Among the witnesses at the hearing will be Dr. Tod Cooperman, president of ConsumerLab.com, a company that has tested over 2,000 dietary supplements made by more than 300 manufacturers and has found that one in four have quality problems. According to Dr. Cooperman’s written testimony, the most common problems are supplements that lack adequate quantities of the indicated ingredients and those contaminated with heavy metals.

Travis T. Tygart, chief executive of the United States Anti-Doping Agency, wrote a letter to the committee saying that some athletes have been rendered ineligible for international competitions because they took supplements that contained steroids not listed on the products’ labels. There are thousands of supplements available for sale that contain steroids or other harmful ingredients, he wrote.

“The F.D.A. is operating in a regulatory environment that is simply too burdensome to allow for effective post-market regulation of these products,” Mr. Tygart wrote.

Half of the nation’s adult population takes vitamin supplements regularly, and about a quarter take herbal supplements at least occasionally. Annual sales are about $25 billion a year, and the growing popularity has led to an increasing number of imported supplements spiked with illegal drugs.

In 1994, Congress passed legislation that allowed supplement makers to sell products without first getting approval from the F.D.A. for their ingredients or for basic health claims. But scientific organizations have warned repeatedly since then that the F.D.A. should do more to ensure that the supplements are safe and that their health claims are substantiated.

In recent years, a vast majority of supplement suppliers have located overseas — principally in China. Nearly all of the vitamin C and many other supplements consumed in the United States are made from ingredients made in Chinese plants. Those plants are almost never inspected by the F.D.A. because the agency is not required to do so, has little money to do so and does not view the plants as particularly risky.

Mr. Mister said supplement sellers tested ingredients before using them, but he agreed that testing could not ensure quality. He called on Congress to provide the F.D.A. with more money to inspect foreign and domestic supplement plants. “I think you’ll see more and more inspections,” Mr. Mister said.