But the opposite seems to be true regarding the causal link with obesity, according to a scientific review by the International Agency for Research on Cancer.
Fourteen years ago when the IARC
, based in Lyon, France, first reviewed relevant studies, its expert panel issued a report finding sufficient evidence that excess body fat increases the risk of certain cancers. Now, the group's latest reassessment
, published Wednesday in the New England Journal of Medicine, reaffirms those findings -- and adds eight more cancers to the list.
"Since 2002 there have been a lot of new studies conducted. We felt like it was the right time to review the literature and maybe confirm the science that has been established," said Beatrice Lauby-Secretan
, lead author of the article and an IARC scientist responsible for the agency's Handbooks of Cancer Prevention Series. The IARC is part of the World Health Organization.
A working group of 21 independent international cancer experts reviewed more than 1,000 studies on cancer risk and excess body fat published since the IARC's 2002 report. That evaluation identified that preventing weight gain can reduce the risk of colon and rectum cancer; a stomach cancer called esophagus adenocarcinoma; kidney or renal cell carcinoma; postmenopausal breast cancer and cancer in the endometrium of the uterus. This year's reassessment added to this list gastric cancer, liver, gallbladder, pancreas, ovary and thyroid cancers as well as the blood cancer multiple myeloma
-- cancer that affects the tissue surrounding the brain and spine.
The risks are highest for corpus uteri
, a cancer in the uterus, and esophagus adenocarcinoma
"The number of cancers that are linked to obesity has increased a lot, which means a much higher proportion of cancer that occurs today is due to obesity," Lauby-Secretan said. Public health messages should be tailored to raising awareness about this fact, she added.
Results also were consistent for children, adolescents and adults younger than 25.
Overweight adults are defined in the study as those with body mass indexes
(BMI) of 25.0 to 29.9, while obese adults have BMIs above 30. According to the study, an estimated 640 million adults worldwide were obese in 2014, which is six times more than in 1975. Around 110 million children and adolescents were obese in 2013, two times more than in 1980.
Meanwhile, the report also found that an estimated 4.5 million deaths in 2013 were related to overweight and obesity, a number that may increase as more cancers are found to be related to the condition.
"The epidemic of obesity has become a global concern," Lauby-Secretan said.
Not all cancers have positive correlations with obesity, though. For example, there is only limited evidence of this link for fatal cancer of the prostate, breast cancer in men and diffuse large B-cell lymphoma
, the most common blood cancer. Evidence is inadequate for cancers of the lung, testis, urinary bladder, brain or spinal cord. While excess fat does lead to higher risks of postmenopausal breast cancer, it does not have the same effect for premenopausal breast cancer.
The reason obesity may increase cancer risks, Lauby-Secretan said, is because excess body fat has been known to trigger chronic inflammation
. It also disturbs the regulation of sex hormones. Both are common pathways for the development of cancer cells in the body.
But the awareness about the link can be low especially when Americans are inundated with news about how many substances -- coffee or sugar, for instance -- may or may not cause cancer. According to the American Institute for Cancer Research
's biennial survey
released in February 2015, a little more than half of Americans realize that being overweight can increase cancer risk, a slight increase over prior surveys.
, head of nutrition programs for AICR, said there will always be studies that disprove or prove links -- but when you look at the whole body of research, there is a scientific consensus on this particular point. And people can actively reduce cancer risks from excess body fat, Bender said, by eating healthier meals and exercising more.
"Oftentimes people are concerned about exposures in the environment or genetics or things you can't control," she said. "Rather than increase fear about this, we can see this as an empowering message: Here is something I can do to help myself lower the risk for many of these cancers ... here are some lifestyle changes that I can make."