Obesity and Cancer: What's the Link?

While much of your cancer risk is out of your hands, you may be able to lower your odds by keeping your weight under control.

Doctor measuring waist of obese patient

It’s widely known that rates of overweight and obesity are on the rise in the United States, affecting 71.6 percent of adults as of 2015 to 2016. Meanwhile, diagnoses of most obesity-linked cancers have jumped, as well. Rates of these diagnoses rose by 7 percent from 2005 to 2014, according to the Centers for Disease Control and Prevention (CDC). At the same time, rates of cancers not related to obesity or being overweight dropped by 13 percent.

What’s going on—and what’s the connection?

“Excess body fatness is a known risk factor for 13 cancers,” says Alpa Patel, PhD, an epidemiologist with the American Cancer Society (ACS). These include cancers of the stomach, pancreas, ovaries, colon and thyroid as well as post-menopausal breast cancer and one type of brain cancer.

All told, nearly 8 percent of all cancers and close to 7 percent of all cancer deaths in the U.S. are attributable to excess body weight, according to research from the ACS. For comparison’s sake, excess weight ranks second among modifiable risk factors for cancer, after smoking and just ahead of alcohol consumption.

What is my BMI?

Under current definitions, body mass index, or BMI, is the most common measure used to define weight status.

You can calculate your own BMI by dividing your weight (in pounds) by the square of your height (in inches) and multiplying the result by 703. For metric values, the equation is simpler: divide your weight in kilograms by the square of your height in meters. Online tables and calculators are also available if you’d like to avoid the math.

For adults age 20 years or more, a result from 18.5 to 24.9 indicates a normal-weight BMI. Anything over 24.9 is considered overweight and values of 30 or greater indicate obesity.

The obesity-cancer connection

How closely obesity is linked to higher cancer risk varies with the specific type of cancer. For cancer of the endometrium, or uterine lining, obesity or overweight can quadruple risk and being extremely obese can increase risk seven-fold. Excess body weight roughly doubles the risk for some stomach, esophageal and kidney cancers.

The increased risk that high BMI carries for some other cancers is more modest. In postmenopausal women, for example, a BMI increase of 5—a difference of roughly 30 pounds for a woman who is 5 feet, 5 inches tall—translates into a 12 percent increased breast cancer risk. Being obese, however, confers a higher breast cancer risk in this population as well as in men.

There’s one apparent outlier among obesity-related cancers: Rates of colorectal cancer have dropped 23 percent between 2005 and 2014. But the major reason for this drop isn’t because colorectal cancer and obesity suddenly became unlinked, but rather as a result of enormously successful colorectal cancer screening programs that have helped doctors catch many cases of the disease in its earliest stages.

How body weight may influence cancer risk

How can excess body weight raise cancer risk? Patel cites several possible explanations. In each case, she says, the effects can disrupt the controls that ordinarily put the brakes on cell growth.

Inflammation: For one, the chronic, low-level, body-wide inflammation that often accompanies obesity can trigger DNA damage. If the damaged DNA plays a role in cell growth, the affected cells can begin to grow unchecked. This uncontrolled growth can in turn be the start of cancer.

Derailed immune system: The immune system might normally be able to deal with small uprisings of uncontrolled cell expansion. But an immune system derailed by the effects of obesity may be less effective in this battle and may become unable to defuse the situation.

Hormones: The extra fat tissue associated with obesity produces excess hormones, including estrogen, which is closely linked to breast, endometrial and ovarian cancers. Obesity and problems regulating blood sugar also often go hand in hand. High levels of insulin—which the body uses to try to control blood sugar levels—may stimulate some cancers to develop, including kidney and colon cancers.

More younger adults are affected

Over the past 40 years, the escalating obesity epidemic has exposed members of younger generations to more excess body fat over their lifetimes than in previous decades. This could help explain why cancers associated with obesity are on the rise among younger adults in the U.S., according to a February 2019 analysis from the ACS.

The study, published in The Lancet Public Health, examined data compiled between 1995 and 2014 on 30 different forms of cancer, including 12 types linked to obesity. The researchers analyzed incidence of invasive cancers among adults between 25 and 84 years old from 25 state registries which covered 67 percent of the U.S. population.

They identified a significant increase in six obesity-related cancers among younger adults while rates of eight non-obesity-related cancers—including diseases linked to smoking and infections—declined or held steady.

For example, the analysis found that the average annual increase for pancreatic cancer was highest among younger adults between 25 and 29 years old compared to older age groups. The average annual increases for colorectal, endometrial, gallbladder and kidney cancers were also largest among adults 25 to 29. And incidence of multiple myeloma was highest among adults between 30 and 34 years old. These findings are surprising given that the risk for these forms of cancer usually increases as people age.

The researchers suggest that the rise in obesity among American youth could help explain why more young people are being diagnosed with these types of cancers, noting that obesity surged by more than 100 percent among kids and teens between 1980 and 2014. They point out, however, that this association doesn’t prove a cause and effect relationship, since other variables—such as sedentary behavior, poor diet (not eating enough fresh fruits and vegetables and eating too much red or processed meats and sugar), diabetes, gallstones and inflammatory bowel disease—may have also played a role.

Prevention and risk reduction

If you calculate your BMI and get a number that suggests you’re carrying excess weight, you might be wondering if losing weight will help reduce cancer-related risks. Patel says the answer isn’t clear, but that evidence suggests some benefit of weight loss. Issues with studies so far, she says, are that most people don’t lose weight and then keep it off, so it’s hard to pin down a consistent weight-loss effect.

Some findings indicate that people with obesity who have bariatric surgery—which reduces stomach size to induce weight loss—have lower cancer risk compared with those who do not have the surgery. But that scenario may not hold for all cancers.

One large study found that women who were already overweight or obese didn’t see a change in their breast cancer risk if they lost weight. What may have an impact is avoiding weight gain in the first place: Women who were at normal weight when the study began but gained more than 5 percent of their body weight saw their risk of breast cancer increase.

Meanwhile, more research is looking at the relationship between weight loss and cancer risk, with some evidence suggesting that weight loss may reduce the risk of post-menopausal breast cancer and certain aggressive forms of prostate cancer, among other cancers.

Where the fat builds up seems to play some part, too. Fat accumulated around the belly (as opposed to the hips, for example) specifically seems to be linked to increased risk for some cancers, including colorectal cancers.

What you can do

The research may not be clear cut, but it doesn’t mean you should throw up your hands and not keep your weight under control or try to drop pounds if your BMI climbs upward. “Given the negative health effects of excess body fatness,” says Patel, “people should strive to lose weight if they are overweight or obese.”

And losing weight isn’t the only goal when it comes to lowering one’s cancer risk. Lifestyle changes, such as to diet and activity, can also be important, says Patel. These changes can include being more physically active, limiting sedentary behavior, and eating a healthy diet that includes fruits and vegetables, lean proteins and whole grains.

Using a tracking app like Sharecare (available for iOS and Android) can help you keep tabs on your weight, diet and daily activity.

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