The moment of truth is near for olestra, the fat substitute in which Procter & Gamble has invested 25 years and almost $300 million in research.

An FDA advisory panel, which has reviewed more than 150,000 pages of data on the proposed new food ingredient, has recommended approval of P&G's petition for limited use of olestra.The Food and Drug Administration should approve olestra but should also acknowledge, first, that olestra is very different from other food additives today - that indeed it might more accurately be called a new food rather than a new additive; and second, that the ingestion of olestra, unlike the ingestion of artificial sweeteners, potentially could cause health or nutritional problems if olestra is not used prudently.

Olestra is different: It is basically made of table sugar and vegetable oil, but its molecules are much larger than the molecules in ordinary fats. Because of its large molecules, olestra is not absorbed by the body. Olestra thus adds rich taste and texture to food without adding calories.

Olestra is extremely versatile: It is stable in frozen products (ice cream) and holds up at high temperatures (french fries).

Basically, olestra has the smoothness and the functional properties of fat because it is fat. Other additives - colors, flavors and preservatives - are present in relatively low levels in our food. But olestra has the potential to replace a substantial portion of the fat in our diet.

Olestra's health effects: Critics are claiming that olestra threatens our health because it causes disturbances of the gastrointestinal tract and diminishes the body's absorption of both fat-soluble vitamins and substances such as beta-carotene. There is some truth to these claims, but they need to be put in perspective.

Olestra adds bulk that will not be absorbed to the diet, thus passing through the digestive track unchanged. Olestra's opponents have pounced on olestra's gastrointestinal effect to call it "basically a laxative." That scaremongering piece of hyperbole is unfounded, but the fact remains that more consumers would find that a diet very high in olestra (olestra foods at every meal, for example) would have the effect of any other diet high in nondigestable fiber.

As for olestra's diminishing the body's absorption of valuable vitamins and other substances: Foods interact with each other in both positive and negative ways. In a balanced diet, any losses that occur in one meal are generally made up in another. Having a glass of milk can reduce iron uptake from a typical breakfast by 50 percent, but nutritionists express no concern about this. Similarly, eating foods that contain olestra can block the absorption of some of the fat-soluble vitamins A, D, E and K.

Procter & Gamble proposes, in its regulatory approval petition, to offset this effect by attaching all four fat-soluble vitamins to the olestra molecule. Problem solved! But there is more.

Olestra can negatively impact beta-carotene and related nutrients - nutrients that are being studied as potential cancer inhibitors. For example, the consumption of olestra products (say, 15 to 20 potato chips) as part of a meal that also contains carotene-rich foods such as carrots will block the consumption of around half of the beta-carotene. And beta-carotene, for technical reasons, cannot be added back easily.

There is a sensible scientific and regulatory response to this possible problem of the depleted absorption of one class of nutrients: Approve olestra for one food category at a time to limit the amount consumers will eat and thus reduce the threat to beta carotene.

Olestra's benefits: It is basically good news about what the fun foods of the future might be: chips, fries, cakes, spreads - all the foods we love but with less than half the calories. It's almost like getting something for nothing.