The health risks for women who are extremely obese may be underestimated as a new study indicates they have a higher prevalence of hypertension, diabetes, and High cholesterol than women at lower levels of obesity, according to a study in the July 5 issue of JAMA.

Obesity diagnosis and treatment are typically based on body mass index (BMI) of at least 30. BMI is calculated as weight in kilograms divided by height in meters squared. However, three categories of obesity are defined: obesity 1 (30-34.9); obesity 2 (35-39.9); and extreme obesity (40 and greater). (A 5’4” person would have a BMI of 40 if they weighed 233 lbs). The latter 2 categories, sometimes termed severe obesity, are reported to be increasing especially rapidly in the United States, according to background information in the article. From 1986 to 2000, prevalence of BMI of 30 or higher approximately doubled, while that of BMI of 40 or higher quadrupled and that of BMI of 50 or higher increased 5-fold. In 2000, 2.8 percent of all U.S. women, and 6 percent of black women reported measurements consistent with extreme obesity. Estimates of obesity-related risks in women have generally been based on weight data that preceded the increase in extreme obesity. It has been unclear whether health risk increases or plateaus as body weight increases throughout the obese range.

Kathleen McTigue, M.D., M.S., M.P.H., of the University of Pittsburgh, and colleagues conducted a study to examine the relationship between weight category and risk of death and coronary heart disease (CHD) in a large population-based sample of U.S. women, focusing on risk across degree of obesity. The researchers analyzed data on incident death and cardiovascular outcomes by weight status in 90,185 women recruited from 40 U.S. centers for the Women’s Health Initiative-Observational Study who were followed-up for an average of 7.0 years (Oct. 1993 to Aug. 2004).

The researchers found that extreme obesity prevalence differed with race/ethnicity, from 1 percent among Asian and Pacific Islanders to 10 percent among black women. “In this diverse population-based sample of older women, we found that obesity was linked with considerable health risk and that accounting for degree of excess weight is important in understanding weight-related health risk. Overall, extremely obese women were more likely to die over the average 7.0 years of follow-up than were women in other examined weight categories. Modeling analyses adjusted for age, smoking status, educational achievement, U.S. region, and physical activity level showed that weight-related risk for all-cause mortality, CHD mortality, and CHD incidence did not differ by race/ethnicity.”

“There was a positive trend in all-cause mortality risk and CHD incidence with increasing weight category. This trend had borderline significance for CHD mortality among black women, likely reflecting sample size limitations. Much of the obesity-related mortality and CHD risk was mediated by diabetes, hypertension, and hyperlipidemia [High cholesterol levels]. In white women, as other studies have found, weight-related all-cause mortality risk was modified by age, with obesity conferring less risk among older women. Smoking may modify weight-related risk in black women, but further study is needed to understand the nature of this relationship,” the authors write.

“Our findings have important clinical and policy implications. The escalating prevalence of extreme obesity may exacerbate the health effects and health-related expenditures resulting from the U.S. obesity epidemic. Calculating the weight-related risks of morbidity and mortality based on findings in earlier population samples, which tended to reflect lower degrees of obesity, may underestimate the risks for extremely obese individuals and overestimate the risks for mildly obese individuals in diverse groups,” the researchers write. “More accurately assessing weight-related health risk may both improve policy decisions about obesity and assist women in making informed decisions about their health.”