NEW ORLEANS -- The more obese a person is, the more likely the risk of low back pain, but it's possible to reduce the odds by engaging in a moderate amount of exercise, according to research presented here.

The paper, presented Thursday at the North American Spine Society's annual meeting, is the latest to link weight and exercise to one of the most common conditions afflicting Americans and one of the first large studies to use an objective measure to study back pain: accelerometers that track a person's daily exercise levels.

Researchers Matthew Smuck, MD, of Stanford University Medical Center, and colleagues analyzed data on 6,796 participants in the National Health and Nutrition Examination Survey.

They found that in people who were normal weight, defined as a body mass index of 20 to 25, the risk of low back pain was 2.9%. (A 5' 10" person who weighs 174 lbs has a BMI of 25.)

In those who were overweight (BMI of 26 to 30), low back pain risk was 5.2%. In the obese, a BMI of 31 to 35, the risk grew to 7.7%. And in the morbidly obese, a BMI of 36 or more, the risk was 11.6%. (A 5' 10" person weighing 251 lbs has a BMI of 36.)

"We showed both increased BMI and inactivity were independent risk of low back pain," said Smuck. "Perhaps the best news out of this study is that big gains can be made by making some incredibly modest changes in activity."

The researchers found different ways to achieve the improvements:

The typical overweight person increasing their amount of moderate activity such as brisk walking, riding a bike, or general gardening by less than 20 minutes a day can reduce back pain risk by 32%.

For people with BMIs of 36 or more, the average duration of time spent during a bout of moderate activity was 1.3 minutes. However, by increasing that time by 1 minute, the risk of back pain dropped by 38%.

For years, anecdotal evidence has led spine specialists to tell overweight people to lose weight and exercise; now there is hard data to back up those beliefs, said Michael Reed, a physical therapist and spine specialist with the Hospital for Special Surgery in Jupiter, Fla.

One question the study could not answer, however, is why obesity increases the risk of low back pain.

Two leading theories are that it causes mechanical changes that affect the spine or that it causes metabolic changes that lead to varying levels of hormones and inflammation.

"I suspect it could be a combination of the two," Reed said.

Either way, Reed said, increasing activity, which can be as simple as gardening, heavy house work, or walking around the neighborhood, can help reduce low back pain.