A new study finds that clinically underweight people have almost twice the risk of death, compared to obese individuals.

You can’t be too rich or too thin, right? Too rich, yes, but, when it comes to being too thin, maybe not. A new study has shown that excessive thinness is bad for your health. In fact, clinically underweight people have a higher risk of death than obese individuals.

The study, led by Dr. Joel Ray, a physician and researcher at St. Michael’s Hospital in Toronto, Canada, was published in the Journal of Epidemiology & Community Health.

Ray evaluated 51 studies that focused on the connections between body mass index (BMI) and death from any cause. The studies observed people for five or more years, in order to rule out people who were underweight as a result of having cancer, chronic lung disease, or heart failure.

Malnourishment, heavy use of alcohol or drugs, smoking, low-income status, poor mental health, and poor self-care can contribute to a person being underweight, according to the researchers.

While also examining data on newborn weight and stillbirths, researchers found that the association between being underweight and a higher risk of dying applies to both adults and fetuses, even when smoking, alcohol use, and lung disease are taken into consideration and when adults with a chronic or terminal illness are excluded.

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Adults who are underweight, with a BMI under 18.5, have a 1.8 times greater risk of dying than those with a BMI of 18.5 to 24.9, according to the researchers. BMI is a rough estimate of the amount of body fat a person carries.

What’s more, the risk of dying is 1.2 times higher for people who are obese (BMI of 30-34.9) and 1.3 times higher for those who are severely obese (a BMI of 35 or higher).

Commenting on the findings, Louise Parent-Stevens, Pharm.D., BCPS, a clinical assistant professor at the College of Pharmacy at the University of Illinois at Chicago, told Heathline she was skeptical.

“The paper raises some interesting questions, but until we have further information, I don’t think that we can definitely say that low BMI, in and of itself, is associated with increased mortality in a person who is otherwise in good health,” Parent-Stevens said. “We do know that anorectic eating disorders are associated with an increased mortality, in part related to the underlying psychiatric issues in these patients, but some of it directly attributable to inadequate nutritional intake and low BMI.”

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Ray said that it’s important to realize that a robust and healthy person has a reasonable amount of body fat, as well as sufficient bone and muscle. “If our focus is more on the ills of excess body fat, then we need to replace BMI with a proper measure, like waist circumference,” said Ray.

Alissa Rumsey, RD, CDN, CNSC, CSCS, and spokesperson for the New York State Dietetic Association, agreed with Ray, telling Healthline, “BMI not only reflects the amount of body fat someone has, but it also reflects muscle mass.”

Pointing out that people with a low BMI may not have sufficient muscle mass, Rumsey said there is evidence to show that when underweight people get ill, with pneumonia, for example, or have a chronic illness, they don’t have the extra energy reserves needed to help them beat the disease.

While agreeing with Ray that waist circumference is a good way to measure excess body fat, Rumsey cautioned, “There are a lot of studies about the visceral fat in the abdominal area around the organs putting you at higher risk for death and different diseases. If someone is very muscular they might have an overweight or obese BMI, even though they have very little body fat. Many football players (not the very large ones), who are muscular often have an obese or overweight BMI, yet they have six or seven percent body fat.”

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In a separate study, published in the Journal of the American Medical Association, Katherine M. Flegal, Ph.D. and her colleagues found that higher levels of obesity are associated with an increased risk of death, while being overweight is linked to a lower risk of death.

Analyzing almost 100 studies involving approximately three million adults, the researchers found that, relative to a normal weight, overall obesity and higher levels of obesity were both associated with a significantly higher risk of death, while being overweight was associated with a significantly lower mortality risk.

Not so fast, according to Rumsey. “They didn’t separate the people who were underweight due to an illness. Clearly, someone who is underweight because he or she is sick is going to have a higher rate of death,” Rumsey said.

Emphasizing that being underweight or obese is certainly not the goal, Rumsey said, “It’s important to get to a normal BMI and to get to a healthy weight for your body. People should just aim to have a balance of exercise and of healthy eating and not take it to the extreme.”

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