People often associate the United States with obesity, and for good reason; more than two-thirds of the population is overweight or obese. The UK isn’t far behind, though, with the number of obese nearly matching the U.S. While it’s predicted that the UK population will become more obese or overweight in the next 15 years, a new study shows its average BMI has been growing for the past 20 years, especially in those who were already obese.

Researchers from the University of Liverpool’s Department of Geography used data from the Health Survey England, which looks at yearly changes in health and lifestyle among people throughout the UK, to study BMI trends across social groups from 1992 to 2013. They found the average BMI level of the population steadily increased over 11 years, with the 95th percentile of people (those who were heaviest) experiencing the largest increase in BMI.

BMI is a calculation that uses a person’s height and weight to gauge a their body fat. It divides people into four categories; underweight (less than 18.5); normal weight (18.5 to 24.9); overweight (25 to 29.9); and obese (30 and over).

Over 20 years, the average BMI levels of more than 10,418 participants rose by nearly a whole point in both men and women. Looking deeper, the researchers found those who were uneducated had an average point-and-a-half higher BMI than those who had achieved a higher education.

More importantly, the study showed that BMI increased throughout the 1990s, but began to taper in men and women by 2001. This slowing did not occur among those in the 95th percentile. Rather, their BMI increased for the next 12 years, with the less educated experiencing the largest increases. Conversely, those within the 5th percentile (with the lowest BMIs) showed little change over 20 years, regardless of education.

The 5 percent of participants with the highest BMI are most at risk for health problems, the researchers said. They cited a 2013 study that found the higher a person’s BMI, the greater their chances of mortality by all causes. “If this pattern continues to rise following current trends, then we may expect increased prevalence of ill health associated with excess body weight,” they wrote. “The slowing down of median BMI hailed by some public health officials may not be quite the success it first appeared.”

While the results may appear concerning, there’s a growing body of research that suggests BMI isn’t the most perfect way to measure body fat and overall health. For example, it doesn’t account for where in the person’s body the fat is located (an important distinction), or the higher weight of muscle. For this reason, a recent study found many Americans can be mislabeled as obese, while normal weight people might actually be the ones who are unhealthy.

Source: Green M, Subramanian S, Razak F. Population level trends in the distribution of body mass index in England, 1992-2013. Journal of Epidemiology and Community Health. 2016.