What to know about obesity and cancer

Being overweight or obese is common in the United States. About 2 in 5 adults are considered to be obese, according to the Centers for Disease Control and Prevention (CDC). An estimated 1 in 5 children between ages 2 and 19 are also obese. Obesity is a chronic condition in which a person has too much body fat.

Meanwhile, diagnoses of most cancers linked to obesity or being overweight have jumped. The CDC reports that cases rose by 6 percent from 2005 to 2014. At the same time, rates of cancers not related to obesity or being overweight dropped 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:

• Stomach cancer
• Pancreatic cancer
• Ovarian cancer
• Colorectal cancer
• Thyroid cancer
• Postmenopausal breast cancer
• A certain type of brain cancer

All told, according to the ACS, excess body weight is thought to be responsible for:

• About 11 percent of all cancers in women
• About 5 percent of cancers in men
• About 7 percent of all cancer deaths

In the 2023 Cancer Progress Report published by the American Association for Cancer Research (AACR), excess weight ranks second among all modifiable risk factors for cancer. It comes after smoking and just ahead of alcohol consumption.

The basics of BMI

Under current definitions, body mass index (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. (The square is what you get when you multiply your height in inches by itself. if you’re 70 inches tall, it’s 70 times 70, or 4,900.)

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 aged 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 and the level of overweight or obesity. For cancer of the endometrium, or uterine lining, obesity or overweight can quadruple risk. Being extremely obese can increase risk seven-fold. Excess body weight roughly doubles the risk for some stomach, liver, 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 five—a difference of roughly 30 pounds for a woman who is 5 feet, 5 inches tall—translates into a 20 percent increased breast cancer risk. Being obese, however, increases breast cancer risk even more.

There’s one apparent outlier among obesity-related cancers: colorectal cancer. Though they have risen in recent years, rates have dropped overall since 1998. But the major reason for this drop isn’t because colorectal cancer and obesity suddenly became unrelated. Rather, it’s the result of enormously successful colorectal cancer screening programs that have helped healthcare providers 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? There are several possible explanations. In each case, the effects can disrupt the controls that ordinarily stop or limit 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 expansion. 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. This includes 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 result of a 40-year-long obesity epidemic, 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 linked to 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 annual increase of multiple myeloma (cancer of white blood cells in the immune system) 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 youth in the U.S. could help explain why more young people are being diagnosed with these types of cancers. They note that obesity surged by more than 100 percent among kids and teens between 1980 and 2014.

The researchers also point out, however, that this association doesn’t prove that obesity causes these cancers. Other things may have also played a role, including:

• Sedentary behavior
• Unhealthy diet, or not eating enough fresh fruits and vegetables and eating too much red or processed meats and sugar
• Diabetes
• Gallstones
• Inflammatory bowel disease

In another large 2023 analysis, this one published in Digestive Diseases and Sciences, researchers looked at almost 3 million adults with four different obesity-related cancers (colorectal, non-colorectal gastrointestinal, uterine, and breast). They confirmed earlier research, finding a link between younger ages, higher body fat, and early-onset gastrointestinal and uterine cancers.

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.

One large 2019 study published in JNCI Cancer Spectrum found that postmenopausal women who intentionally lost more than 5 percent of their body weight had a lower risk of developing obesity-related cancers, particularly endometrial cancer.

A 2020 study published in the Journal of the National Cancer Institute found that women who were already overweight or obese didn’t see a change in their breast cancer risk if they lost weight. But what may have an impact is keeping the weight off in the long-term. The ACS noted that research points to lowered risk of breast cancer in women who can sustain their weight loss.

Some findings also indicate that people with obesity who have bariatric surgery—which reduces stomach size to induce weight loss—have lower overall cancer risk and lowered risk of both hormone-related cancers and obesity-related cancers.

Where the fat builds up seems 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. It should also minimize 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.