When I interviewed a dietitian recently, she mentioned the modifiable risk factor that’s responsible for more cancers than any other except smoking. I was surprised to hear what it was.  

I unscientifically surveyed my Facebook friends to see if they were more clued in to their cancer risk than I was.

“Less than half of the people we asked were aware that obesity was related to cancer risk at all”
Nigel Brockton
American Institute for Cancer Research

I got a lot of good guesses—air pollution, red meat, environmental toxins, food additives and pesticides, radon, and unprotected sex for women. Wind turbines got a mention, with a wink.

But of 30 people who replied, only three were right. It’s obesity, according to the American Institute for Cancer Research (AICR). And it’s not well recognized as a risk factor for cancer. 

When the AICR surveyed people in 2017, “Less than half of the people we asked were aware that obesity was related to cancer risk at all,” says Nigel Brockton, AICR’s vice president of research. 

Cancers linked with obesity

Researchers have found strong links between excess body fat and these 12 cancers: 

  • esophagus
  • pancreas
  • colon and rectum
  • endometrium
  • kidney
  • postmenopausal breast
  • gallbladder
  • ovarian
  • liver
  • prostate (advanced)
  • stomach 
  • mouth/larynx/pharynx

Brockton points out that for most of these cancers, risk increases with higher body mass index (BMI). In other words, the more overweight you are, the greater your risk.

And for some cancers the news is even more worrisome—even a BMI at the higher end of the healthy range is linked with increased risk. 

The AICR reports that one in four people will develop cancer, and about 40% of cancer cases are preventable. 

Fat cells are active, and that’s a problem

Fat cells aren’t just inactive substances we carry around. Fat is metabolically active in ways that increase our risk for cancer. Brockton notes how fat contributes to cancer risk:

  • It’s linked with chronic inflammation
  • It can raise levels of insulin and other hormones, and these hormones can encourage cancer cells to grow
  • In postmenopausal women, fat is the largest source of estrogen, which is connected with higher rates of breast, endometrial, and ovarian cancers

According to the AICR, excess body fat at the waist, called visceral fat, is more closely linked with increased cancer risk than fat elsewhere in the body. 

Losing weight might help

We don’t yet have good data on how losing weight affects your cancer risk. “We know there’s a lower risk of these 12 types of cancer with a lower body weight, and it’s never too late to make healthy changes,” Brockton says. 

But some research in mice shows that there’s still an elevated cancer risk linked to former obesity.

Brockton says the AICR recommends losing weight if you’re overweight or obese. If you are diagnosed with cancer, you’ll generally have a better outcome if you are at a healthy weight, he says. 

His food choices will likely sound familiar—a cancer-fighting diet is similar to one that helps fight heart disease and dementia. He says good choices are fruit, vegetables, whole grains, beans and legumes. 

He steers us away from fast foods that are high in fats, starches, and sugars as well as red meat, processed meat, and alcohol.

“It’s not that you can never have these things. They just shouldn’t be part of your habitual diet,” he says.

Other factors increase your cancer risk, too

After smoking and obesity, Brockton says the list of top modifiable factors that increase your cancer risk are alcohol use, ultraviolet radiation, physical inactivity, low fruit and vegetable intake, HPV infection, low fiber intake, high processed meat intake, high red meat intake, and viral and bacterial infections. 

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