TUESDAY, April 5 (HealthDay News) -- Extreme weight gain increases breast cancer survivors' risk of cancer recurrence and death, but moderate weight gain has no effect on outcomes, a new study finds.

Researchers analyzed data from 18,336 breast cancer survivors in the United States and China, ages 20 to 83, whose weight was assessed 18 to 48 months after diagnosis and compared with their pre-diagnosis weight.

Extreme weight gain (10 percent or greater than pre-diagnosis weight) occurred in 16 percent of the women. They were 14 percent more likely to suffer a cancer recurrence than women whose weight remained stable (within 5 percent of their pre-diagnosis weight).

Among the women with extreme weight gain, 19.4 percent had a body mass index (BMI) lower than 25, which is considered normal weight, before their diagnosis of breast cancer. These women had a 25 percent increased risk of cancer death and also had an increased risk of cancer recurrence.

The study is to be presented Tuesday at the annual meeting of the American Association for Cancer Research, in Orlando, Fla. Research presented at meetings is considered preliminary until it is published in a peer-reviewed journal.

"Most women are not gaining a large amount of weight following breast cancer diagnosis. But our analysis of the pooled data showed an association with poorer outcomes overall for those who do," lead researcher Bette Caan, senior research scientist at the Kaiser Permanente Division of Research, said in an AACR news release.

Further research is needed to learn why extreme weight gain puts breast cancer survivors at increased risk for worse outcomes, Caan added.

More information

The U.S. National Cancer Institute has more about breast cancer.