Currently, 70% of American adults are either overweight or obese, with approximately 37% of Americans defined as having obesity.
While we often focus on the cardiovascular and diabetes risks associated with increased body weight, being overweight or obese has also been shown to increase risk for other serious, life-threatening conditions, including various forms of dementia and cancer.
Approximately 38% of men and women will be diagnosed with cancer in their lifetime, with overweight or obese individuals at an increased risk for a cancer diagnosis. An estimated 7% of all cancer deaths are attributed to overweight or obesity.
Being overweight or obese is associated with a significantly increased risk for cancers of the breast, colon, rectum, kidneys, pancreas, and endometrium, as well as a moderately increased risk for cancers of the gallbladder, liver, cervix, ovaries, and prostate.
While the association between body weight and cancer is unclear, researchers theorize that the mode of effect may vary depending on cancer type.
Most scientists agree that the high levels of cellular inflammation caused by excess body fat and the impact of fat on immune function play a key role in the development of various forms of cancer. The disrupted hormone status of overweight or obese individuals is also thought to play a role, specifically with regards to insulin, estrogen, and insulin-like growth factors.
How much greater is the risk for overweight or obese individuals? It varies largely depending on cancer type.
Overweight and obese women are estimated to have a risk factor 2-4 times as high as healthily weight women for endometrial cancer, for example, while risk for pancreatic cancer is about 1.5 times as high.
Most cancers have also shown to have a dose-dependent risk relationship with body weight. In other words, the greater the weight, the higher the risk. For example, while overweight or obese women have an risk increase of up to 300% for endometrial cancers, morbidly obese women have an increased risk of up to 800%.
Currently, the associations between weight loss and risk reduction are not as clear as the association between weight gain and increased risk. However, the available data shows that losing weight can significantly reduce risk for certain types of cancer, particularly for cancers of the breast and prostate.
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Cancers that are hormonally influenced tend to benefit most from weight loss, given the hormonal disruption that occurs with overweight or obesity and subsequent hormonal rebalancing that can occur with adequate weight loss and maintenance.
Studies on cancer risk have found that weight loss benefits individuals who are overweight or obese to a significant degree in long-term interventions.
Studies on cellular changes in the colon, rectum, and gastrointestinal tract have shown that some of the cellular changes associated with overweight and obesity that influence cancer risk are reversible with weight loss and maintenance.
Shorter term interventions have also shown some degree of improvement in risk reduction, but not to as significant degree as longer intervention periods. This is likely given the relatively small amount of weight loss and inflammation reduction that occurs in a shorter intervention period.
The risk reduction associated with weight loss also appears to be dose-dependent, much like the risk increase associated with weight gain: overweight or obese individuals who lose a higher amount of their starting weight benefit from a greater risk reduction than those losing less.
Clearly, diet and exercise patterns vary in overweight and obese individuals: just as some healthily weighted individuals are active and eat a healthy diet, so too do many overweight or obese individuals. The same might be said of unhealthy diet and exercise patterns.
With that being said, how do diet and activity habits impact cancer risk for overweight or obese individuals? Is risk still high?
As with many areas of cancer research, the statistics are not firm, but diet quality, physical activity, and body weight have all been shown to play a role in cancer development. In other words, in overweight or obese individuals, risk will still be heightened, regardless of the overall healthfulness of diet or quality of physical activity.
This is largely due to the inescapable levels of inflammation that occur in overweight or obesity, which can only be fully remedied with weight loss and maintenance.
However, for overweight or obese individuals who do engage in healthy dietary patterns and sufficient physical activity, risk will not be as high as it is for overweight or obese individuals who do not engage in healthy lifestyle patterns. Healthy lifestyle patterns have also been shown to effect rates of recurrence and survivability for individuals who have been diagnosed with cancer.
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