Oct. 29 (HealthDay News) -- Anabolic steroids, taken by some athletes to build muscle and strength, can cause kidney disease, new research suggests.

Doctors haven't previously realized that the steroids, which are known to cause many health problems, also can seriously harm the kidneys.

In the new study, Dr. Leal Herlitz, of Columbia University Medical Center in New York City, and her colleagues studied 10 bodybuilders who were long-time users of steroids. The patients had begun to leak protein into their urine and had lost kidney function.

The findings were scheduled to be presented at the annual meeting of the American Society of Nephrology, held Oct. 27 to Nov. 1 in San Diego.

Tests showed that all but one of the bodybuilders developed a kidney-scarring condition known as focal segmental glomerulosclerosis.

According to the researchers, the condition develops when the kidneys are overworked. The kidney scarring in the study patients was worse than is sometimes seen in morbidly obese people, the study authors noted in a news release from the American Society of Nephrology.

However, there was some good news: Except for one, who went on to develop end-stage kidney failure and needed dialysis, the kidneys of the bodybuilders improved when they stopped taking the steroids, although one patient relapsed upon taking steroids again.

The researchers suspect that rapidly increasing muscle mass puts a lot of strain on kidneys, and the steroids themselves can be toxic to the organs.

"Athletes who use anabolic steroids and the doctors caring for them need to be aware of the potentially serious risks to the kidney," Herlitz said in the news release.

SOURCE: American Society of Nephrology, news release, Oct. 29, 2009