We know that obesity is associated with an increased risk of many types of cancer (including breast, endometrial/uterine, ovarian, esophageal, colon, kidney, liver, pancreas, thyroid , and many others).  There are many mechanisms involved in the higher cancer risk, including increase in hormones released by excess fat tissue, higher insulin levels and insulin resistance,  inflammatory cytokines, changes in gut bacteria, and epigenetic changes.


While we know that obesity is associated with a higher cancer risk, we don’t have definitive data to show that weight loss reduces cancer risk.  This is in part because historically, there hasn’t been much in the way of treatment options to produce enough weight loss to see a beneficial effect.


As bariatric surgery results in 25-30% weight loss, it’s an important opportunity to study whether a larger magnitude of weight loss can reduce cancer risk. 


Just such a study was recently published in JAMA.  This was an observational, matched cohort study of over 5,000 patients who had gastric bypass surgery or sleeve gastrectomy (the two most commonly performed bariatric surgeries), and compared them with over 25,000 matched controls.


During the average 6.1 year follow up, the obesity-associated cancer rates were 3 per 1000 people in the surgery group, vs 4.6 per 1000 people in the control group.  At 10 years, there was a 32% lower risk of obesity-associated cancer in the bariatric surgery group, and a lower risk of cancer-related death (0.8% in the bariatric surgery group vs 1.4% in the control group). It took about 6 years for these differences to emerge. Their analysis suggested that weight loss, and not the type of procedure (sleeve vs gastric bypass) was responsible for the reduced cancer risk.


Among all cancer types, endometrial cancer has the strongest association with obesity.  This is in part because women with obesity are at a higher risk of developing polycystic ovary syndrome (PCOS), which can result in infrequent menstrual periods.  It is important that these women are having periods at least every 3 months (either spontaneously, or with the help of medication), to prevent the lining of the uterus (the endometrium) from building up.  In this study, while most individual obesity-associated cancer types were numerically less common in the bariatric surgery group, it was only endometrial cancer that was statistically significantly lower (53% lower).


Similar benefits have been reported in other studies looking at the benefits of bariatric surgery. That being said, the data is not entirely consistent.  Regarding colon cancer, some data have suggested a decrease in risk, whereas other data suggests an increased risk after gastric bypass surgery.  Randomized clinical trials could better answer these questions, but such studies are very difficult to do in the area of bariatric surgery.


Because these data are observational and not from a randomized clinical trial, there are factors other than bariatric surgery that could have an impact on the results.  The people in this study who had bariatric surgery were more likely to have screening tests for breast, colon, and prostate cancer than those who didn’t have surgery.   People who choose to have (or have access to) bariatric surgery may be different than those who don’t in other ways as well.  The people in this study were mostly female and Caucasian; data in more diverse populations is needed.


The above being said, it does make sense that having bariatric surgery would be associated with a lower risk of cancer.  The 25-30% weight loss seen with bariatric surgery markedly improves many of the mechanisms by which obesity is associated with a higher risk of cancer (see above).  As noted in the accompanying editorial, we need to learn more about the specific biological mechanisms that may be involved in reducing cancer risk with bariatric surgery.  Whether potential benefits are purely weight loss related, or something specific to bariatric surgery is not known.


BOTTOM LINE:  The available data suggests that bariatric surgery is associated with a lower risk of developing obesity-associated cancer down the road, most likely related to the metabolic benefits that occur with the magnitude of weight loss achieved While bariatric surgery has traditionally been the only type of obesity treatment to achieve 20% or greater weight loss, emerging medications are now achieving this goal post for weight loss.  It will also be important to study whether these medications can reduce cancer rates over the long term.


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