BMI may not predict health risk accurately. A new study finds body fat percentage, measured by bioelectrical impedance, is a better predictor of early death.
Study Challenges Longstanding Weight Metric
A new study is questioning the decades-long dominance of Body Mass Index (BMI) as the standard tool for assessing health risks tied to weight. Published this week in the Annals of Family Medicine, the research finds that body fat percentage—measured using bioelectrical impedance analysis (BIA)—is a far more accurate predictor of early mortality, especially among adults aged 20 to 49.
“We found body fat percentage to be a much stronger predictor of 15-year mortality risk than BMI,” said lead author Dr. Arch Mainous III, a professor at the University of Florida College of Medicine.
Why Body Fat Matters More
Unlike BMI, which simply divides weight by height, BIA uses weak electric currents to estimate fat mass, muscle mass, and water composition. The study found that younger adults with high body fat, even if their BMI was in the normal range, were 262% more likely to die of heart disease over 15 years.
Co-author Dr. Frank Orlando emphasized that BMI missed these risks entirely in many younger adults. “This could change how we screen for metabolic and cardiac risks much earlier,” he said.
How the Research Was Conducted
Researchers analyzed data from 4,252 adults who participated in the National Health and Nutrition Examination Survey (NHANES) between 1999 and 2004. Each participant underwent detailed body composition tests, including BIA, and their mortality data was tracked through the National Death Index up to 2019.
Even after adjusting for age, race, and income, BMI classifications showed no statistically significant increase in risk of death, while high body fat levels correlated with a 78% higher risk of dying from any cause.
The Flaws in BMI
While BMI remains useful at a population level, it frequently fails individuals, especially those with high muscle mass or low muscle and high fat (so-called “skinny fat”). Dr. Mainous points out that visceral fat—fat wrapped around organs—can go undetected with BMI but leads to higher risks of diabetes, hypertension, and liver disease.
“BMI is easy and cheap,” he said. “But we now have better tools like BIA that can personalize risk assessments more accurately.”
A Cheaper, Better Alternative to DEXA
Though DEXA scans are the gold standard for body composition analysis, their high cost and limited availability keep them out of most clinics. In contrast, modern BIA machines offer a viable, lower-cost alternative with reliable results in clinical settings.
However, experts caution that at-home BIA devices are less accurate. “Hydration levels can throw off the numbers,” said Dr. Andrew Freeman, a cardiologist not involved in the study. “For meaningful results, it needs to be done in a clinical setting.”
Rethinking the Doctor’s Office Visit
This study underscores the need for a shift in how doctors assess weight-related risk. “Imagine if patients were routinely given their body fat percentage and a personalized risk report,” Freeman suggested. “It could open doors to early lifestyle interventions and, when necessary, medical support.”
With findings showing a massive underestimation of risk using BMI alone, researchers hope this new evidence prompts wider adoption of BIA in primary care. “This could be the beginning of more personalized, accurate, and life-saving care,” said Orlando.
