A lot of people hear overweight and think 'fat person', where you look at these pictures of people in the overweight range and they look perfectly normal," Harding said. "
In the 1980s, doctors began to use BMI as an individual health metric for determining if a person was of normal weight or overweight, but that use of BMI has several limitations.
Harding pointed to the classic example of the athlete with a lot of muscle and a low body fat percentage who has a BMI in the overweight range. The measurement also doesn't account for frame size, bone density, age or ethnicity, she said.
As well, BMI and cardiovascular health have a J-shaped relationship, de Lemos said, where the best body mass index isn't the lowest.
Those with BMIs under 23 actually show a slightly higher risk of heart disease, probably related to other illnesses associated with bad heart outcomes that reduce body and muscle mass.
"People who are normal or even slightly overweight might be even better from a cardiovascular standpoint," he said. The risk then increases again as weight relative to height increases.
Body mass index does have some value outside of clinical settings, de Lemos said, because it allows people to track changes in body mass over time. However, there are other measurements that more accurately indicate health risk, he said.
In a study comparing body mass index, waist circumference and waist-to-hip ratio in terms of association with plaque build-up in arteries of the heart and aorta, de Lemos and his research team found that the waist-to-hip ratio measurement remained associated with heart risk even when obesity-associated conditions like diabetes, hypertension and cardiovascular risk factors were accounted for.
Body mass index didn't hold the same association. "There's a much deeper association between waist-and-hip ratio and atherosclerosis than we saw for body mass index and atherosclerosis," he said.
Waist-to-hip ratio is calculated by dividing the circumference of your waist by your hips. Women should get a number below 0.8, and the healthy ratio for men is below 0.95. More belly fat results in higher ratios.
Excess fat around the belly can be dangerous for the heart because it behaves differently in the body. "It's very biologically active," de Lemos said, "and the fat on the hips seems to be at least inert and maybe even protective."
Few people have an ideal waist-to-hip ratio, de Lemos said, including those with a BMI in the normal range. While small changes to reduce waist circumference, and therefore the waist-to-hip ratio, can be helpful and are worth aiming for, he said, the associated risk is also small. "This isn't something that should panic people."
Labels like "overweight" and "obese" can lead to panic too, Harding argued. They can also be stigmatizing, she said, when a person feels and looks healthy but has a BMI that places them in the "overweight" or "obese" category. "I think a lot of people are beating themselves up."
Some insurance companies in the United States now use BMI to classify risk, and adjust their rates accordingly, which means that a person with a body mass index of 25 or 30 may pay more for health insurance than one with a BMI of 23. If a person is 5'6", the difference between a BMI of 23 and 25 is 7.5 pounds.
In her writing, Harding advocates the concept of "Health At Every Size" which focuses on exercise and intuitive eating instead of dieting, focusing on wellness and fitness as a goal instead of thinness, and saying that fat doesn't necessarily equal unhealthy.