Why Does Obesity Increase the Risk for Cancer? Explaining the Link

Weight — and having a BMI over 30 — is mostly determined by things outside someone’s control. But if you’re living in a larger body, it’s helpful to know how this changes your risk for certain conditions, like diabetes and heart disease. 

People who have a BMI over 30 also have an increased chance of developing certain types of cancers. According to the American Cancer Society, 11% of cancers in women and 5% of cancers in men are linked to increased body weight. But this isn’t true for all types of cancer. And other factors, like your family history, also have a role in determining your cancer risk. 

So, why is excess body weight linked to an increased cancer risk? And what types of cancer are linked to larger body sizes? Here’s what the latest evidence says. 

Why does body fat increase cancer risk?

The link between body weight and cancer is a complicated one. And we don’t have all the answers yet. Increased body fat raises the risk of some cancers but not others. Even the age at which you gain weight can change your risk. For example, a BMI over 30 is linked to an increased risk of breast cancer, but only after menopause. And a larger body size as a teenager seems to increase the risk of developing ovarian cancer more than a larger body size in adulthood.

That said, scientists have identified some potential ways a BMI over 30 increases cancer risk including:

• Widespread growth of cells, and the blood vessels that supply them 

• Increased overall body inflammation

• Changes in hormones, like estrogen, insulin, and leptin

• Dysregulation of other molecules that balance cell growth and are related to body fat

• Decreased physical activity (that can sometimes be linked to a larger body size) may promote cellular changes that are favorable for cancer growth

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A larger body size also seems to play a role in allowing cancer cells to spread more easily to other parts of the body than in people without extra body weight. Scientists haven’t figured out the reason for this yet either.

What types of cancer are linked to body fat?

A group of researchers studied the risk between increased body fat and cancer, and found a connection between body weight and about 13 different types of cancers. 

It’s important to point out that these are only associations. Most of the studies linking body weight to cancer risk are observational studies. This means the studies can find a link between the two, but they can’t determine cause and effect. There could be factors other than increased body weight that increase the risk for cancer. 

The following are the main types of cancer linked to having a BMI over 30.

Endometrial cancer

Endometrial cancer is cancer of the lining of the uterus. Up to 50% of cases of endometrial cancer are attributed to having a BMI over 30. People with a BMI of 40 or higher — which some may consider “severe obesity” — are seven times as likely to develop endometrial cancer. This may be because a larger body size reduces your chances of releasing an egg from your ovary and having regular menstrual cycles. Also, not ovulating is a known risk factor for endometrial cancer, regardless of body size. 

Colon cancer 

Colon and rectal cancer are about 30% more likely to occur in people with a larger body size. There are several reasons why this might be. It may be related to hormonal changes and increased inflammation associated with having a BMI over 30. Plus, a higher body weight can increase the risk for diabetes. This can lead to higher levels of insulin in the blood and increase the risk for colon cancer. 

Esophageal and other cancers linked to the GI tract

Several cancers throughout the gastrointestinal (GI) tract have been linked to excessive body weight. Up to 30% of esophageal cancers in men are attributed to excess body weight. Weight is also linked to a specific type of stomach cancer, called gastric cardia cancer. 

Cancer of the gallbladder is also seen more commonly in people who are considered obese. Having a larger body size is known to cause gallstones. And people with gallstones, regardless of body size, are more likely to develop cancer of the gallbladder.

Liver cancer

Up to 50% of liver cancer cases are attributed to excess body weight. People who are classified as overweight or obese are more likely to have diabetes and nonalcoholic fatty liver disease (NAFLD). Both conditions also increase the risk of liver cancer. 

Kidney cancer

Kidney cancer is twice as likely to develop in people with a BMI over 25. In addition to excess body weight, kidney cancer is linked to high levels of insulin, diabetes, and high blood pressure.

Other cancers

Several other types of cancer are associated with a larger body size as well. These include:

• Multiple myeloma (a cancer of blood cells)

• Meningioma (a type of brain tumor)

• Pancreatic cancer

• Thyroid cancer

• Breast cancer, but only after menopause

• Ovarian cancer

Common misconceptions about obesity and cancer

A lot of the research on body size and cancer uses the BMI to classify people’s weight. BMI is a public health tool that categorizes people as “normal weight,” “overweight,” or “obese.” It takes into account someone’s height and weight. While BMI categories are meant to classify body fatness, they aren’t perfect. They’re also not good indicators of individual health. For example, a bodybuilder with a lot of muscle and minimal body fat may be categorized as obese based only on their BMI. 

These problems with BMI also complicate clinical research. They make it challenging to really figure out the link between body size and cancer risk. 

Plus, much of the data that links larger body sizes with cancer comes from observational studies. This means scientists can’t say for sure that having excess body fat is the reason why there are more cases of cancer in people who are considered obese. Things like increased inflammation, changes in hormones, and higher rates of diabetes may play more of a role in the development of cancer than body weight alone. 

There’s also concern that people with larger body sizes are less likely to get screened for cancer. The frequency of cancer screening varies by gender, cancer type, and race. But this can lead to signs of pre-cancer being missed — like polyps that later develop into colon cancer, or cervical dysplasia that can become cervical cancer. If these things are missed in people with larger body sizes, it increases their risk of being diagnosed with cancer.

So until we know more, it’s unfair to say that having increased body weight is a direct cause of cancer.

Does weight loss reduce cancer risk?

Just like scientists aren’t exactly sure how having a larger body size causes cancer, they aren’t sure if weight loss reduces this risk. Studies on this topic are complicated, and the results have been mixed. 

For example, the Women’s Health Initiative didn’t find a reduced risk of breast cancer when women who were considered obese lost weight. But a larger study found a reduction in breast cancer risk in women over the age of 50 who lost weight. 

Even though there’s a lot more to learn about weight loss and reduced cancer risk, experts do know some things:

• Losing weight leads to a decrease in insulin and estrogen, two hormones that promote the growth of some cancers. 

• People who lose excess body weight likely lower their risk of developing endometrial, colon, and prostate cancers. 

• Weight reduction in people with larger body sizes has many health benefits, like reducing the risk of diabetes, stroke, and heart disease. 

• Regardless of your weight, staying active and eating a nutritious diet helps reduce your cancer risk. 

Focusing on your overall health journey will benefit you in your cancer risk and beyond. 

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Written by:

Nicole Andonian, MD

Nicole Andonian, MD, is a private practice anesthesiologist in Orange County, California. Her focus is in general anesthesia, regional anesthesia, and acute pain management.

Edited by:

Katie E. Golden, MD

Katie E. Golden, MD, is a board-certified emergency medicine physician and a medical editor at GoodRx.

Reviewed by:

Mandy Armitage, MD

Mandy Armitage, MD, has combined her interests in clinical medicine with her passion for education and content development for many years. She served as medical director for the health technology companies HealthLoop (now Get Well) and Doximity.

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