Did you think starch blockers were too good to be true? Well so does the Food and Drug Administration.

Manufacturers--there may be as many as 100 of the latest have-your-cake-and-pig-out-on-it-too fad--have been out there raking in fast profits.

Some doctors have felt right along that the only thing the blockers would flatten is your wallet.

Yesterday the Food and Drug Administration decided that the starch blockers are to be considered "unapproved new drugs."

Now the manufacturers and distributors have 10 days to get the pills off the market and, if they really want to sell them, manufacturers will have to prove to the FDA that they are both safe and effective.

If you're thinking about stocking up, better give it a second thought.

Whether it is simple flatulence or pancreatic toxicity or just that the so-called anti-starch pills don't work as promised, starch blockers appear destined, experts believe, to eventual oblivion, along with liquid diets and other fat fads.

Meanwhile, backed by saturation advertising, starch blockers have been selling like the hotcakes you're supposed to be able to eat with virtually no caloric consequence.

Starch blockers, under a variety of names, are a concentrate of a derivative from raw legumes--beans--that somehow inhibits the enzyme (beta amylase) that digests starch in humans.

"Listen," says Dr. Aaron Altschul, director of the Georgetown University Diet Manangement Center, "my prediction is that nobody, or practically nobody, will finish the whole 30 pills you got in a bottle."

It is, he says, a matter of simple physiology.

"The way our digestive system works is that most everything is absorbed before it hits the bowel . . . so what gets into the bowel is fiber and maybe some percentage of stuff that isn't absorbed. Then in the bowel you have bacteria which work on this stuff.

"For example, in the case of lactose intolerance inability to digest the sugars in milk , the lactose milk sugar is not absorbed where it normally should be--in the small intestine--and it gets into the bowel, is acted on by bacteria and you get flatulence and serious stomach discomfort . . .

"Okay," he says, "now here you come along and you deliberately block the absorption of starch. What's going to happen to us? It's going to get into the bowel, be worked on by the bacteria in the bowel and cause great flatulence and great stomach discomfort and goodness knows what else."

FDA already is processing a number of pill users' complaints of diarrhea, nausea, vomiting, gastrointestinal distress and stomach pains, and is investigating five admissions to emergency rooms possibly involving use (or abuse) of the pills.

There are ominous hints from scientists who have been doing studies of starch blockers in rats since the early 1970s.

Dr. John Whitaker of the University of California at Davis says that tests of "very high levels" of starch blockers in weanling rats showed that those rats that got the blockers grew quite as well as those without. In other words, they didn't seem to prevent the rats from getting what they needed; they didn't block enough starch, at even the highest levels, to make a difference between the two rat families.

More recently, says Whitaker, he was asked to analyze one of the products on the market.

"In addition to small amounts of amylase inhibitor starch blocker it contains inhibitors of protein digestion. As a consequence of interfering with protein digestion there is a feedback message to the pancreas which provides the digestive enzymes. The message is 'give us more enzymes.' As a result the pancreas is overworked and becomes enlarged."

Whitaker, a professor of food science and technology, also found substances called lactins which "cause two things to happen. First, they prevent the normal absorption of food constituents across the small intestine and second, they cause a sluffing-off of the lining of the small intestine.

"We have well-documented literature with animal experiments and since the starch blockers contain these (lactins)," he says, "we should have some concern about their use in humans.

"We need a great deal more clinical studies with humans to see if these compounds are effective in controlling weight under well-controlled conditions and to look at the potential harmful effects." By declaring the blockers an unapproved new drug, the FDA has required these very studies.

Advertisements for the blockers have been proliferating for the past few weeks in newspapers, magazines and in TV ads of the operators-are-waiting-for-your-call variety. Prices had ranged from about $20 to $50 per 100 at a probable cost to manufacturers of less than a dollar.

The FDA is phoning manufacturers and writing distributors that they have 10 days to get the pills off shelves and out of the mail.

Some of the scientists involved in the initial development of the blockers have defended their usefulness as diet adjuncts, as well as their safety. They will now need to document those claims. Manufacturers suggest that inasmuch as "people have eaten beans for centuries," there is nothing to worry about.

Counters Whitaker, "Yes, but it fails to point out that they've been eating them cooked and in this case the active ingredient is from the raw product."

Dr. Sanford Miller, director of the FDA's Bureau of Foods, notes that lima beans contain cyanide, but nobody suggests that a portion will harm you. "But if you took several tons of limas you could get a concentration with which you could kill yourself."

FDA officials now have moved into high gear to minimize the impact of this "unusually heavy" nationwide campaign. FDA spokesman Bruce Brown, whose FDA-designated specialty is "quackery," says that usually the FDA regards the ever-present fad diets as "a little bit of foolishness," but "the blockers were being advertised as promoting a clearly experimental medical theory." Brown says there is especial concern for diabetics or anybody taking any regular medicine. These people should not take any of the pills they may have already purchased.

"There is work on all sorts of blockers," says Altschul, "but they are being tested, not marketed. I don't understand how they got away with this. Losing weight is a tough battle, so these guys come along and they mislead the poor suckers looking for the easy way. It is trespassing on people's desires, their fervent wish to do it with a pill . . ."