering the content and distribution of body composition in people with obesity across the different age groups, since those in middle- and older-age have higher central adiposity and lower muscle mass with respect to the younger group. Our findings open new directions for future research as we suggest that people with obesity might experience a sort of redistribution within their body compartments (i.e. fat and muscle) as they age, without meaningful changes in their BMI. Namely an increased fat trunk accumulation and a decrease in muscle mass in the extremities.”

The authors explain that this redistribution may have major negative health consequences – such as low-grade chronic inflammation, insulin resistance, and increase in the risk of several cardiometabolic diseases – without significant changes in BMI. Professor El Ghoch explains: “Accordingly, the use of BMI becomes useless and misleading, and future research is needed to identify new tools that are able to detect these changes in fat and muscle mass in this specific population. However, our findings need confirmation through longitudinal studies.”

Professor El Ghoch adds: “Instead of BMI, we need to rely on new simple-to-use tools that are able to detect these changes in fat mass and distribution - such as waist-to-height ratio -  as well muscle mass and strength, including the handgrip test.”

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