Taking high doses of vitamins seems to increase the risk of prostate cancer, according to a new study.

The research at the National Cancer Institute followed nearly 300,000 men for five years. In that time, just over 10,000 -- about 3.5 percent -- were diagnosed with prostate cancer.

Dr. Karla Lawson said that men were about equally likely to have localized prostate cancer, regardless of vitamin intake. But those who took high doses -- such as taking a multivitamin more than seven times a week -- were almost twice as likely to have advanced or fatal cancers.

Researchers speculated that high-dose vitamins may have little effect until a tumor appears but then could spur its growth.

The association was strongest in men with a family history of prostate cancer and men who also took selenium, beta-carotene, or zinc supplements.