The body mass index (BMI) is a rough proxy of body composition that is widely used to predict the risk of declining health, including early death, but BMI is an unreliable metric that does not actually predict the risk of death, new research suggests.
The study, published in the Journal of Family Medicine on June 24, found that BMI cannot reliably predict the risk of death from any cause. However, body fat percentage, an alternative measure of body composition, is found to be much more accurate, indicating a clear correlation with the risk of death for an individual.
“Ultimately, the current definition of obesity needs to change to something based on body fat percentage,” says Dr. Frank Orlando, medical director at UF Health Family Medicine.
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It could change the way anti-obesity drugs, such as Ozempic-style drugs, are prescribed, Orlando told Live Science via email.
Incomplete measurements
There are several ways to assess body composition, but none are as cheap and easy as BMI. This calculates an indirect measure of body fat using a person’s height and weight. It classifies people as being underweight, normal weight, overweight or obese.
“The assumption is that weighting above a certain amount of height can lead to excess body fat around,” said Adam Collins, a nutrition professor at the University of Surrey in the UK who was not involved in the new research. “The associations make excessive body fat more likely to increase the risk of illness,” Collins told Live Science.
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The study has led to over 25 BMIs in increased risk for a variety of conditions, including type 25 diabetes and heart disease. However, previous studies have found an inconsistent relationship between BMI and early death.
BMI has also recently been recognized as a “incomplete” measure of fat by medical institutions, including the American Medical Association. Experts argue that this metric can misclassify people who have many muscles as being overweight or obese, even at low or normal levels of body fat, for example. The BMI category is also primarily based on non-Hispanic white populations. This means that excess fat may not reflect the different levels of risk pose in all patients.
However, despite these limitations, BMI continues to be used, as there are no well-tested and inexpensive alternatives. In their new study, researchers wanted to see how BMI stacked up on various body composition measurements in predicting the risk of death for people over the next 15 years.
They used data from a representative sample of 4,252 US adults aged 20-49 at the start of monitoring. The team pitted BMI against two metrics around the body fat percentage and waist. Having a high waisted circumference is likely to lead to death from heart disease, but not all causes, research has found.
To calculate body fat percentage, researchers used devices that send small currents around the body and measure the amount of resistance encountered in different tissues. Tissues like fat and muscle have different water content, so they carry electricity in a variety of ways. This means that current moves more easily through the muscles. This assessment, called bioelectrical impedance analysis (BIA), estimates the overall body composition, namely fat-free mass and body fat percentage.
Over the course of 15 years, people with high body fat percentages (more than 27% for men and more than 44% for women) were 78% more likely to die than people with low body fat levels.
What’s next?
BIA is not the perfect measure either. “It’s not just about how much body fat you have in total, but where that fat is,” Collins said. “If it’s just below the skin, subcutaneous fat is not that much of a problem. It’s intraperitoneal fat, fat around or inside the organs, and you start to have the real problems with metabolic disease.”
Any replacement for a BMI must be as accessible as a BMI. The patient can calculate the BMI themselves. BIA devices are becoming cheaper and more accessible due to US primary care practices, he added. Bioelectrical impedance is also used on smart scales that can be purchased for home use.
Until now, we have based our definition of obesity on BMI. This is because this scale is simple and inexpensive. “But now we have research like ours that shows that BIA is accurate and reliable when compared to BMI.”
Access to obesity-treating medications such as Wegovy, which has the same active ingredients as Diabetes Ozempic, is available to people with BMI in the obesity category and those who are overweight with additional risk factors such as diabetes. However, Orlando says that patients with BMI under BMI still see patients with high body fat percentages.
“Our research proves why anti-obesity drug cutoffs are necessary. [change to] Body fat percentages are otherwise missing as patients with normal BMI but high body fat are otherwise missing.
This article is for informational purposes only and is not intended to provide medical advice.
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