We’ve come to rely on body mass index (BMI), as an indicator of health, but this number doesn’t always tell the whole story. According to a new study, one of the biggest characteristics of metabolically unhealthy people, who are also lean, is a lack of fat accumulation in their legs.

Having less fat in the lower half of your body despite having a healthy BMI may be an indication of other health problems: These individuals are three times more likely to experience mortality and/or cardiovascular disease when compared to other healthy weight individuals without this unequal fat accumulation. Researchers hope these findings may help to dispel the myth that low weight and health always go hand-in-hand, and serve as a prevention tools for lean individuals who may not be aware of the risk factors for metabolism problems, they said in a statement.

Metabolic health relates to the health of chemical processes we use to turn food into energy--in other words, our metabolism. Enzymes in our bodies break down nutrition found in food, such as proteins and fat, and carry them to use as fuel in other parts of the body or store them in the liver, body fat, or muscles for later use, Healthline reported. When something in this process goes wrong, an individual's metabolic health is at risk.

Normally, metabolic health is associated with body weight, as a high accumulation of body mass is one of the first and most notable signs of metabolic problems. However, not all people with excess body fat are metabolically unhealthy, and as shown in the new study, not all people who are of healthy body mass are metabolically healthy.

The study was based on health information from 981 individuals of various body sizes. The researchers measured metabolic health as having more than two factors associated with metabolic syndrome. Of the 981 subjects, 18 percent of the lean subjects were found to be metabolically unhealthy as measured by having two or more metabolic health indicators.

According to the National Institutes of Health, metabolic syndrome is a group of risk factors that raise your risk for heart disease and other health problems, such as diabetes and stroke. Some of these factors include: a large waist size, also known as abdominal obesity, high blood pressure, and high triglyceride levels (a type of fat found in the blood).

The results also showed that having low amount of fat storage in the legs was the strongest determinant of metabolic risk in lean subjects, while intra-abdominal fat (fat located around the body organs rather than just under the skin) and nonalcoholic fatty liver disease were the biggest determinants of metabolic risk in obese individuals.

Past research has shown that about 20 percent of the US population that is considered of healthy body weight according to their BMI is actually metabolically unhealthy and at higher risk of death or disease, the researchers explained in a statement. 

This finding does not mean that having thin legs is a certain sign of metabolic disease; however, doctors and patients may consider the trait another risk factor to consider. 

Source: Stefan N, Haring HU, Schick F, et al. Causes, Characteristics, and Consequences of Metabolically Unhealthy Normal Weight in Humans. Cell Metabolism . 2017