Obesity's Cancer Link Is Worse Than You Think

WebMD

5 days ago

July 31, 2025 — Cancer deaths are dropping overall, but not the ones linked to obesity.
That's what mounting evidence now shows. A sweeping new report on U.S. cancer trends, published in April, revealed that cancers linked to obesity are becoming more common. Another study, presented in July at the Endocrine Society Annual Meeting in San Francisco, found that deaths from obesity-related cancers have more than tripled over the past two decades.
These include esophageal, colon and rectal, breast (postmenopausal), uterine, gallbladder, upper stomach, kidney, liver, ovarian, pancreatic, thyroid, meningioma (brain), and multiple myeloma — 13 types in all, now accounting for 40% of new cancer diagnoses in the U.S. Women, older adults, Native Americans, and Black Americans are especially vulnerable.
Second only to smoking, obesity ranks as one of the leading preventable causes of cancer. Yet even as the public's awareness of smoking's risks has dramatically increased, experts warn we've underestimated just how much excess weight — and the complex biology behind it — can fuel the disease.
What exactly drives this link is not fully understood, but experts are homing in on some strong possibilities. It could be estrogen, fat cells, the microbiome, insulin resistance, or all of the above. One thing is sure: The public health threat of obesity is only increasing.
So what does this mean for cancer prevention, and how can you protect yourself?
A Risk for Women
Behind the drop in overall cancer rates are significant declines in the number of smokers — and smoking-related cancers. Between 1965 and 2015, the smoking rate fell from 42% of the population to 15%, putting a major dent in rates of lung cancer, which is still the deadliest form. Meanwhile, obesity rates have gone the other way, rising from 13% in 1960 to more than 40% today.
Estimates reveal a crucial gender difference. While overall cancer diagnoses in men are down —decreasing from 2001 through 2013, and plateauing after that — the same is not true for women, according to the April report, jointly published by top health organizations like the CDC, the National Cancer Institute, the American Cancer Society.
Cancer diagnoses among women increased every year from 2003 to 2021. That includes upticks among women in every major racial and ethnic group since 2017, even while cancer rates have remained stable among men in those groups. According to the American Cancer Society, women under 50 are now 82% more likely to be diagnosed with cancer than are men of the same age — up from a 51% difference in 2002.
'One of my most common referrals is cancer doctors,' said Ethan Lazarus, MD, an obesity medicine specialist in Colorado, who is often recruited by oncologists to help patients in remission, commonly from breast cancer, lose weight. Evidence has repeatedly shown that higher amounts of fat tissue increase the risk of cancer recurrence. Weight loss is linked to improvements in cancer survival, particularly with breast and colon cancer.
Women tend to have higher rates of obesity than men — and higher rates of severe obesity, in particular — which may partly explain why they're more affected by obesity-related cancers. Also worth highlighting is that several of the obesity-related cancers are female-specific or far more common in women.
But that doesn't explain the bigger question: Why does obesity raise the risk of cancer in women — or in anyone?
When Fat Makes You Sick
'The truth is, no one really knows why obesity causes cancer,' said Scott Summers, PhD, co-executive director of the Diabetes and Metabolism Research Center at the University of Utah Health. But Summers' research group believes fat cells are a big part of the problem. 'Fat cells are weird, and they can be nasty,' he said.
Obesity experts have coined a term for the tipping point when fat cells start to affect your health: adiposopathy — 'adipos' meaning fat, and 'pathy' meaning disease, Lazarus said.
For one, they may drive cancer simply by proximity. Almost all of the obesity-related cancers occur in or next to parts of the body where fat tissue is more concentrated, like the abdomen and breast, according to a 2023 review in Frontiers in Endocrinology. And research in mice shows that breast cancer is more aggressive when surrounded by more fat tissue.
One leading theory is that fat tissue is 'hormonally active,' especially after menopause — meaning it releases hormones like estrogen that can cause certain cancers like breast or endometrial to grow, said Priya Jaisinghani, MD, an obesity medicine specialist in New York City. In fact, postmenopausal breast cancer is the most common obesity-associated cancer in women.
The Role of Inflammation and Insulin
Inflammation is another suspect. As fat cells accumulate and grow — or as a person gains weight — those tissues stimulate an immune response. 'There are a few ways that happens,' Summers said. 'One is immune cells infiltrate fat tissue because they are clearing out dying fat cells.' This immune activation produces a chronic, low-grade inflammation, which raises the risk of DNA damage and cellular mutations — potentially priming the body for cancer.
Chronic inflammation may also contribute to insulin malfunctions, which can promote cancer. People with obesity often have insulin resistance, meaning their cells don't respond properly to normal levels of insulin, the hormone secreted by the pancreas to regulate blood sugar. When that happens, the pancreas releases more insulin to try to keep blood sugar under control, elevating blood levels of insulin, which can stimulate tissue growth, including cancer tissue, Summers said.
What's Happening in the Gut
Summers's team at U Health has also uncovered a potential driver in the gut: a type of fat-storage molecule called sphingolipids, which build up in the body during obesity. 'We think they do a lot of damage in obesity, and they're a sign the other pathways for [fat] storage are full,' Summers said.
In mouse studies, removing sphingolipids eliminated obesity-related diseases like kidney disease, fatty liver disease, diabetes, and heart failure. One study showed that sphingolipids drive up the rapid production of stem cells in the intestine — an unexpected finding that suggests the fat molecules could trigger excessive cell proliferation in the colon, increasing the risk for colon cancer, Summers said.
This is critically important given the unexplained, global rise in colon cancer among people under 50. Data shows persistent obesity increases the risk of colon cancer by as much as 57%.
Big Problem With Bias
Forty-two percent of Americans now live with obesity, and that number is projected to hit 50% by 2030. That leaves us with two options for cancer prevention, Lazarus said. 'Do we want to go after all the cancers [individually] or recognize obesity as a serious health threat?'
Jaisinghani agreed. Preventing obesity-related cancers starts with equitable access to obesity care, 'regular physical activity, balanced and nutritious eating, and early treatment of metabolic conditions like insulin resistance or prediabetes,' she said. 'And prevention shouldn't be a privilege,' or it can't just be for those who can afford it.
Only about 10% of people with obesity receive medical treatment, compared with 85% of patients with diabetes, Lazarus said. Many people, doctors included, still see obesity as a lifestyle issue instead of a medical one. 'Most don't get any care, much less comprehensive.' Early cancer symptoms are often overlooked and blamed on excess body weight, and machines used for screening, such as MRIs and mammography machines, don't always fit properly.
For these reasons, obesity often goes untreated, and cancers in people with obesity go undetected.
Experts say it's a public health crisis that, like smoking and lung cancer, calls for a public health approach. 'It's policy-level interventions that shape the environments in which people live, work, and make decisions about their health,' Jaisinghani said.
But you can take steps to protect yourself too. Know that if you have health concerns with obesity, that is a medical issue and seeking treatment is completely appropriate. Consider seeing an obesity medicine specialist. This subspecialty is rapidly expanding, and doctors with this training can help you avoid bias and inadequate care. You can find one by searching the Obesity Medicine Association's provider directory.