Story highlights A new study finds that adults who have either low BMI or high body fat have similarly higher death rates

The best survival was among individuals with high BMI and a medium amount of body fat

The findings could help explain the "obesity paradox," that overweight people have lower death rates

(CNN) Many health experts will tell you that being skinny is not necessarily healthy. Lending support to that argument, a new study finds that the thinnest people, similar to those who have the most body fat, have higher rates of death.

Researchers looked at the connection between body mass index and death rates over several years among nearly 50,000 women and 5,000 men age 40 and older in the Canadian province of Manitoba.

Unlike many previous studies, the researchers did not rely on BMI -- which is a measure of weight that includes both fat and muscle -- as a proxy for fat. They were also able to estimate total body fat directly, because they specifically selected individuals who had previously undergone an X-ray test to determine if they had decreased bone density and possibly osteoporosis.

The researchers found that the skinniest women, with a BMI less than about 22.5, a group that includes both those who are underweight and normal weight, had a 44% higher risk of dying during the approximately seven-year follow-up period. At the other end of the spectrum, women with more than 38.7% total body fat had 19% higher death rates.

Among men the thinnest group, those with a BMI less than about 23.8, had 45% higher death rates during a follow-up period of about 4.5 years, whereas men in the highest body fat group, with more than 36% total body fat, were at 59% higher risk of dying during the study period.