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Physical Fitness Versus Body Mass Index
Physical inactivity and a body mass index (BMI) greater than 25 can each lead to increased health risks such as heart disease, stroke, and type 2 diabetes. But when they are considered together, which presents the greater risk? Two recent studies examining the combined effect of BMI and physical activity on health reached different conclusions.
A prospective study of 37,878 healthy women enrolled in the Women’s Health Study (WHS) looked at the association between BMI, physical activity, and the development of type 2 diabetes. Each year for 7 years, researchers gathered information on the women’s BMI and physical activity levels, and whether they had been diagnosed with type 2 diabetes in the last year. They divided study participants into six groups based on BMI and activity levels: normal-weight active, normal-weight inactive, overweight active, overweight inactive, obese active, and obese inactive.
As expected, when the researchers looked at BMI and physical activity levels separately, they found that women who were overweight or obese had a higher risk of developing type 2 diabetes than normal-weight women, and inactive women had a higher risk than active women. However, when they examined the effects of BMI and physical activity together, increased activity within the same BMI group—normal-weight, overweight, or obese—offered a negligible reduction in the risk of developing type 2 diabetes, but increased BMI, regardless of activity level, led to a significantly higher risk.
A second study that examined the relationship of BMI and physical fitness to the development of cardiovascular disease in women found the opposite effect. Researchers assessed BMI and fitness among 906 women with suspected coronary artery disease (CAD) who were enrolled in the Women’s Ischemia Syndrome Evaluation for diagnostic testing. The women completed two questionnaires, one to assess fitness based on self-reported ability to perform various activities that correlate with treadmill test results, and one to assess average physical activity levels at home, work, and leisure.
Both high BMI and low fitness and activity scores were associated with CAD risk factors such as hypertension, diabetes, and dyslipidemia. However, diagnostic tests showed no difference in the presence or severity of CAD across BMI categories. In contrast, women with low fitness scores were significantly more likely to have obstructive and severe CAD. Follow-up over the course of 4 years revealed that these women suffered more adverse events including stroke, congestive heart failure, and death, regardless of BMI.
In a commentary on the two studies, researchers from the Cooper Institute, a research and education center that focuses on physical activity and health, noted differences in the studies that could partially account for the divergent results. Each one measured different outcomes—development of type 2 diabetes versus adverse cardiovascular events. The diabetes study followed healthy women; the CAD study followed women with suspected heart disease. Each study used different measures of self-reported physical activity with potentially unequal levels of accuracy.
Regardless of the disparities in study methods and results, the Cooper Institute researchers conclude that physical activity is the common denominator in achieving both increased fitness and long-term weight management. Rather than focus on the “fit vs. fat” debate, they suggest that the medical community support physical activity to promote both health and weight control.
Both studies were supported by the National Institutes of Health. The full reports, “Relationship of physical activity vs body mass index with type 2 diabetes in women” and “Relationship of physical fitness vs body mass index with coronary artery disease and cardiovascular events in women,” appear in the September 8, 2004 issue of JAMA, as does the commentary.
Source: National Institute of Health
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