Scientists have linked being overweight or obese to many diseases and health conditions, including heart disease, several types of cancer, and even mental health disorders like depression, anxiety, and substance abuse. Beyond specific diseases, obesity is also linked to an increased risk of earlier mortality.
Health organizations in the US and abroad use body mass index, or BMI, to determine what constitutes being overweight or obese. For example, the Center for Disease Control and the World Health Organization both define being overweight as having a BMI of over 25, and obesity as having a BMI of over 30. In simple terms, a person’s BMI is calculated by dividing their weight in kilograms by the square of their height in meters, meaning that someone who is 1.8 meters tall (about 5’11”) and 90.7 kilograms (about 200 lbs) would have a BMI of 28.
Doctors acknowledge that BMI can be a useful tool in healthcare, but it’s not without its drawbacks. Athletes with high amounts of muscle can fall into the overweight category just because muscle weighs more than fat. Additionally, body fat can vary depending on ethnicity and sex, meaning a blanket approach of using BMI is not necessarily accurate for everyone.
Recently, researchers at the University of Florida set out to explore whether other measures of body composition are more accurate than BMI in predicting mortality risk among young adults. To do this, they analyzed survey data from the National Health and Nutrition Examination Survey (NHANES). This survey was conducted in the US between 1999 and 2004, and linked to an index that reported whether participants died before 2020. The researchers used this survey to examine data from 4,252 adults between the ages of 20 and 49. A major benefit of the NHANES data was that it included complete body composition data, like BMI and body fat percentage, for all participants.
The researchers next assessed whether a high BMI, a high body fat percentage, or a large waist circumference was a stronger predictor of whether a participant died within 15 years after the survey. The researchers defined a higher risk body composition as (1) a BMI over 25, which was considered as being overweight or obese, (2) a body fat percentage of 27% or higher in men and 44% or higher in women, which was considered a high body fat percentage, and (3) a waist circumference of more than 40 inches for men and more than 35 inches for women, which was considered a large waist circumference. The causes of mortality that the researchers examined included death from any cause, referred to as all-cause, heart disease, and cancer.
The researchers were intrigued to find that body fat percentage was a better predictor of mortality risk in young adults than BMI. More specifically, they found no statistically significant association between overweight or obese BMIs and either cancer-related death or all-cause death. Instead, both high body fat percentage and large waist circumference were more statistically related to all-cause and heart-disease-related death. However, none of the 3 body composition measures were statistically associated with cancer death.
The researchers noted a few limitations to their study. First, the body fat percentage thresholds they used were taken from another study, and aren’t universally accepted metrics like BMI. Next, they only explored mortality risk in young adults, so BMI could still be a strong predictor of mortality in older adults. Finally, they only looked at mortality as an outcome, while multiple diseases and health issues, like cardiovascular disease, are still linked to higher BMI.
Nonetheless, the team concluded that BMI may not be the perfect metric of body composition, and that other measures like body fat percentage could be more useful in healthcare and clinical contexts. They suggested that future researchers aim to validate these results in older adults and explore other health outcomes such as cardiovascular disease.
