Increased physical activity cannot overcome the potentially deadly effects of obesity, conclude researchers at the Harvard School of Public Health.

The Study

The 24-year study of more than 116,000 women found that obesity and lack of physical activity were independent predictors of the risk of death. A high level of physical activity did not eliminate the risk of premature death associated with obesity.

Excess weight plus less than 3.5 hours per week of physical activity accounted for 31% of all premature deaths. Of these, 59% were from cardiovascular disease and 21% were from cancer among nonsmoking women.

As expected, women who were physically inactive and obese fared the worst. Their risk of death was 2.5 times higher than for physically active, lean women.

Controversial Finding

A contradictory theory is "if you are fat but physically fit, you don't have to worry about your weight because physical activity can cancel out the risk associated with obesity," says study author Dr. Frank B. Hu, an associate professor of nutrition and epidemiology at Harvard. "This is not supported by our data. Our data suggest that obesity is a risk factor independent of physical activity."

However, not everyone agrees with this conclusion. Mark A. Pereira, an assistant professor of epidemiology at the University of Minnesota, notes, "One large study that looked at men found that physical activity was a much truer predictor of mortality rate than body mass index (BMI)."

One flaw in the new study that might explain the different results, Pereira says, is that it relied on self-reporting of physical activity by the participants. In contrast, a treadmill test was used to measure physical fitness in the study of men.

"A related limitation (of the new study) is that they did not assess how much time women spent in sedentary activities, such as watching television," Pereira adds.

BMI is a factor in mortality, Pereira acknowledges, but it is certainly not the only or most important factor. "BMI is a good marker of disease risk and mortality, but a far from perfect marker. In and of itself, BMI says nothing about body composition and lifestyle. Lifestyle is going to predict longevity."

Hu believes that lifestyle and weight both play an important role. "Some people cannot maintain a healthy weight even with exercise, if they don't pay attention to diet," he says. "It is an issue of both diet and exercise, not of one or another. Really, both are important."

BMI, a ratio of weight to height, is a standard measurement used to determine if someone is at a healthy weight. A person who has a BMI between 25 and 29.9 is defined as overweight; someone who has a BMI of 30 or greater is considered obese.

Help Your Teen Get Moving

Only 25% of children in the United States get significant physical activity each day. This lack of exercise is contributing to the nation's childhood obesity epidemic.

Inactive teens may already have the start of serious health problems that are associated with obesity.

The National Association for Sport and Physical Education ( offers these suggestions to encourage teenagers to get moving…

  • Enroll your teen in an exercise class. Analyze your child's likes and dislikes to find a program that will appeal to him/her. Check colleges and community programs for fun and unusual classes.
  • Suggest to your teen that you drive him and his friends to the ice-skating rink or community swimming pool instead of the video store. For older teens, suggest hikes or long walks.
  • Plan family activities that include physical exercise. Challenge kids to a game of Frisbee or volleyball. You might also consider riding bikes and exploring new surroundings.
  • Set limits on passive pastimes, such as watching TV or playing video games.

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