A University of Cincinnati medical team yesterday described a substitute for fat that contains no calories and is indistinguishable from the real thing, saying that it had helped very obese men lose almost half a pound a day.

But the experiment was conducted on only 10 men who used the substitute for only 20 days. The principle researcher, Dr. Charles J. Glueck, said it could be years before the substitute was available on the market.

The fat substitute, called sucrose polyester, can be used in milkshakes, salad dressing and a spread that looks and tastes like margarine. Researchers said it could also be used as a cooking oil.

"Patients literally can have their cake and eat it, too," Glueck said.

Sucrose polyester, which is not related to the polyester in clothes, consists of a sucrose molecule attached to a group of eight fatty acids. Glueck said the body does not know what to do with this peculiar combination, so the fat passes through the body undigested.

The medical team tested sucrose polyester on 10 men who were obese but did not have glandular disorders. For 20 days they were fed regular foods and for 20 days foods with the fat substitute. Some nutritionists--but neither the subjects nor the medical researchers--knew when the sucrose polyester was in the food.

In an article in the American Journal of Clinical Nutrition, Glueck and his colleagues said that eight of the 10 subjects could not tell when they were eating foods with fat and when with sucrose polyester.

The men were allowed to eat snacks--brownies, potato chips, chocolate-chip cookies and jelly beans. However, they did not eat more snacks when they were on the substitute than when they were on fats.

In the special diet period, nutritionists each day substituted about two ounces of sucrose polyester for other fats in each person's food. As a result, the number of calories each person consumed at meals dropped 30 percent, and the men lost an average of four-tenths of a pound per day.

Glueck said the substitute would lower blood cholesterol and triglycerides, thus reducing the chance of heart disease. He said that the only known side effect is a slight drop in vitamins A and E in the body, but vitamin supplements were not necessary with any of the subjects. Glueck said sucrose polyester should be available only by prescription.

Experiments with many more people and lasting several months are planned. But Glueck said it could be years before the Food and Drug Administration approves the substitute.

Procter & Gamble holds the patent for sucrose polyester, which researchers have been experimenting with for seven years. Most of the money for the experiments in Cincinnati came from the National Institutes of Health.

Other experts said the research was promising but noted that other fat substitutes are available. Dr. George L. Blackburn of the Harvard Medical School said sucrose polyester probably has a future, but he emphasized that it is not a panacea.

"It's an interesting compound, and it appears to be safe," said Gilbert A. Lebeille, director of nutrition and health sciences for General Foods Corp. in White Plains, N.Y. But Lebeille added that sucrose polyester might not be easily substituted for cooking oils and its extensive use could cause diarrhea.