Exercise has been shown to lower the risk of certain cancers, and just this week an ambitious study of exercise and cancer was published. The authors were surprised that exercise had such a profound effect on lowering the risk of so many types of cancer, from 10 to 42%.
Published in the Journal of the American Medical Association on 16 May, researchers from the National Cancer Institute (USA) and Harvard Medical School analyzed data from 12 previous research studies that looked at exercise and cancer. These studies combined included over 1.4 million people, and the average person in the study was followed for 11 years.
The researchers looked at this: did people who exercised more compared with people who exercised little or not at all have a reduced risk of cancer, and if so, how much was the reduced risk? Here are the 13 cancers and the percentage of reduced risk for a person who exercised at least moderately:
- Esophageal cancer — 42% reduced risk
- Liver cancer — 27%
- Lung cancer — 26%
- Kidney cancer — 23%
- Stomach — 22%
- Endometrial cancer — 21%
- Myeloid leukemia — 20%
- Myeloma — 17%
- Colon cancer — 16%
- Head and neck cancer — 15%
- Rectal cancer — 13%
- Bladder cancer — 13%
- Breast cancer — 10%
The people who exercised spent on average 150 minutes per week exercising during their leisure hours, which maybe sounds like a lot, but it works out to only 30 minutes 5 times per week. The exercise was sometimes vigorous such as running or biking, but could also be moderate or fast paced walking. The research showed that the more time the person spent exercising, the greater was the benefit in cancer risk reduction.
How exercise reduces cancer risk
The researchers believe that exercise helped for three reasons. First, some cancers—such as breast and endometrial —are more common with high levels of sex hormones, particularly estrogen, and exercise generally reduces estrogen levels. Second, people who exercise have lower levels of insulin. High insulin levels may predispose to cancer. Third, exercise lowers the overall level of inflammation in the body, and inflammation sets the stage for many cancers.
Interestingly two types of cancer were greater in the exercisers: the risk of malignant melanoma was 27% greater and prostate cancer 5% higher. The increased melanoma risk was only seen in geographic areas with high UV indexes from more sunlight. Physically active people spend more time outside, and sunlight is a known factor that leads to melanoma skin cancer, so this makes sense. This emphasizes the importance of sunscreen for active people.
The slight increase in prostate cancer risk was unexpected, but the researchers did not feel that the exercise actually increased the risk of that cancer, but that the men who were physically active were the ones who were more likely to be checked for that cancer via blood tests and rectal exams. Therefore, these men had smaller cancers discovered early.
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