For decades, the body mass index (BMI) has been the preferred indicator of weight and health. But a new study suggests it may lack markers, especially for young people.
Researchers at the University of Florida found that the percentage of body fat in the United States, compared with BMI, has much more predicted the risk of 15-year deaths among American adults aged 20 to 49. Posted in annthe study adds to growing evidence that BMI may mask actual health risks, especially for people who seem “normal” outside.
Fat is more important than quantity on scale
Using data from 4252 participants in the National Health and Nutrition Examination Survey (1999-2004), researchers have been tracking deaths from all-cause and heart disease in 2019. The human components of all participants were directly measured by bioelectrical impedance analysis, a method that can use mild currents to estimate body fat. This approach allows the team to compare BMI, body fat percentage and waist circumference as predictors of mortality.
The results are surprising:
- Adults with high body fat (≥27% for men and ≥44% for women) are 1.78 times more likely to die than healthy fat levels.
- They are also 3.62 times more likely to die from a heart disease.
- By contrast, BMI with overweight or obese (≥25) does no Significantly increase the risk of death.
- High waist circumference (>40 inches for men,>35 inches for women) also predicted higher mortality: a 1.59-fold increase in the likelihood of dying and a 4.01-fold increase in the likelihood of dying from heart disease.
“The percentage of body fat is better predicting the 15-year risk of death in young people than BMI,” the author wrote.
Normal BMI, hidden dangers
BMI has long been a standard tool because it can be calculated simply using height and weight. But this simplicity is also its flaw. The metric system classifies people with muscle classification as overweight and ignores people with low muscle mass but high fat content, a condition known as “normal weight obesity.” This hidden fat quietly increases the risk of metabolic diseases and heart disease.
“There are significant limitations in using BMI as a measure of body composition,” the researchers noted. “Adults with normal BMI and elevated BF% may not be aware of their significant increase in their risk of metabolic syndrome, type 2 diabetes and cardiovascular disease.”
Why waist circumference is still important
In terms of predicting risk, waist circumference performance was slightly higher than BMI, consistent with other studies, indicating that fat distribution is important. In particular, abdominal fat is associated with higher inflammation and metabolic dysfunction. However, measuring waist circumference in a clinic is not always easy or consistent.
“Wait circumference measurements have significant problems with inter-evaluator measurement error and reliability, so they lack both repeatability and the ability to identify clinically relevant changes,” the authors explained.
Bioelectric impedance may change the game
The study relies on BIA devices from two decades ago — but even these older tools outperformed BMI. Today’s BIA devices are faster, cheaper and more accurate, making them a promising option for routine checks.
“As BIA prices drop and accuracy improve, the technology will become increasingly attractive to clinicians who want to replace BMI, with more accurate risk measurements,” the researchers said.
Looking to the future: Transformation in clinical practice?
Although BMI remains a standard recognized by major health institutions, this study provides momentum for the need for better indicators. The authors call for large-scale prospective studies to determine standardized ranges of healthy body fat percentages. If these ranges can be verified, the clinic may soon swap the BMI chart for something more displayed.
“These results suggest a rethinking of how we should measure body composition in a clinical setting,” they concluded.
Magazine: Family Medicine Annual Review
doi: https://doi.org/10.1370/afm.240330
title: Body mass index and body fat percentage as a predictor of mortality in adults aged 20-49
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