(USA TODAY) -- About one in five people can train all they want but, because of their genetic makeup, are not likely to see much improvement in their endurance levels, an international team of researchers reported Thursday.

Still, the authors caution, their findings shouldn't be an excuse to throw away the running shoes.

"There is a whole host of other physiological responses" to exercise, such as heart rate, cholesterol levels, blood pressure and insulin metabolism, that are related to genes other than those implicated in the aerobic response, says co-author Tuomo Rankinen, a scientist in the human genomics laboratory at the Pennington Biomedical Research Center in Baton Rouge.

Even if you're genetically predisposed to see little improvement in endurance, "it doesn't mean that you don't get any benefit from exercise," Rankinen says. "That's an important public health message."

He and his co-authors conducted a genomewide association study, in which the complete genomes of many people are scanned to find genetic variations linked to common diseases or, in this case, physiological traits.

In the Journal of Applied Physiology, they report that a combination of about 30 genes predict "to a significant extent" an individual's aerobic response -- or VO{-2} max, which refers to the body's ability to take in and use oxygen during maximum exercise -- to endurance training. A low level is known to be a strong risk factor for premature illness and death, the scientists write.

The researchers scanned the DNA in muscle biopsies from 473 sedentary volunteers, split about evenly between men and women. Sedentary is defined as not having engaged in regular physical activity for the previous six months

After their DNA was scanned, the volunteers completed 20 weeks of individually customized, three-times-a-week endurance training sessions that increased in intensity over time. The researchers then again took muscle biopsies and measured their VO{-2} max and compared it with their pre-training levels. About 15% to 20% of them had much smaller improvements than expected, and the scientists saw no change in the function of the genes linked to aerobic response.

The new findings "may ultimately help us to personalize exercise prescriptions," says epidemiologist Molly Bray of the University of Alabama, who has conducted other research into the link between genetics and exercise response. "Having genetic information about a person can at least give us some informed guidance."

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