Rethinking body mass index standards: Addressing hidden nutritional risks across populations

In March 2024, The Lancet published a widely recognized article based on a metanalysis of worldwide trends in malnutrition during the period 1990-2022 authored by the NCD-RisC initiative. In brief, the article showed that increased malnutrition (under- and over-nutrition) was mainly driven by obesity defined as body mass index (BMI) of or above 30 kg/m².

Two researchers from the Global Health Section, Associate Professor Dirk L. Christensen and Professor Emeritus Ib C. Bygbjerg, had a Letter-to-the Editor shortlisted and accepted for publication in The Lancet (26^th of October, vol. 404) in which they discuss the limitation of using BMI of 30 kg/m² as cut-off. The authors argue that it may underestimate the risk of disease in some populations, especially in South-East Asia, as a so-called thin-fat phenotype seems to be common here.

The term thin-fat refers to a person with a disproportionately high fat percentage and low lean body mass for a given BMI, which makes such an individual at increased risk of cardio-metabolic disease below the generally recognised BMI cut-off standards of overweight (BMI ≥25 kg/²) and obesity (BMI ≥30 kg/m²). The thin-fat type may be related to undernutrition and infection during pregnancy, both of which were prevalent in South-East Asia until recently.

The authors do recognise that cut-offs for BMI are difficult to implement for policy purposes and that ethnic/population-specific cut-offs are extremely difficult to get across as a public health message. Especially in countries with a high degree of ethnic heterogeneity. Nonetheless, for the clinician it may be important to recognise ethnic-specific risk factors such as unhealthy fat accumulation despite having BMI below 30 kg/m² or even having a “normal” BMI.

Finally, the authors point to the fact that current WHO recommendations for (un)healthy weight are still predominantly or disproportionately based on data collected in White/Caucasian populations. Therefore, they recommend that prospective population-based data using BMI as well as other measures and tools for the assessment of fat accumulation-based diseases are being prioritised and carried out in populations from low- and middle-income countries and areas as well as migrant populations from these countries/areas living in high-income settings. Apart from this, further research on how, when and why unhealthy fat accumulation occurs is needed, including the importance of early prevention addressing somatic as well as mental health issues.

Read the Letter-to-the Editor “Assessing worldwide trends of underweight and obesity” here.