Manufacturers and distributors of a new diet aid, "starch blockers," have become embroiled in a controversy with the Food and Drug Administration over whether the product should be marketed.

The FDA, saying that the product is an "unapproved new drug," has written companies threatening possible regulatory action unless they stop distributing it. But the companies say that the product is a food and not a drug, and hence is not subject to the same standard of approval. They have filed suits against the FDA.

Starch blockers, which have been on the market for about a year, are usually made of a concentrate extracted from beans that inhibits the digestion of potatoes, spaghetti, bread and other high-starch foods. Because these foods are largely undigested and pass through the system, the consumer is supposed to lose weight.

The FDA says that it is concerned about the safety of the product, which it says is suspected of causing nausea, vomiting, stomach cramps, gas, and bowel blockage in some people. The agency wants further tests conducted to prove whether it is safe and effective.

One of the companies that is continuing to manufacture and distribute starch blockers is the General Nutrition Center, a nationwide chain of health food stores with about a dozen outlets in the Washington area.

David B. Shakarian, chairman of the board of GNC, said he understands the letter from the FDA to be "just a request, not an order." He said his researchers "assure us the product is absolutely safe." He added that his company does not want to "deprive the public of a product they want and need."

The government agency can get a court order to enforce its position, but has declined to do so because the agency is not convinced that health hazards warrant it, according to Bruce Brown, an FDA spokesman.

On July 1, after complaints from consumers and doctors, the FDA began sending letters to manufacturers and distributors informing them that because the makers claim "that starch blockers affect the digestive function of the body, this causes them to be considered a drug . . . . "

The FDA letter said that the product must meet FDA approval. The letter also said that so far the agency is not aware of any "substantive scientific evidence" that proves or disproves the product's safety and effectiveness.

Brown said that several retailers of vitamins and health foods have continued to market the product. The FDA letter was sent only to manufacturers (about 12 nationwide) and distributors and not directly to retailers.

At the GNC store in Georgetown, 3222 M St. NW, as in other GNC area stores, a bottle of 30 starch-blocker tablets--marketed under the GNC brand name, "Advantage"--sells for $9.95. The bottle features a Pac-Manlike creature gobbling up letters on the label.

Ronald Camp, manager of Vita Foods, a health food store selling starch blockers under the trade name STA-TRIM, at 1010 F St. NW, said he was aware of the FDA letter, but said he had received a letter from the STA-TRIM distributors telling him to keep selling it until further notice. Camp said he doesn't believe that starch blockers are dangerous. "If I listen to everything they the FDA say, I would have closed my door years ago," he said.

Elizabeth Rappaport, manager of the Good Diet Shop at 1226 H St., which also sells STA-TRIM, says the product is selling slowly. She said she is "not thrilled about selling a product involved in so much controversy."