The risk likely owes to shared risk factors between the two cancers – of which obesity is one – as well as genetic susceptibility and long-term effects of breast cancer treatment, the study authors said.
"The risk is comparable to what we would see for an initial breast cancer," said Heather Spencer Feigelson, senior investigator at the Kaiser Permanente Colorado Institute for Health Research, in Aurora. "It's just another piece of evidence showing us how [excess weight] is really important."
For the study, the researchers reviewed data from nearly 6,500 women treated at Kaiser Permanente in Colorado and Washington state. Roughly equal percentages were normal weight, overweight and obese.
Women who had an invasive breast cancer had a small, but significantly higher risk for a second cancer as their body mass index (BMI) increased, the study found. (BMI is an estimate of body fat based on height and weight.)
That link was more pronounced when the analysis focused on obesity-related cancers or second breast cancers, the researchers said. The link was strongest for a diagnosis of estrogen receptor-positive second breast cancer.
Of the 14 cancers listed by the International Agency for Research on Cancer as obesity-related, some are common and some are harder to treat, Feigelson said.
The investigators found that 822 (nearly 13%) of the women developed a second cancer after an average follow-up of just over seven years. Of those, nearly 62% were an obesity-related cancer and 40% were a second breast cancer.
The 508 obesity-related cancers included 283 postmenopausal breast cancers; 70 colon/rectal cancers; 68 uterine cancers; 21 ovarian cancers; 23 pancreatic cancers; and 14 kidney cancers. There were fewer than 10 cases each of thyroid, esophageal, gallbladder, multiple myeloma, meningioma, liver and upper stomach cancers.
Though having excess weight appears to increase risk, evidence that shedding pounds and keeping them off reduces risk is limited, because losing weight is hard, Feigelson said.
"The science suggests that, yes, if you lose weight you should reduce your risk, but really the best studies … are studies of women who have gotten bariatric [weight-loss] surgery, and those who lose that large amount of weight do have lower risk of cancer," Feigelson said.
About 55% of all cancers in women occur in those who are overweight or obese.
Feigelson noted there are a lot of breast cancer risk factors that women can't do much about.
"For example, for these second breast cancers or second cancers after breast cancer, one risk factor is treatment, and obviously you're not going to forgo treatment," she said. "But this is something that women actually can have control over. And I think if you're worried about cancer or you're a cancer survivor, having those things that you can control and do something about can be very important to you."
Building some healthy habits into your everyday life can help with cancer prevention. Maintain a healthy body weight, be active and don't sit so much, Feigelson advised.
The findings were recently published in the Journal of the National Cancer Institute.
The researchers noted that one limitation of the study was a lack of diversity, because about 82% of the participants were white women.
Dr. Jennifer Ligibel, director of the Zakim Center for Integrative Therapies and Healthy Living at Dana-Farber Cancer Institute in Boston, reviewed the findings.
"I think this paper really provides a compelling rationale for why thinking about weight loss after being diagnosed with breast cancer is important," she said.
Excess weight has a multipronged effect on a person's body, increasing levels of insulin and other metabolic markers, as well as inflammation, Ligibel said. It probably also depresses the immune system, she added.
In addition, she noted that excess weight raises levels of sex hormones that can also lead to the development of certain types of cancer.
"It's probably not one thing, but the complex interplay between these different systems," said Ligibel, who is part of another study that is investigating whether a weight-loss program as part of breast cancer treatment can lead to lower rates of new cancers.
For many years, she noted, the American Cancer Society has made recommendations about nutrition, physical activity and weight for cancer prevention and for cancer survivors. They include trying to achieve and maintain a healthy weight.
"Unfortunately, [a lot of people have] gained weight as a result of the quarantining and everything else through this last year, but I think that this is a goal that we really need to be thinking about on a societal level," Ligibel said.
The U.S. Centers for Disease Control and Prevention has more on the causes and consequences of adult obesity.
SOURCES: Heather Spencer Feigelson, PhD, cancer epidemiologist and senior investigator, Kaiser Permanente Institute for Health Research, Aurora, Colo.; Jennifer Ligibel, MD, associate professor, medicine, Harvard Medical School, and director, Leonard P. Zakim Center for Integrative Therapies and Healthy Living, Dana-Farber Cancer Institute, Boston; Journal of the National Cancer Institute, April 5, 2021