Tuesday, September 13, 2016

Is Cancer Preventable?

Most of the major chronic diseases, like heart disease and type 2 diabetes, are easy to study. Since they occur often, cohort studies have enough power to provide very useful information. Since they have intermediate endpoints (HDL, LDL, triglycerides, insulin resistance, etc), more short term study designs like randomized trials have been extremely important. As research methods have become more advanced, we have learned a great deal about heart disease, type 2 diabetes and how to prevent them both.

Cancer is something of a different story. It is not just one disease but 100 different diseases. Lung cancer has a very different story than skin cancer when it comes to causes, prevention and effective treatments. This is true for most cancers. This is what makes cancer so very hard to study. Cohort studies generally won’t have enough cases of distinct cancers to perform meaningful analysis for decades, if ever. Randomized trials are not really effective because cancer takes years to develop and doesn’t have hard and fast identifiable intermediates to study. 

For these reasons, we don’t know nearly as much about preventing cancer as we do about most other diseases. Since cancer is the 2nd leading cause of death in the U.S., it is imperative to learn as much as we can about its prevention. In the last year or so, some very influential studies were published. I thought I would summarize this research for readers of my newsletter.

Tomasetti, et al.

This study was published a year and a half ago and was quite controversial (Reference 1). It proposes that the majority of cancers have nothing to do with genetics or lifestyle factors. They are simply a function of the number of times a tissue’s cells divide. The more divisions, the more likely a random act of mutation will occur.

The authors of this study theorize that one third of cancer risk is lifestyle or genetic and that two-thirds is simply the bad luck of random cell division. What this study is basically saying is that almost 70% of your risk of cancer has nothing to do with how you live your life.

Song, et al.

This was a very large study that combined both the Nurses’ Health Cohort with the Health Professional Follow-up Study (Reference 2). Over 135,000 men and women were included in this investigation. Subjects were considered to have a healthy lifestyle pattern if they met the following 4 criteria:

1) Nonsmoker.

2) Moderate alcohol consumption (≤1 drink per day for women, ≤2 drinks per day for men).

3) A BMI between 18.5 and 27.5.

4) Weekly aerobic exercise totaling 75 minutes/week of vigorous cardio or 150 minutes/week of moderate cardio.

Rates of death due to cancer were compared between those with a healthy lifestyle and those without a healthy lifestyle. The population attributable risk for the healthy lifestyle score was 48% of cancer deaths for women and 44% in men. What this means is that if everyone in the cohort adopted these 5 habits, roughly half of all cancer deaths would be prevented.

These 2 cohorts, made up of mostly doctors and nurses, are generally more healthy than the average American. When they extrapolated the score to the U.S. population as a whole, these 5 factors would prevent 59% of cancer deaths in women and 67% of cancers deaths in men.

This study shows that lifestyle is very important when it comes to reducing the risk of cancer mortality and contradicts the first study presented. Two other interesting notes: 1) This study did not include dietary variables and 2) Some of the factors were a little relaxed. For example, a BMI of 27 or less was considered a healthy range where a BMI or 25 would have been a bit more strict. Therefore, the results from this study may actually underestimate the protective effects of a healthy lifestyle.

Moore, et al.

In this large study, 12 separate cohorts were combined to investigate the association between self reported physical activity and incidence of 26 different types of cancer (Reference 3). When all of the cohorts were combined, there were 186,932 cancers reported. The combination of the sample size and number of cancers reported provided a very unique research opportunity.

High versus low levels of physical activity were associated with significantly lower risk of 13 different cancers, including: esophageal, liver, lung, kidney, gastric, endometrial, leukemia, myeloma, colon, head and neck, rectal, bladder and breast.

Three other types of cancer were borderline significant: gall bladder, small intestine and non-Hodgkin’s lymphoma.

There was actually a slight increase in melanoma among those more physically active, so be sure to use sunscreen if exercising outdoors.

Previously, physical activity was thought to reduce risk of just a handful of cancers. This study showed that it does a lot more to protect us than originally thought.

Conclusions
Cancer is difficult to study. There is a lot of inconsistency in the literature due to methodological issues. I would say that there is definitely an element of bad luck with cancer that you don’t see in other diseases. However, it appears that a large percentage of cancers are preventable.

Here is what I recommend:
#1 Don’t smoke.
#2 Maintain a healthy weight.
#3 Exercise regularly, every day even.
#4 Eat a healthy Mediterranean style diet, limiting red meat and processed meats.

These 4 areas are a very good place to start if you are trying to reduce your risk of not only cancer, but other major chronic disease as well.

References
1) Tomasetti, et al. Variation in cancer risk among tissues can be explained by the number of stem cell divisions. Science 2015; 347:79-81.

2) Song, et al. Preventable incidence and mortality of carcinoma associated with lifestyle factors among white adults in the United States. JAMA Oncology. doi:10.1001/jamaoncol.2016.0843.

3) Moore, et al. Association of leisure time physical activity with risk of 26 types of cancer in 1.44 million adults. JAMA Internal Medicine. doi:10.1001/jamainternmed.2016.1548.

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