Fitness level, not size or weight, may be more important determinant of women's risk of death

American College of Sports MedicineNov 2 2010

Obesity alone may no longer be linked to a high risk of death in women, according to research published in the Nov. 2010 Medicine & Science in Sports & Exercise, the official journal of the American College of Sports Medicine. Researchers now suggest cardiovascular fitness level, not just a woman's size, may actually be the key predictor of health level and overall risk for death.

The study, titled "Cardiorespiratory Fitness, Adiposity, and All-Cause Mortality in Women," measured body mass index (BMI), body fat percentage, waist-to-hip ratio, waist circumference, waist-to-height ratio and cardiorespiratory fitness in 11,335 women from 1970 to 2005. Participants were divided into groups based on their cardiorespiratory fitness levels – low fit equals the lowest 20 percent; moderate fit equals the middle 40 percent; high fit equals the highest 40 percent. Researchers tracked death rates among all participants, and 292 deaths from all causes occurred during the study period.

Researchers found that death rates were significantly lower for fit women than for unfit women. Additionally, fit women with high values (based on standard clinical measures) for BMI, body fat percentage, waist circumference and waist-to-height ratio had no greater risk of death than their fit counterparts with normal adiposity values. These results suggest that fitness is a stronger predictor than thinness for predicting a long and healthy life.

"In other studies, failure to measure cardiorespiratory fitness levels may be due in part to an underlying assumption that all overweight individuals are unfit and at high risk for mortality," said Dr. Stephen W. Farrell, lead author of the study, ACSM member, and science officer in the education division at The Cooper Institute. "This study makes clear that this assumption is not always valid."

While other studies have examined the relationship between death rates and various measures of obesity, this study is among the first to consider the significance of cardiorespiratory fitness level. Cardiorespiratory fitness was quantified through a treadmill exercise test in which all participants were encouraged to provide a maximal effort performance.

The findings suggest that even modest levels of cardiorespiratory fitness are associated with lower risk of death in women. This level of fitness can be achieved by performing moderate amounts of regular aerobic activity such as brisk walking.

"Physicians and other health care professionals should make a concerted effort to estimate or measure cardiorespiratory fitness levels before categorizing patients' risk status on obesity measures alone," said Dr. Farrell. "We would agree however, that morbid obesity (BMI>40) does significantly increase the risk of mortality in women. We also acknowledge that our sample of women does not represent a random sample of the population."  

Researchers conducted baseline examinations of all participants at the Cooper Clinic in Dallas, Texas and employed the National Death Index to determine which participants were deceased as of Dec. 31, 2006.