It is perhaps a surprising fact that the second most important environmental cause of cancer after smoking is obesity. Obesity has been spreading like wildfire through the Western world over the last half a century – with experts pointing the finger at changes in our daily habits and available diet as main culprits. In short, our diets have gotten richer in sugar and fat and have a much higher calorie content than they did in the first half on the twentieth century. This in turn has lead to an dramatic increase in the number of the overweight – and of the morbidly obese. Obesity carries a whole host of awful medical consequences, including type-2 diabetes, heart disease and, crucially, cancer.
The most logical group of cancers to be affected by obesity are those that occur in organs that are somehow linked to the digestive tract. Obese patients put a much higher burden on their digestive tracts – which leads to higher levels of intestinal inflammation and to a higher rate of cell turn-over, both of which are crucial for tumor formation. For instance, obese people are twice as likely to be diagnosed with cancer of the upper-part of the stomach and the esophagus. At the other end of the digestive system, people with morbidly high BMIs are twice as likely to have kidney cancer. They are also 30% more likely to develop cancer of the colon and 20% more likely to have gallbladder cancer. Obese people are also about twice as likely to get liver and pancreatic cancer, both of which are heavily involved in digestion and metabolism. An interesting fact is that the correlation between obesity and liver cancer is much stronger in men than in women. One theory to explain this gender difference is that men are much more likely to get their excess calories from alcohol than women (although much less so now than a few decades ago), putting a higher burden on the liver than on the rest of the body.
A second – are more biologically interesting – category of cancers are those that are not directly related to the digestive system but that more likely to affect the obese. The most dramatic case is endometrial cancer, which is two to four times more frequent in women with elevated BMIs. Interestingly, the risk of endometrial cancer increases with weight for postmenopausal women who have not had hormone therapy, which suggest that something about an increasing BMI is directly causing endometrial malignancies. Obese patients are also slightly more likely to contract a variety of other malignancies, including brain tumors, myeloma, ovarian cancer and tyroid malignancies. While a lot still needs to be understood regarding the biological reasons why obesity is related to breast cancer, these statistics speak for themselves.
The current obesity epidemic in the western world is shaping a whole new high-risk demographic for many very severe malignancies. Since high-BMIs in many cases a direct result of lifestyle choices, this is an area where public health can make a huge impact in decreasing the number of morbidly obese patients – and therefore of those who are diagnosed with cancer every year.