Muscle-building supplements claim to be the natural alternative to steroids for men looking to bulk up their muscles, but a new study finds reason to be concerned about certain ingredients.

According to new research published in the British Journal of Cancer, men who used pills and powders that contain creatine or androstenedione once a week or more had a 65 percent greater likelihood of developing testicular cancer than men who didn’t use supplements.

Senior author Tongzhang Zheng, who led the study at Yale University, said the risk is even greater among men who began using the supplements before age 25, those who take multiple kinds of supplements and longtime users.

"The observed relationship was strong," said Zheng in a statement. "If you used at earlier age, you had a higher risk. If you used them longer, you had a higher risk. If you used multiple types, you had a higher risk."

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Though still one of the rarer forms of cancer, the rate of testicular cancer rose to 5.9 cases per 100,000 men in 2011, from 3.7 cases in 100,000 in 1975, according to Zheng. Researchers are struggling to explain the 59 percent increase, and there is evidence that some of the ingredients in supplements damage the testes.

The supplements alone accounted for a 65 percent increase in risk; for men who took more than one, that risk jumped to 177 percent; and for men who began using them before age 25 it was 221 percent. Men who had used supplements for more than three years had the highest risk, at 256 percent higher than men who didn’t use them at all.

"Testicular cancer is a very mysterious cancer," Zheng said. "None of the factors we've suspected can explain the increase."

Our bodies naturally make creatine, mostly in our muscles, and it can also be found in foods like fish. It's considered likely safe by the National Institutes of Health, though at high doses the organization warns it can be possibly unsafe, with concerns (but no definitive proof) that it could harm the kidneys, liver and heart. Androstenedione is also naturally made by the bodies of both men and women, and is a precursor chemical to testosterone.

The study’s results are based on interviews with almost 900 men, 356 of whom had developed testicular cancer. The men were asked about their supplement use, but also other possible contributing habits such as family history, smoking habits, exercise routines and past groin injuries.