One-third (33.1 percent) of people age 45 and older who are of healthy weight—defined by BMI in the 18.5 to 24.99 range—now have prediabetes, says Arch Mainous, PhD, researcher and department chair of Health Services Research, Management and Policy at the University of Florida. According to the study, this percentage increased from 22 percent in 1994, and it accounts for about 6 million to 7 million people.

What’s the reason for this trend of diabetes in normal weight people? Well, it’s complicated, but Mainous first points to people’s overall reduction in physical activity and an increasingly common body composition that trends toward softening: less lean muscle tissue and more fatty tissue. In healthy weight people with prediabetes and type 2 diabetes, sedentary lifestyle, activity level and sitting time distinguishes them, he says. Mainous’ research shows this group has lower grip strength than its no-diabetes counterparts, grip strength being a proxy for lean muscle mass. “While losing muscle mass as we age is a natural process, it is accelerated by inactivity,” he says.

Other factors contributing to a growing number of healthy weight individuals with elevated blood sugar? One culprit could be limitations with the BMI test. BMI is a measure of body mass index based on a basic equation of a person’s height and weight, but doesn’t take muscle mass, fat tissue, age or other factors into account. Some experts believe BMI is not a good measurement of how metabolically healthy a person is.

“BMI doesn’t measure the fat tissue we carry,” says Joel Zonszein, M.D., professor of clinical medicine (endocrinology) at Albert Einstein College of Medicine in New York. “You can be thin and still have too much fat tissue, especially around the middle of the body where it has metabolic effects,” he says.

“Other factors for the trend,” says Zonszein, “include eating too many processed foods, exposure to chemicals and pollutants, and even the health of the microbiome,” the trillions of gut bacteria which were once believed to be isolated in the gut but which we now know communicate with the rest of the body and can influence the development of diseases.