As global populations age and the prevalence of obesity and metabolic disorders rises, understanding the complex relationships between body mass index (BMI), metabolic health, and aging becomes increasingly critical.

In a new thesis from Karolinska Institutet, Peggy Ler at the Department of Medical Epidemiology and Biostatistics, sought to unravel these connections, focusing on how BMI and metabolic health are associated with biological aging and all-cause mortality while considering the nonlinear effects of BMI and age differences.

By employing measures of biological aging—encompassing functional (functional aging index, FAI), physiological (frailty index—FI), and cellular (epigenetic age acceleration—EAA) levels—she aimed to provide a comprehensive examination of the BMI, metabolic health, and biological aging connections.

What are the most important results in your thesis?

One of the key takeaways from this thesis is that body mass index both high and low, as well as unhealthy metabolic status, are linked to increased biological aging and risk of mortality. I think most people are aware of the health risks associated with high BMI. However, one of our studies found that BMI tends to decrease especially among older people, and this BMI decline seems to be driven by an acceleration of biological aging.

Given that low BMI is also consistently associated with higher biological aging and mortality risk in this thesis—perhaps we should pay as much attention to low BMI as we do to high BMI. Integrating metabolic health into assessments may help us better understand and manage health risks among older adults, regardless of where they fall on the BMI spectrum.

Why did you become interested in this topic?

I became interested in this topic because of the rising prevalence of obesity and the aging population. It felt that understanding the relationship between obesity and aging is of great relevance and importance. Initially, I assumed that the relationship would be straightforward—that higher BMI equals greater health risks.

Little did I know that obesity turns out to be a complex and heterogeneous condition and can present with varying metabolic health profiles. What caught me by surprise was also learning that low BMI can be just as concerning as high BMI. This whole process of gradually unraveling how BMI, metabolic health, and aging connect, and the nuances in the connections, are what make the entire topic truly interesting.

What do you think should be done in future research?

While this thesis highlights the limitations of using BMI as a sole tool for assessing health risks, since both low and high BMI signal potential health problems—it is simple, painless, and cost-effective. I believe we should continue to develop similar, easy-to-use, noninvasive, affordable tools to facilitate health evaluations.

I still dream of the day when we will have a device, like the medical tricorder from Star Trek, that can quickly scan a part of the body and provide a comprehensive health assessment. Imagine something inexpensive and accessible to everyone. That would be truly amazing!