The investigation, which looked at the association of post-diagnosis weight gain and breast cancer outcomes, was conducted by researchers at Kaiser Permanente in Northern California. Data for the study came from the After Breast Cancer (ABC) Pooling Project, which includes 18,336 breast cancer survivors from four prospective cohorts -- three in the United States and one in Shanghai, China.

Participants were diagnosed with invasive primary breast cancer between 1976 and 2006; their ages ranged from 20 to 83 years. Weight and body mass index (BMI) were assessed 18 to 48 months after diagnosis and were compared with each woman's pre-diagnosis weight.

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

While extreme weight gain occurred in 16 percent of the women overall, 19.4 percent of women with a BMI lower than 25 before diagnosis fell into this category. Breast cancer survivors who gained the most (10 percent or more over their pre-diagnosis weight was considered extreme) were 14 percent more likely to experience a cancer recurrence compared with women whose weight remained stable (within 5 percent of pre-diagnosis weight) following diagnosis.

"Women tend to worry about gaining weight after a breast cancer diagnosis," said Caan. "But it's actually only the larger weight gains that increase the risk of poor outcomes."

Moderate weight gain (a 5 to 10 percent increase post-diagnosis) was also more common among normal or underweight women, but was unrelated to breast cancer outcomes. Only 11.1 percent of women who were overweight or obese before diagnosis had extreme weight gains after their diagnosis.

Women who were leaner to begin with at diagnosis (BMI lower than 25) and who later gained 10 percent or more had a 25 percent higher risk of cancer death and also had a higher risk of recurrence. The risk of overall death was also greater for women whose tumors were ER-positive.

Continued research is needed to understand those women most at risk for extreme weight gain and those whose weight gain puts them at risk for poorer cancer outcomes, according to Caan.

Note: If no author is given, the source is cited instead.

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