NEW YORK (Reuters Health) - Women who are more physically fit may have lower levels of inflammation in their bodies--and, therefore, a lower risk of heart attack--than those who exercise less, according to researchers.

This finding suggests that the benefits of exercise on the heart may, in fact, stem from its ability to reduce inflammation within blood vessels, a supposed risk factor for heart attack, the authors note.

"The health benefits from enhanced fitness may have an anti-inflammatory mechanism," Dr. Michael J. LaMonte of the University of Utah in Salt Lake City and colleagues write.

LaMonte and his team investigated the effect of exercise on inflammation by comparing a woman's fitness to her blood levels of a substance known as C-reactive protein (CRP), which is a marker for inflammation.

The body releases CRP as part of its response to infection and injury. During infection, for instance, blood CRP levels temporarily soar as the immune system jumps into action. More subtly, chronic CRP elevations have been linked to an increased risk of heart disease, and inflammation is believed to play a key role in the hardening and narrowing of arteries that can lead to heart attack and stroke.

In this study, the authors did not measure a woman's fitness by simply asking her how much she exercised, for those reports are often inaccurate. Rather, LaMonte and his team determined how in shape a woman was by testing her stamina on a treadmill. While each woman was on the treadmill, the machine sped up and increased its elevation periodically during the test. The authors measured how long each woman could keep exercising on the machine, with those lasting the longest designated as the most physically fit.

The researchers tested fitness and CRP levels in 135 African-American, Native-American, and white women.

Reporting in the recent issue of Circulation: Journal of the American Heart Association, LaMonte and his team found that Native-American women and whites who were the most physically fit also had the lowest levels of CRP in their blood. Surprisingly, the authors did not detect a link between CRP blood levels and exercise stamina in African-American women.

African-American women had the highest levels of CRP in their blood, with an average of 0.43 milligrams per deciliter of blood, contrasted with the 0.25 and 0.23 milligrams per deciliter measured in the blood of Native-American and white women, respectively.

CRP levels also varied according to a woman's body mass index (BMI), a measurement of obesity that factors in height and weight. Women with normal BMIs exhibited the lowest levels of CRP in their blood, while those who were considered obese had the highest CRP levels.

Low levels of CRP have been associated with a lower risk of developing diabetes, the authors note, and the anti-inflammatory benefits of exercise may be responsible for that effect, as well.

"Higher fitness levels appear to have an anti-inflammatory effect that may be a mechanism for lowering [heart disease] and type 2 diabetes," LaMonte and his colleagues conclude.

SOURCE: Circulation: Journal of the American Heart Association 2002;106.

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