The authors found seventy three separate studies on breast cancer (they say separate, I assume, to differentiate multiple publications on the same data set). Three quarters of the studies found physical activity to have a positive effect on breast cancer risk; within those, the risk reduction tended to be around twenty five percent when comparing most to least active participants. Interestingly, postmenopausal physical activity seemed to have the strongest effect, which was a bit of a surprise to me. What this says, to me, is that adjustments to lifestyle made late in life can still have significant effects on health. If you were a couch potato when young but are committed to physical activity now, all is not lost.
For endometrial cancer, the authors concluded that “physical activity probably protects against endometrial cancer” (p. 2594, anyone else find that wording odd?). There were twenty studies of endometrial cancer, so far fewer than of breast cancer, but the reduction in risk was very similar, of twenty to thirty percent. Sedentary behavior appears to increase risk (like jobs where you sit all day), and more intense or longer bouts of activity tend to have more positive effects.
Here, the authors found conflicting evidence, where some studies found risk reduction with physical activity, some found no effect, and one found a risk increase with physical activity. There were about twenty studies that looked at ovarian cancer, but the authors claim that the sample size was often small.
What surprised me about this section was what little attention they paid to mechanism. Ovarian cancer is different from breast or endometrial cancer: a big part of what researchers think causes it is the “incessant ovulation” of industrial societies. That is, women in industrial populations, due to a positive energy balance, low rates of childbearing and low rates of breastfeeding, have as many as 400 menstrual cycles in their lives, compared to only 50 in a forager population, which would have a later age at menarche and women who are often pregnant or breastfeeding. Continual insults to the ovarian tissue, from the rupture of the ovarian wall during ovulation, and its subsequent repair, increase the risk of cell mutations. Small differences in sedentary or active behavior don’t tend to be enough to keep a woman from ovulating.
Why parse out physical activity and sedentary behavior?
You may have noticed that I mention different kinds of physical activity, but also sedentary behavior. In the paper the authors go into far more detail about different types of behavior, from domestic activities to occupational activities to recreational ones, and whether they are physically active or sedentary. It is important to consider both of these factors because they contribute to an overall lifestyle that can tip the balance towards a more positive (more calories in than out) energy balance, or a more negative (more calories out than in) energy balance.
Let’s take my Polish field site for instance, the Mogielica Human Ecology Study Site, directed by Grazyna Jasienska. These women burn on average more calories than urban US women (Clancy et al 2009). Women there work alongside the men to do all the agricultural work needed to run their farms. You could just stop there and say that is very different from many populations within the US. But these women also do all their own housework and cooking, they have gardens, if they work they walk or take the bus. They are often sweeping and mopping instead of vacuuming. They spend a lot less time in front of screens, like televisions or computers (they have them for sure, but there are fewer desk jobs in front of a computer there). At least from my observations, they are less likely to order things online, and instead go to the store.
So the lifestyle differences, comparing an urban, sedentary person with a desk job in the US to a farmer in Poland, become far more striking, as do the number of calories they likely burn each day in their daily lives.
Is there population variation in cancer rates?
I thought you’d never ask! The main reason I decided to write about this article was so that I could highlight one of the most elegant, simple little graphs I have ever read, from a 2001 article by Jasienska and Thune. They compare population averages in progesterone (a female reproductive hormone) with breast cancer rates. Here’s the graph:
|Jasienska and Thune 2001|
Guess what is one of the biggest predictors of progesterone concentrations? Physical activity.
Finally, why someone else should always read your manuscripts
There were a few tough sentences in here that a copyeditor could have really improved. The authors wrote:
“There is strong and consistent evidence that physical activity reduces the risk of several of the major cancer sites, and that between 9% and 19% of cancer cases could be attributed to lack of sufficient physical activity in Europe” (p. 2593, abstract)Which I tweeted, and to which Alex Wild of Myrmecos responded:
“@KateClancy So if Europeans were more active we'd all have less cancer?#wordingfail”And then as I was preparing this post, I found:
“Physical activity reduces breast cancer risk when performed at any age throughout life, but activity done after the age of 50 years does have a stronger effect on risk than activity done earlier in life and sustained lifetime activity is of benefit” (p. 2594).So, sustained lifetime activity isn’t as good as activity after 50, or this is a point independent of which time of activity matters? I thought perhaps they were trying to make points about both? Copyeditor, please!
Anyway, this article was a fun way to get to share a broader perspective on reproductive cancer rates in women across the world. Share your thoughts or questions in the comments!
Clancy, K., Ellison, P., Jasienska, G., & Bribiescas, R. (2009). Endometrial thickness is not independent of luteal phase day in a rural Polish population Anthropological Science, 117 (3), 157-163 DOI: 10.1537/ase.090130
Friedenreich CM, Neilson HK, & Lynch BM (2010). State of the epidemiological evidence on physical activity and cancer prevention. European journal of cancer (Oxford, England : 1990), 46 (14), 2593-604 PMID: 20843488
Jasienska, G., & Thune, I. (2001). Research pointers: Lifestyle, hormones, and risk of breast cancer BMJ, 322 (7286), 586-587 DOI: 10.1136/bmj.322.7286.586