22 Scientific American, October 2019THE SCIENCE
OF HEALTHClaudia Wallis is an award-winning science journalist whose
work has appeared in the New York Times, Time, Fortune and the
New Republic. She was science editor at Time and managing editor
of Scientific American Mind.Illustration by Celia KrampienA Ticking Cancer
Time Bomb
Malignancies are on the rise in
the most obese generation in historyBy Claudia WallisMost of us recognize that obesity is not a benign condition. Dia-
betes, arthritis, plus heart, liver and gallbladder diseases com-
monly plague folks who carry major excess poundage. Less famil-
iar is the risk of cancer. Being overweight or obese has been linked
to at least 13 types of cancer. Obesity more than doubles the risk of
the most common forms of uterine and esophageal cancer. It rais-
es the risk of tumors of the colon, gallbladder, kidney, liver, pan-
creas, upper stomach and brain membranes by 50 to 80 percent
compared with adults at a healthy weight, and it ups the odds for
multiple myeloma and cancers of the breast, ovary and thyroid.
The danger tends to rise with the number on the scale: ex-
tremely obese women, for instance, face seven times the risk of
uterine cancer. Obesity also makes it more likely that certain can-
cers, including breast and prostate, will prove fatal or not respond
optimally to treatment. Given that most cancers take decades to
develop, one has to wonder what the eventual cancer toll will look
like now that nearly 80 percent of American adults and a third of
children are overweight or obese—up 60 percent and more than
100 percent, respectively, from 1980.
An analysis released this year by researchers at the AmericanCancer Society paints an alarming picture. Their study, published
in the Lancet Public Health, looks at how cancer rates have
changed over the past 20 years among younger adults—Ameri-
cans who came of age during the steepest rise in obesity—com-
pared with older adults. Hyuna Sung and her co-authors exam-
ined trends for the 30 most common cancers, 12 of which occur
more often in overweight people. Six of those 12—colorectal, gall-
bladder, kidney, multiple myeloma, pancreatic and uterine—were
found to be rising more rapidly in younger Americans (ages 25 to
49). The sharpest jumps—between 2 and 6 percent annually—
were in the youngest adults (ages 25 to 35). “This is not negligi-
ble,” Sung says. “It’s a huge and very fast increase.”
Trends in young adults can be seen as a “bellwether for future
disease burden,” Sung notes. She also points to growing evidence
that obesity that starts in childhood or adolescence may present a
particular risk for some cancers. That said, Sung’s study does not
address causality and therefore does not prove that the uptick in
certain malignancies is a direct consequence of rising obesity.
Other investigators, however, are looking directly at how obe-
sity might promote cancer. Turns out, excess body fat impacts the
body in multiple ways that may aid and abet a developing tumor.
“The whole hormonal milieu changes dramatically with obesity,”
says Stephen Hursting, professor of nutrition at the University of
North Carolina’s Lineberger Comprehensive Cancer Center. The
first “big basket” of changes includes a rise in growth factors, in-
cluding insulin and those that promote blood vessel formation, he
explains. A second big basket involves substances that promote
inflammation. “The obese state is a kind of smoldering, low but
insistent inflammatory state,” Hursting says. A third alteration is
suppression of the immune responses that fight incipient cancers.
And there are other impacts, including obesity-related changes to
the microbiome, metabolism and gene expression. Reading Hurst-
ing’s 2018 review of the many mechanisms linking obesity with
cancer was one of the more disturbing things I’ve done as a health
reporter. It made me want to scream at the big food industry,
which has done so much to engineer our obesity epidemic.
Screaming is of little use, so the question is: How can we dif-
fuse this time bomb? Clues come from studies of patients who
have slimmed down after bariatric surgery and trimmed their
cancer rate as well. A study with 88,625 obese women found, for
example, that those who underwent such operations had a 50 per-
cent lower rate of postmenopausal breast cancer than those who
did not. Newer data suggest the surgery may also lower the risk of
virulent “triple-negative” breast cancer in younger obese women.
Whether less drastic measures will do the trick is a hot research
topic. At the Dana-Farber Cancer Institute, oncologist Jennifer Li-
gibel is leading a randomized, controlled trial with 3,136 obese
breast cancer patients to see if losing just 7 to 10 percent of their
body mass with diet and exercise will lower their risk of cancer re-
currence and mortality. That level of weight loss has a big benefit
for people with type 2 diabetes, she notes. Results will not be out
for a few years, but Ligibel is hopeful: “Wouldn’t it be great if we
could find a treatment for breast cancer where the side effects are
that you have less arthritis and diabetes and you feel better?”