- 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
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.
"Being underweight is a marker for illness in some individuals at the same time that being overweight and obese is not optimal for health," said Dr. William D. Leslie, professor of medicine and radiology at the University of Manitoba and one of the authors of the study, which was published on Monday in the Annals of Internal Medicine
The idea is that being too thin is associated with not being fit and not preserving muscle mass, which is an "unhealthy situation," Leslie said. However, the individuals who fell into this category were admittedly in the minority. Only 2% of women and 1.3% of men were underweight, and another 38.3% of women and 28.9% of men were normal weight. "The reality is we are dealing with the obesity epidemic in our country as in other countries," Leslie said.
There were no clear health differences between individuals of different weights to suggest why those at the extremes had higher rates of death. Rates of conditions such as diabetes, heart and lung disease were similar between the BMI groups.
The researchers did not parse out the causes of death and only looked at whether an individual died during the follow-up period.