According to the Centers for Disease Control and Prevention (CDC), over 684,000 Americans are diagnosed with an "obesity-associated" cancer each year.
The incidence of many of these cancers has been rising in recent years, particularly among younger people — a trend that sits in contrast with the overall decline in cancers with no established relationship to excess weight, such as lung and skin cancers.
Is obesity the new smoking? Not exactly.
Tracing a direct line between excess fat and cancer is much less clear-cut than it is with tobacco. While about 42% of cancers — including common ones such as colorectal and postmenopausal breast cancers — are considered obesity-related, only about 8% of incident cancers are attributed to excess body weight. People often develop those diseases regardless of weight.
Although plenty of evidence points to excess body fat as a cancer risk factor, it's unclear at what point excess weight has an effect. Is gaining weight later in life, for instance, better or worse for cancer risk than being overweight or obese from a young age?
There's another glaring knowledge gap: Does losing weight at some point in adulthood change the picture? In other words, how many of those 684,000 diagnoses might have been prevented if people shed excess pounds?
When it comes to weight and cancer risk, "there's a lot we don't know," said Jennifer W. Bea, PhD, associate professor, health promotion sciences, University of Arizona, Tucson, Arizona.
Consistent Complicated Relationship
Given the growing incidence of obesity — which currently affects about 42% of US adults and 20% of children and teenagers — it's no surprise that many studies have delved into the potential effects of excess weight on cancer rates.
Although virtually all the evidence comes from large cohort studies, leaving the cause-effect question open, certain associations keep showing up.
"What we know is that, consistently, a higher body mass index [BMI] — particularly in the obese category — leads to a higher risk of multiple cancers," said Jeffrey A. Meyerhardt, MD, MPH, codirector, Colon and Rectal Cancer Center, Dana-Farber Cancer Institute, Boston.
In a widely cited report published in The New England Journal of Medicine in 2016, the International Agency for Research on Cancer (IARC) analyzed over 1000 epidemiologic studies on body fat and cancer. The agency pointed to over a dozen cancers, including some of the most common and deadly, linked to excess body weight.
That list includes esophageal adenocarcinoma and endometrial cancer — associated with the highest risk — along with kidney, liver, stomach (gastric cardia), pancreatic, colorectal, postmenopausal breast, gallbladder, ovarian, and thyroid cancers, plus multiple myeloma and meningioma. There's also "limited" evidence linking excess weight to additional cancer types, including aggressive prostate cancer and certain head and neck cancers.
At the same time, Meyerhardt said, many of those same cancers are also associated with issues that lead to, or coexist with, overweight and obesity, including poor diet, lack of exercise, and metabolic conditions such as diabetes.
It's a complicated web, and it's likely, Meyerhardt said, that high BMI both directly affects cancer risk and is part of a "causal pathway" of other factors that do.
Regarding direct effects, preclinical research has pointed to multiple ways in which excess body fat could contribute to cancer, said Karen M. Basen-Engquist, PhD, MPH, professor, Division of Cancer Prevention and Population Services, The University of Texas MD Anderson Cancer Center, Houston.
One broad mechanism to help explain the obesity-cancer link is chronic systemic inflammation because excess fat tissue can raise levels of substances in the body, such as tumor necrosis factor alpha and interleukin 6, which fuel inflammation. Excess fat also contributes to hyperinsulinemia — too much insulin in the blood — which can help promote the growth and spread of tumor cells.
But the underlying reasons also appear to vary by cancer type, Basen-Engquist said. With hormonally driven cancer types, such as breast and endometrial, excess body fat may alter hormone levels in ways that spur tumor growth. Extra fat tissue may, for example, convert androgens into estrogens, which could help feed estrogen-dependent tumors.
That, Basen-Engquist noted, could be why excess weight is associated with postmenopausal, not premenopausal, breast cancer: Before menopause, body fat is a relatively minor contributor to estrogen levels but becomes more important after menopause.
How Big Is the Effect?
While more than a dozen cancers have been consistently linked to excess weight, the strength of those associations varies considerably.
Endometrial and esophageal cancers are two that stand out. In the 2016 IARC analysis, people with severe obesity had a seven-times greater risk for endometrial cancer and 4.8-times greater risk for esophageal adenocarcinoma vs people with a normal BMI.
With other cancers, the risk increases for those with severe obesity compared with a normal BMI were far more modest: 10% for ovarian cancer, 30% for colorectal cancer, and 80% for kidney and stomach cancers, for example. For postmenopausal breast cancer, every five-unit increase in BMI was associated with a 10% relative risk increase.
A 2018 study from the American Cancer Society, which attempted to estimate the proportion of cancers in the US attributable to modifiable risk factors — including alcohol consumption, ultraviolet rays exposure, and physical inactivity — found that smoking accounted for the highest proportion of cancer cases by a wide margin (19%), but excess weight came in second (7.8%).
Again, weight appeared to play a bigger role in certain cancers than others: An estimated 60% of endometrial cancers were linked to excess weight, as were roughly one third of esophageal, kidney, and liver cancers. At the other end of the spectrum, just over 11% of breast, 5% of colorectal, and 4% of ovarian cancers were attributable to excess weight.
Even at the lower end, those rates could make a big difference on the population level, especially for groups with higher rates of obesity.
CDC data show that obesity-related cancers are rising among women younger than 50 years, most rapidly among Hispanic women, and some less common obesity-related cancers, such as stomach, thyroid and pancreatic, are also rising among Black individuals and Hispanic Americans.
Obesity may be one reason for growing cancer disparities, said Leah Ferrucci, PhD, MPH, assistant professor, epidemiology, Yale School of Public Health, New Haven, Connecticut. But, she added, the evidence is limited because Black individuals and Hispanic Americans are understudied.
When Do Extra Pounds Matter?
When it comes to cancer risk, at what point in life does excess weight, or weight gain, matter? Is the standard weight gain in middle age, for instance, as hazardous as being overweight or obese from a young age?
Some evidence suggests there's no "safe" time for putting on excess pounds.
A recent meta-analysis concluded that weight gain at any point after age 18 years is associated with incremental increases in the risk for postmenopausal breast cancer. A 2023 study in JAMA Network Open found a similar pattern with colorectal and other gastrointestinal cancers: People who had sustained overweight or obesity from age 20 years through middle age faced an increased risk of developing those cancers after age 55 years.
The timing of weight gain didn't seem to matter either. The same elevated risk held among people who were normal weight in their younger years but became overweight after age 55 years.
Those studies focused on later-onset disease. But, in recent years, experts have tracked a troubling rise in early-onset cancers — those diagnosed before age 50 years — particularly gastrointestinal cancers.
An obvious question, Meyerhardt said, is whether the growing prevalence of obesity among young people is partly to blame.
There's some data to support that, he said. An analysis from the Nurses' Health Study II found that women with obesity had double the risk for early-onset colorectal cancer as those with a normal BMI. And every 5-kg increase in weight after age 18 years was associated with a 9% increase in colorectal cancer risk.
But while obesity trends probably partly explain the rise in early-onset cancers, there is likely more to the story, Meyerhardt said.
"I think all of us who see an increasing number of patients under 50 with colorectal cancer know there's a fair number who do not fit that [high BMI] profile," he said. "There's a fair number over 50 who don't either."
Does Weight Loss Help?
With all the evidence pointing to high BMI as a cancer risk factor, a logical conclusion is that weight loss should reduce that excess risk. However, Bea said, there's actually little data to support that, and what exists comes from observational studies.
Some research has focused on people who had substantial weight loss after bariatric surgery, with encouraging results. A study published in JAMA found that among 5053 people who underwent bariatric surgery, 2.9% developed an obesity-related cancer over 10 years compared with 4.9% in the nonsurgery group.
Most people, however, aim for less dramatic weight loss, with the help of diet and exercise or sometimes medication. Some evidence shows that a modest degree of weight loss may lower the risks for postmenopausal breast and endometrial cancers.
A 2020 pooled analysis found, for instance, that among women aged ≥ 50 years, those who lost as little as 2.0-4.5 kg, or 4.4-10.0 pounds, and kept it off for 10 years had a lower risk for breast cancer than women whose weight remained stable. And losing more weight — 9 kg, or about 20 pounds, or more — was even better for lowering cancer risk.
But other research suggests the opposite. A recent analysis found that people who lost weight within the past 2 years through diet and exercise had a higher risk for a range of cancers compared with those who did not lose weight. Overall, though, the increased risk was quite low.
Whatever the research does, or doesn't, show about weight and cancer risk, Basen-Engquist said, it's important that risk factors, obesity and otherwise, aren't "used as blame tools."
"With obesity, behavior certainly plays into it," she said. "But there are so many influences on our behavior that are socially determined."
Both Basen-Engquist and Meyerhardt said it's important for clinicians to consider the individual in front of them and for everyone to set realistic expectations.
People with obesity should not feel they have to become thin to be healthier, and no one has to leap from being sedentary to exercising several hours a week.
"We don't want patients to feel that if they don't get to a stated goal in a guideline, it's all for naught," Meyerhardt said.
https://www.medscape.com/viewarticle/obesity-and-cancer-untangling-complex-web-2024a10009zi
