Low and High BMI Increases Risk of Death

While studies have linked being either underweight or overweight to poor health, the effect of being overweight or obese on the risk of dying has been a topic of recent controversy. Researchers have long used the body-mass index (BMI), weight in kilograms divided by the square of height in meters, as a measure of the appropriateness of weight in relation to height. Researchers from Yonsei University, in Seoul, South Korea, and the Johns Hopkins Bloomberg School of Public Health report in one of the largest studies to date (over 1.2 million study participants) that having either a high or low BMI increases risk of death. The researchers found that the effect of BMI on the risk of dying varied among major causes of death and that the risk of death from being overweight or obese was greater in younger people. The study is published in the Aug. 24, 2006, edition of the New England Journal of Medicine.

Sun Ha Jee

“The relationship between being underweight or overweight and the risk of death is controversial because, of the few studies on this topic, most were not able to pinpoint at what BMI the risk of death increased. The results from these other studies were also mixed; some of them did not show an increase in risk of death,” said Sun Ha Jee, PhD, lead author of the study and an associate professor of Epidemiology at Yonsei University.

The authors examined the effect of body weight on the risk of death of over 1.2 million 30- to 95-year-old Koreans over a 12-year time span. They looked at death from any cause and those from specific diseases, such as cancer, atherosclerotic cardiovascular disease and respiratory disease. The study participants, who were part of the Korean Cancer Prevention Study, had an average BMI of 23.2, regardless of sex; the majority of participants had a BMI below 25, which is considered a normal weight. By comparison, the average BMI in the United States is currently 27.8 for men 20 years and older and 28.1 for women 20 years and older. Compared to normal-weight Koreans, underweight, overweight and obese men and women had higher rates of death. Men and women who had never smoked with a BMI of 23.0 to 24.9 had the lowest risk of death from any cause.

The researchers found that the relationship of BMI with risk of dying varied among the major causes of death considered. The risk of death from cancer increased beginning at BMI levels of 26.0-28.0 and rose further at higher levels, according to the researchers. Risk for death from respiratory causes was highest at the lowest BMI values and decreased with higher BMI values, whereas the risk of death from atherosclerotic cardiovascular disease increased progressively with higher BMI values. Information on cardiovascular risk factors showed an increasingly unfavorable profile with increasing BMI values. Study participants younger than 50 years of age had the highest relative risk of death associated with a high BMI. The researchers report no evidence of an increased risk of death for 65-year-old and above, obese individuals.

Jonathan Samet

“Past studies were conducted primarily in Western populations and we did not know if these results would apply in Asian populations, which tend to be thinner but have a higher percentage of body fat than do their Western counterparts,” said Jonathan M. Samet, MD, senior author of the study and professor and chair of the Bloomberg School’s Department of Epidemiology.

The amount of respiratory causes of death, such as tuberculosis, pneumonia, chronic obstructive pulmonary disease and asthma, differ between Koreans and inhabitants of Western countries. Jee, who is also an adjunct associate professor at the Bloomberg School, noted that the study results can not be generalized to other populations.

“Body-Mass Index and Mortality in Korean Men and Women” was supported by grants from the National Cancer Institute, the National Institutes of Health, the Department of Health and Human Services and the Korean Seoul City Research and Development program.

In addition to Jee and Samet, the study is co-authored by Eliseo Guallar, MD, DrPH, and Jae Woong Sull, PhD, with the Bloomberg School of Public Health, Jungyong Park, PhD, and Heechoul Ohrr, MD, with Yonsei University, and Sang-Yi Lee, MD, with Cheju National University and National Health Insurance Corporation, Korea.