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Obesity is a significant risk factor for several types of cancer, including cancers of the colon and rectum. In fact, according to the National Cancer Institute, obese individuals are about 30 times more likely to develop colorectal cancer than people who are at a healthy weight, and the increased risk is higher for men than women. Being obese means you have too much body fat, or your fat is disproportionately distributed around your hips and abdomen.
Unfortunately, this puts many Americans at unnecessary risk for the fourth most commonly diagnosed cancer. The NCI says about 70 percent of adults over 20 are overweight or obese, and nearly 37 percent are obese. In 2012, nearly 4 percent of new cancer cases in men and 10 percent in women were attributed to being overweight or obese.
Obesity is a huge problem in the U.S. – even an epidemic, says Dr. Darrell M. Gray, a gastroenterologist with The Ohio State University Comprehensive Cancer Center – Arthur G. James Cancer Hospital and Richard J. Solove Research Institute.
Wht's the Link Between Obesity and Colorectal Cancer?
Doctors aren't entirely sure exactly how obesity raises the risk for colorectal cancer, but they have a number of possible explanations, Gray says. Let's start with inflammation. "Obese people tend to have chronic, low levels of inflammation, which can lead to DNA damage and cancer," he says. According to the NCI, obese individuals may also be more likely to have other health conditions associated with inflammation, such as ulcerative colitis, an inflammatory bowel condition that's also a risk factor for colorectal cancer, regardless of weight.
Gray says obese individuals have increased blood levels of insulin and insulin-like growth factor-1, or IGF-1, that may promote CRC. Insulin and IGF-1 are associated with diabetes, which is more likely to occur in someone who is overweight or obese.
A third hypothesis, Gray says, is that fat cells produce adipokines, which are hormones that inhibit or promote cell growth.
There are secondary factors that may also play a role, says Dr. Mark Pochapin, professor and director of the division of gastroenterology at NYU Langone Medical Center. People who are obese typically eat less healthy diets, which are higher in fat and red meat (too much red meat is also a colorectal cancer risk factor). "Obesity is associated with less physical activity," Pochapin says. "If you are obese, you get less exercise, which makes you more obese."
Pochapin says another hypothesis researchers are exploring is how the gut microbiome might predispose someone to CRC. This line of study is still in its infancy, however.
Of course, not all people who are overweight or obese develop colorectal cancer, and doctors aren't exactly sure why this is the case. Whether or not you develop CRC is likely due to the interplay of many factors, of which obesity is just one. Doctors do know, however, that the majority of cases of CRC in people who are obese are sporadic cancers, which means they're not related to genetics, Gray says.
It's also hard to say if losing weight lowers your risk of developing colorectal cancer. No one has studied this, and it's difficult to evaluate because people who lose weight tend to put it back on. However, according to the NCI, people who've undergone weight-loss surgery appear to have lower risks for obesity-related cancers.
Lifestyle Factors Are Important
Pochapin says the focus of CRC prevention should be less on weight loss and more on an overall healthy lifestyle, such as getting enough exercise. "Just walk," he says. "Most people don't realize that walking is a form of exercise. You can incorporate walking into your daily routine. You don't have to go to a gym."
Diet is important, too. "Keep a log of what you eat," Pochapin says. "Most of us don't realize what we eat, especially between meals. Really focus on the big picture: high fiber, low fat." Excessive red meat is a risk factor for colorectal cancer, so consume it in moderation. Eat chicken and fish for protein and plenty of colorful fruits and vegetables. "They have antioxidants and reduce the risk of CRC," Pochapin says.
Are You Obese?
Doctors use Body Mass Index – a ratio of weight to height – as a measurement of obesity. If your BMI is over 25, you're overweight, and if it's over 30, you're obese. Individuals who have a BMI of 34.9 or higher are considered morbidly obese. You can quickly assess your own BMI by entering your height and weight into an online calculator, such as the one on the National Heart Lung and Blood Institute's website. If your BMI is 25 or higher, talk to your doctor about how you can improve your overall health and reduce your risk for colorectal cancer.