Obesity and Cancer Risk: A Complicated Link

We have all heard over and over about the links between obesity and cancer risk. The U.S. Center for Disease Control and Prevention says that being overweight or obese is linked with a higher risk for 13 kinds of cancers; these cancers make up 40% of all cancers diagnosed in the United States.

Recent studies have begun to question, or at least to modulate, this message. Most of the research has been based on data from observational studies, and that means there are many limitations in the findings. A new study, published in the Journal of the National Cancer Institute and based on randomized studies, found a potential causal association between obesity and cancer risk for only (in italics because I know it is still a lot) six kinds of cancer. They are: colorectal, endometrial, ovarian, kidney, pancreatic, and esophageal cancers.

Surprisingly, it also found an inverse relationship for breast cancer, in which obesity in early life (note that the higher risk exists for post-menopausal women who are obese) was associated with a reduced risk, and the connections were unclear for lung and prostate cancers.

In an accompanying editorial, Dr. Song Yao, from the Roswell Park Cancer Institute, wrote extensively about the nuances and puzzles. He wrote that if you want to talk about cancer overall, as one disease, we all know that a clear association with obesity does not exist. It’s not that simple.

He is clear that it is incorrect to state that obesity increases cancer risk. It is true, however, that people who have a higher BMI at the time of a cancer diagnosis or who have survived cancer have higher risks of developing second, unrelated cancers. Common sense suggests that a similar risk may exist in terms of cancer recurrences. This is why our oncologists and other doctors talk with us about maintaining a healthy weight.

The enduring public health message has long been that obesity is dangerous for our health. There are clear links between BMI and conditions like hypertension, diabetes, and cardiovascular disease. It is also believed that the association between obesity and breast cancer risk in older women is due to the presence of estrogen in adipose (fat) tissue.

Obesity is a growing health problem in our country. In 2011, 27.4% of adults 18 or older had obesity or severe obesity. By 2020, that number had increased to 31.9%. What we eat and how much of it we eat is obviously the problem.

As a small example, I have a number of glasses that belonged to my parents. Although I grew up drinking milk or water and, later, gin and tonics, from them, they now seem very small compared to the typical glass size in my cabinet or on the table at a restaurant. If you Google portion size then and now, many links appear, like this one from Your Weight Matters. The differences are astounding.

What do we know about the possible ways that obesity may increase cancer risk? Adipose (fat) tissue produces excessive amounts of estrogen, and we know that high levels of estrogen have been associated with risk of breast, endometrial, ovarian, and some other cancers. People who are obese often have higher blood levels of insulin, and this might increase the risk of colon, kidney, prostate, and endometrial cancers. People who are obese may have other chronic inflammatory conditions that may lead to DNA damage. Fat cells produce hormones that can affect cell growth and repair. The list goes on, but it is clear that the subject is complicated and not easily understood by us non-scientists.

Where does this leave us? First, it leaves me with a healthy respect, once again, for how complicated our bodies are and how much more there is to learn and understand. It reinforces the importance of trying to maintain a healthy weight, but also reassures me that there is likely a little wiggle room. No one is suggesting that we need to eliminate ice cream cones and occasional onion rings from our diets.

Leaving cancer completely out of it, life is short. For me, it is definitely too short to live on carrot sticks and broccoli. I think this concludes with another vote for moderation in all things and doing our best to treat our bodies and our health with the care they deserve.