Body mass index (BMI), which is determined by a person’s height and weight, is a way to measure body fat. Although many have questioned its accuracy, BMI is still considered a rule-of-thumb tool to gauge overall health. It is well documented that people with BMIs of 25 or higher are at an increased risk of high blood pressure, coronary heart disease, and even death. But new research published in JAMA found that the BMI associated with the lowest risk of death had increased, giving more fuel to the ongoing debate abotu using BMI as a proxy for health.

Researchers from Denmark combed through data for more than 120,000 members of the white Danish population who participated in the Copenhagen City Heart Study and the Copenhagen General Population study. The researchers followed up on participants from the time they were included in the studies to whichever came sooner: November 2014, emigration, or death. Over 30 years, from 1976-1978 to 2003-2013, the researchers found that the BMI value associated with the lowest all-cause mortality increased by 3.3 points, from 23.7 to 27. For reference, the Centers for Disease Control and Prevention cites a normal BMI falls between 18.5 and 24.9.

In other words, researchers found that the optimal BMI falls in the overweight category, which spans 25 to 29.9. “This finding was consistent in both the whole population sample [optimal BMI, 27], and in a subgroup of never-smokers without history of cardiovascular disease or cancer [optimal BMI, 26.1],” researchers wrote.

Furthermore, the researchers found that the ratio for mortality associated with those who had a BMI of 30 or greater versus those with a “normal” BMI decreased from 1.3 to 1.0 during this period. Researchers said these findings were present even when the analyses were stratified by age, sex, smoking status, and history of cardiovascular disease or cancer. The current study mirrors previous studies that have indicated that while average BMI has increased over time in most countries, the prevalence of cardiovascular risk factors among obese individuals may be decreasing.

Theses findings come amid an ongoing debate about whether BMI is a good indicator of healthy weight. One study published earlier this year found that using BMI to determine health has miscategorized more than 54 millions of Americans as “unhealthy” when, in fact, those categorized as overweight or obese were actually metabolically healthy.

A possible explanation for the findings could be that people with higher BMIs benefit more from treatments for cardiovascular risk factors. After all, obesity is associated with higher risk for many serious health conditions, including diabetes, heart disease and hypertension.

More research is needed to understand the reason for this change and the implications it may have, but researchers said if their findings are confirmed in other studies, “it would indicate a need to revise the [World Health Organization] categories presently used to define overweight, which are based on data from before the 1990s.”

Source: Afzal S, Tybjaerg A, Jensen G, Nordestgaard B. Change in Body Mass Index Associated With Lowest Mortality in Denmark,1976-2013. JAMA. 2016.