British researchers on Monday raised doubts about a Olestra, the revolutionary fat substitute hailed as a possible antidote to obesity. Sucrose polyester (SPE), which is used in snacks labeled "fat-free" in the United States, has shown worrying side effects that should be investigated before the product is made widely available, according to a new study. "This study has demonstrated important deleterious effects of SPE which need to be carefully examined before this product is made available for widespread long-term consumption in a broad range of foods," the researchers said.

The six-month study of 76 adults was carried out by researchers in the gastroenterolgy department of Addenbrookes hospital, Cambridge, eastern England, and published in the British Journal of Nutrition. SPE is a tasteless, odorless, non-absorbable, fat-like substance that passes undigested through the intestines, reducing the fattening effect of foods. The study said it also produced "anal leakage." The study found that people who ate SPE had lower levels of vitamin E and of nutrients known as carotenoids. Thirty percent suffered significant bowel upsets. Olestra, which is to all intents and purposes the same as SPE, has been approved by the U.S. Food and Drug Administration, which requires that four vitamins, including vitamin E, are added.

Hunter said he had not expected to discover problems with SPE. "We were surprised to find it upset bowels and that when we measured carotenoids, they were low," he said. "You can't laugh it off." Hunter said the researchers had been asked to test SPE by Anglo-Dutch consumer products group Unilever Plc, which had provided the product used in the experiments.

In the U.S., Olestra is sold by Procter and Gamble and by a subsidiary of PepsiCo.