The association between excess body fat and increased risk of various cancers is an undisputed fact. Now, the findings of a working group from the International Agency for Research on Cancer (IARC) adds eight additional types to the list of cancers whose risks are increased by being overweight, showing that the burden of cancer due to being obese is more extensive than previously thought.
The IARC’s special report, “Body Fatness and Cancer — Viewpoint of the IARC Working Group,” published in The New England Journal of Medicine, now shows that the risk of gastric cardia, liver, pancreas, gallbladder, ovary, and thyroid cancers, as well as meningioma and multiple myeloma, is associated with increased body fat.
In 2002, the same working group found that five other tumor sites were linked with body fatness. Those included cancers of the kidney, colon, breast, esophagus and uterus.
Currently, the body mass index (BMI) is used to assess overall body fatness. Having a BMI between 25 and 29.9 is indicative of being overweight, while a BMI of 30 or more classifies people as obese.
In 2014, the World Health Organization estimated that nearly 39% of the adult population worldwide was overweight. In addition, 13% of the adult population was estimated to be obese, accounting for 640 million adults worldwide, a number that was six times higher than observed in 1975.
It was estimated that in 2013 alone, being overweight or obese were responsible for 4.5 million deaths, and recent estimates show that 9% of cancer cases among women in North America, Europe and the Middle East are obesity-related.
To reassess the preventive controls of weight control on cancer risk, the IARC working group examined more than 1,000 epidemiologic studies, most of which were observational studies on cancer risk and excess body fatness, because clinical trials of weight loss or weight-control interventions were scarce.
The new analysis reaffirmed previous findings, showing that the absence of excess body fatness lowered the risk of colon, esophagus, kidney, breast and uterine cancers.
However, researchers found eight additional cancers whose risk also was decreased by the absence of body fatness. When the researchers compared people with normal BMI to people with BMI of 40 or higher, they found the following increase in risk of cancer:
- 80% higher risk for gastric cardia cancer and liver cancer;
- 50% higher risk for pancreatic cancer, meningioma, and multiple myeloma;
- 30% higher risk for gallbladder cancer;
- 10% higher risk for ovarian cancer;
- 10% higher risk for thyroid cancer in each increment of 5 BMI units.
The researchers also found body fatness influenced the risk of cancer in a dose-dependent manner, where people with higher BMIs had greater risk of developing each of these cancer types.
The study assessed several other types of cancers, including male breast cancer, fatal prostate cancer, and diffuse large B-cell lymphoma, for which they had limited evidence of an association. That means they found an association between body fatness and increased risk of cancer, but could not rule out chance or confounding factors.
Eight additional types of cancer, including lung, testis, squamous cell carcinoma, gastric non-cardia, cutaneous melanoma, urinary bladder, and glioma, also were assessed, but the available studies were not of sufficient quality, consistency or statistical power to allow a conclusion regarding the existence of an association between body fatness and cancer risk.
Body fatness in children and adolescents also was associated with increased risk in the same types of cancers as in adults, with the exception of postmenopausal breast cancer.
In addition, although there was considerable variation in the design and time of body fat measurements in the studies assessed, the researchers found a large volume of evidence suggesting that increased BMI at the time of diagnosis leads to reduced breast cancer survival. Knowing whether the BMI at the time of diagnosis affects the survival rates of patients with other types of cancers remains to be addressed.
Nonetheless, the study strongly suggests that modifications in lifestyle, such as eating a healthy diet, exercising and maintaining a healthy weight can significantly reduce the risk of cancer.