A no-calorie fat substitute being considered for government approval causes cancer and other health problems in rats, a consumer group said last week.

The Center for Science in the Public Interest said Olestra, a no-calorie fat substitute made by Proctor & Gamble of Cincinnati, had serious side effects when it was fed in high doses to laboratory rats.

The product, designed so the body does not absorb the fat, combines a molecule of table sugar with a fatty acid into a compound called sucrose polyester. A company spokesman said Olestra tastes and cooks like ordinary cooking fats.

The consumer group, in a 19-page letter to Food and Drug Administration Commissioner Frank E. Young, said data from Proctor & Gamble's own studies showed there were serious problems with Olestra.

The group said the studies showed that compared to control rats receiving no Olestra, male rats that got high doses of the fake fat had shorter lives, with 48 percent of the experimental animals dying before the end of the study. Also, the group said the data showed higher levels of cancer in rats given Olestra.

Proctor & Gamble spokesman Don Tassone said the studies had shown no significant differences in the life spans of the high-dose male rats and control animals. He said there were no significant differences in cancers or birth defects between animals that had and had not been given Olestra.