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 United States 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, Dr. 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, Dr. 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.”