Although most women do not gain a large amount of weight after being diagnosed with breast cancer, those who do are at risk for poorer outcomes.

Data presented at the recent annual meeting of the American Association for Cancer Research, held in Orlando, Florida, showed that breast cancer survivors who gained the most weight (10% or more over prediagnosis weight was considered extreme) were 14% more likely to experience a cancer recurrence compared with women whose weight remained stable (within 5% of prediagnosis weight).

A team led by Bette Caan, DrPH, a senior research scientist at the Kaiser Permanente Division of Research (Oakland, California), pooled data from 18,336 breast cancer survivors from three prospective cohorts in the United States and one in Shanghai, China. The participants were aged 20 to 83 years when diagnosed with invasive primary breast cancer between 1976 and 2006. Their prediagnosis weight was compared to their weight and body mass index (BMI) 18 to 48 months after diagnosis.

Overall, 16% of the women had extreme weight gain. This category included 19.4% of women whose prediagnosis BMI was lower than 25 kg/m2. These lean women had a 25% higher risk of cancer-related death as well as a higher risk of cancer recurrence if they had a weight gain of 10% or more of their prediagnosis weight after diagnosis.

Moderate weight gain postdiagnosis—an increase of 5% to 10%—was also more common among normal or underweight women, but did not affect breast cancer outcomes.

“Women tend to worry about gaining weight after a breast cancer diagnosis, but it’s actually only the larger weight gains that increase the risk of poor outcomes,” commented Dr. Caan in a statement describing her group’s findings.