Women who develop breat cancer apparently live longer if they keep their weight down, a group of Canadian doctors have found.

Breast cancer is the most common female cancer.One women in 11 develops it.

Of more than 700 Toronto women who did, and have been followed for eight to 10 years, 60 percent of those weighing under 140 pounds lived for five years. Fifthy percent have lived for 10 years.

Of women who weighed more than 140, 49 percent lived for five years and 39 percent lived for 10 years.

The difference is marked in this disease, where progress in survival rates has been made only slowly, said Dr. Norman Boyd. He reported the results at Toronto's Princess Margaret Hospital to the American Federation for Clinical Research, meeting here yesterday.

There may be a way of compensating for the excess weight in some cases, Boyd also said.

Doctors destroy the ovaries of some breast cancer patients, because the cancer's development apparently is linked to the body's output of sex hormones. c

When the overweight women's ovaries were destroyed by radiation, the women survived as long as the thinner women, on the average.

Evidently, weight not only affects survival, but also "has something to do with the hormonal machinery," Boyd said. When more is learned about these relationships, more cancers may be cured and perhaps prevented, he said.

Meanwhile, he said, the new data suggest, though they do not yet prove, that weight reduction might improve breast cancer survivial.

Oddly, he said, weighing sharply under or sharply over 140 made only a slight difference in the cancer victims' survival rate.

Why was 140 pounds the cutoff point?

"We don't know," Boyd said, just as "we don't know breast cancer cause, though we do have some clues."

He said that studies suggest that excess weight increases the risk of getting breast cancer i the first place.

But an authority on cancer epidemiology, Dr. Ernst Wynder of the American Health Foundation, denied this. He said studies of women in several countries are showing that devloping breast cancer has no relation to weight or height.

What does seem to be true, he said, is that more women in nations with high fat intake, such as the United States and Canada, develope breast cancer than do women in nations were low-fat diets are common.

In Japan, traditionally a country with low fat consumption, the consumption of high-fat foods has almost doubled in the past, increasingly prosperous, decade. Breast cancer incidence has been doubling, too.