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.
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.
A Consistent but 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, Dr. 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, Dr. 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, Dr. 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, Dr. 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.