Obesity affects hormone levels and inflammation in the body, and can contribute to pre-cancerous conditions like fatty liver disease.
It’s pretty obvious that carrying around extra weight can make you feel sluggish, affect your self-esteem, and put you at increased risk for heart disease and diabetes. But increasingly, researchers are also making connections between obesity and cancer—several different types of cancer, in fact.
Cancer is caused by mutations within cells, which cause those cells to grow and multiply at unnatural rates. Many cases of cancer occur because of genetic traits, or purely because of chance. But for others, obesity can be a big contributing factor.
“We know that a good third of cancers are associated with our lifestyle behaviors, such as what we eat, how much we exercise, and collectively, our weight,” says Melinda Irwin, PhD, director of Cancer Prevention and Control at Yale University. “And obesity is now the leading modifiable risk factor, even ahead of tobacco use, that’s associated with cancer risk and mortality.”
How does obesity encourage cancer growth?
High-levels of long-term inflammation—the immune system’s response to injury, illness, or other disturbances in the body—has been shown to fuel the growth of cancer cells. “And we know that obesity is basically a chronic inflammatory state,” says Irwin.
Not only can obesity itself trigger inflammation; so can some of the the eating behaviors that lead to weight gain in the first place—like high-sugar and high-fat diets. Having too much fat around the belly, regardless of body mass index, increases inflammation in the body, as well.
Some cancers are also linked to sex hormones like estrogen. Women’s bodies produces estrogen in their fat cells, especially after menopause. “The higher levels of body fat you have, the higher levels of estrogen,” says Irwin.
Then there’s the way that obesity contributes to insulin resistance—a condition in which the body loses its sensitivity to the hormone and can’t respond normally. This can lead to excess levels of insulin and insulin-related growth hormones in the body, which has been linked to cell proliferation and several types of cancer.
Which cancers are linked to obesity?
Studies have shown that being overweight or obese is clearly linked to an increased risk of several cancers, including uterine (endometrial), colorectal, esophageal, kidney, pancreatic, and, in post-menopausal women, breast cancer. “The strongest is probably uterine cancer,” says Irwin. “There’s probably a sixfold higher risk of dying of uterine cancer for those with a high BMI versus those with a low BMI.”
It’s also been associated with an increased risk of liver cancer—a cancer whose rates have tripled in the United States since the 1970s. Obesity is a risk factor for nonalcoholic fatty liver disease, which can progress to liver cirrhosis and, in extreme cases, liver cancer.
Several genes influence a person’s likelihood of developing fatty liver disease, but a recent study in Nature Genetics found that people with high-risk gene variants were much more likely to actually get the disease if they were obese than if they were normal weight.
Studies have found less of a direct association between obesity and other cancers, including gallbladder, cervical, ovarian, and prostate cancer, as well as Non-Hodgkin lymphoma and multiple myeloma—but research does suggest it may play a role in these diseases, too.
How to protect yourself
Maintaining a healthy weight, or losing weight if you need to, can reduce your risk of getting cancer—and from dying from cancer if you do get it. “It’s really the weight loss that seems to be so important, rather than following a specific diet or making sure you’re eating a specific food,” says Irwin.
Watching your calories and making healthy food choices may be the most effective way to shave unwanted pounds, but Irwin stresses that exercise is also crucial—especially for middle-age and older adults. “As we age, we lose bone mass and muscle mass, which affects metabolic rate,” she says. Exercise can protect muscles and bones, and helps fight inflammation better than dieting alone.
"What I would tell the general public, or anyone who has been diagnosed with cancer, is that preventing weight gain as we age or trying to promote weight loss if you're overweight is one of the most important things you can do," says Irwin. "It's going to significantly change your risk for cancer, and, importantly, for overall mortality."