Women who gain a lot of weight after being diagnosed with breast cancer are more likely to have a recurrance of cancer, and some of them are more likely to die from it, than women whose weight remains stable, says an Oakland Kaiser Permanente researcher.

It’s unclear why, exactly, there is a connection between weight gain and women’s risk of death and cancer recurrance, said Bette Caan, lead researcher in a study that looked at more than 15,000 women in the United States and China. Caan presented results of her study at a meeting of the American Association of Cancer Research in Orlando, Fla., this morning.

Sixteen percent of women in the study gained a large amount of weight — more than 10 percent of their pre-diagnosis weight — in the two to four years after they were diagnosed and treated for breast cancer. Those women were 14 percent more likley to have a recurrance of cancer than women who didn’t gain much weight.

Women who were slim to begin with were more likely to gain a lot of weight compared to women who were already overweight or obese. The leaner women who gained a lot of weight had a 25 percent higher risk of death from breast cancer than women whose weight remained stable.

Curiously, women who started off heavy did not have an increased risk of death from breast cancer if they gained a lot of weight.

What’s reassuring, Caan said, is that women who gain a moderate amount of weight — less than 10 percent more than their pre-diagnosis weight — don’t appear to have any increased risk.

“Most women want to beat themselves up if they gain five pounds,” Caan said. “But for most women that weight gain is not increasing their risk. It’s only these much larger weight gains that are a problem.”