Updated on April 1, 2024.
For decades, rates of overweight and obesity have been on the rise in the United States, affecting 71.6 percent of adults as of 2015 to 2016. Meanwhile, diagnoses of cancers related to obesity 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 decreased by 13 percent.
Cancers linked to obesity and overweight
“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 one type of brain cancer and breast cancer in people who are post-menopausal and have gone a full year without a menstrual cycle.
All told, nearly 8 percent of all cancers and close to 7 percent of all cancer deaths in the U.S. are linked to excess body weight, according to research from the ACS. As a comparison, excess weight ranks second among risk factors for cancer that can potentially be changed, after smoking and just ahead of alcohol consumption.
How to calculate 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 for calculating BMI.
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 (the tube that leads from the mouth to the stomach) and kidney cancers.
The increased risk that high BMI carries for some other cancers is more modest. In postmenopausal people, for example, a BMI increase of 5—a difference of roughly 30 pounds for someone who is 5 feet, 5 inches tall—translates into a 12 percent increased breast cancer risk. Being obese, however, increases breast cancer risk even more.
There’s one apparent exception among obesity-related cancers: Rates of colorectal cancer (a digestive cancer that affects the large intesting and the rectum, the last several inches of the large intestine) dropped 23 percent between 2005 and 2014. But the major reason for this decrease isn’t because colorectal cancer and obesity suddenly became unrelated, but rather because 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 stop or decrease cell growth.
Inflammation: For one, the chronic, low-level, body-wide inflammation that often accompanies obesity can damage DNA, (deoxyribonucleic acid, the genetic material inside cells). If the damaged DNA plays a role in cell growth, the affected cells can begin to grow unchecked. This uncontrolled growth can lead to cancer.
Impaired immune system: The immune system might normally be able to deal with small amounts of uncontrolled cell growth. But obesity can impair the immune system, making it less effective against cells that are growing uncontrollably.
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 together. High levels of insulin—which regulates blood sugar levels—may stimulate some cancers to develop, including kidney and colon cancers.
More younger adults are affected
As a results of the obesity epidemic over the past 40 years, younger generations have been living with more years of excess body fat over their lifetimes than in previous decades. This could help explain why cancers associated with obesity are increasing 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 new diagnoses of cancers that have spread beyond the original tumor among adults between 25 and 84 years old. Data came from 25 states and 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—decreased or stayed the same.
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 (cancer of white blood cells in the immune system) was highest among adults between 30 and 34 years old. These findings were surprising because the risk for these forms of cancer usually increases as people age.
The researchers suggest that the rise in obesity among youth in the U.S. 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 children and teenagers between 1980 and 2014. The researchers point out, however, that this association doesn’t prove that obesity causes these cancers, since other things—such as sedentary behavior, unhealthy diet (not eating enough fresh fruits and vegetables and eating too much red or processed meats and sugar), diabetes, gallstones and inflammatory bowel disease (inflammation of the digestive tract, especially the small and/or large intestine)—may have also played a role.
Prevention and risk reduction
Patel says research suggests some benefit of weight loss in reducing cancer risk among people who are overweight or obese. Issues with studies so far, she says, are that most people who lose weight don't keep it off. So it’s hard to make a clear connection and confirm whether weight loss really can reduce cancer risk.
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 connection may not hold for all cancers.
One large study found that women who were already overweight or obese didn’t experience 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 healthy weight when the study began but gained more than 5 percent of their body weight experienced an increase in their risk of breast cancer.
Meanwhile, more research is looking at the relationship between weight loss and cancer risk, with some results suggesting that weight loss may reduce the risk of post-menopausal breast cancer and certain fast-growing forms of cancer of the prostate (a male reproductive organ found below the bladder that makes semen), among other cancers.
Where fat accumulates eems to play some part, too. Fat located 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
Beyond the connection with certain types of cancer, there are plenty of benefits to managing your weight.
“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 as you are able, limiting sedentary behavior, and eating a healthy diet that includes whole fruits and vegetables, lean proteins and whole grains, and minimizes salt, added sugar, and saturated fat (which is solid at room temperature, like butter).
Tracking your weight, diet and daily activity can help you build healthier habits. Many apps are available that can help you get started.
