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The longer a woman is overweight, the higher her risk of several cancers, researchers report.
Incentive to avoid excess weight
The study, which followed nearly 74,000 US women, found that the longer a woman carried excess poundage, the greater her risk of breast, endometrial, colon and kidney cancers.
"We've known for a long time that excess weight is important in cancer risk," said Susan Gapstur, vice president of epidemiology for the American Cancer Society.
The cancer society says excess weight contributes to as many as 20 percent of all cancer deaths.
"This study is interesting because it suggests that the number of years you spend being overweight also matters," said Gapstur, who was not involved in the research.
Read: Obesity May Up Death Risk in Older Women With Colon Cancer
And that, she said, should provide further incentive to avoid excess weight gain in the first place.
Currently, about 7 out of 10 adults in the United States are overweight, and more than one-third are obese, according to background notes with the study.
Duration of weight
The new findings are based on data from the Women's Health Initiative – a huge government-sponsored study that followed US women aged 50 to 79. At the outset, researchers measured the women's current weight and height, and asked them to recall their weight at ages 18, 35 and 50.
Over the next dozen years, just over 6,300 women developed a form of cancer that has been linked to obesity, such as breast, ovarian, endometrial, colon, kidney, liver or pancreatic cancer.
It turned out that the risk of developing any of those cancers rose in tandem with the number of years a woman had been overweight.
Read: Weight loss might reduce cancer risk
On average, the study found, the odds rose by 10 percent for every 10 years a woman had been obese. Similarly, they climbed by 7 percent for every decade she'd been overweight.
When the researchers took a closer look, four cancers were clearly connected to the duration of a woman's excess weight: breast, endometrial, colon and kidney. But the findings do not prove excess weight causes these cancers.
Difficult to isolate effects
The findings suggest that when it comes to curbing cancer risk, "obesity and overweight prevention is essential, at any age," said lead researcher Melina Arnold, of the International Agency for Research on Cancer, in Lyon, France.
Why is excess weight related to cancer risk? According to Arnold, it's difficult to isolate effects of obesity, per se, since it often goes hand-in-hand with lifestyle habits and medical conditions that have also been linked to cancer.
Those include smoking, lack of exercise, poor diet and type 2 diabetes, she said.
Her team did, however, account for some of those factors. And there was still a link between cancer risk and the number of years a woman spent in the overweight range.
Read: Obesity linked to two types of cancer
Beyond that, Gapstur said, a body of research supports the idea that excess weight itself influences cancer development.
The bottom line
For instance, extra body fat affects levels of "sex hormones", like oestrogen, she said. This may be one reason excess weight is connected to breast and endometrial cancers.
Plus, Gapstur said, that extra weight can help feed chronic inflammation in the body, which is thought to contribute to a range of diseases, including various cancers.
The bottom line, she said, is simple: "Try to maintain a healthy weight throughout adulthood."
Arnold agreed. "Having a healthy body weight is always beneficial – not only to prevent cancer, but also other diseases associated with [excess weight]," she said.
The study appears in the online journal PLOS Medicine.
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