Philadelphia City Councilwoman Blondell Reynolds Brown leads a group of city workers on one of their twice-weekly walks along the corridors of City Hall. NewsWorks
First some background:
Cancer, Physical Activity, And Exercise, Comprehensive Physiology, October 2012
This review examines the relationship between physical activity and cancer along the cancer continuum, and serves as a synthesis of systematic and meta-analytic reviews conducted to date. There exists a large body of epidemiologic evidence that conclude those who participate in higher levels of physical activity have a reduced likelihood of developing a variety of cancers compared to those who engage in lower levels of physical activity. Despite this observational evidence, the causal pathway underling the association between participation in physical activity and cancer risk reduction remains unclear. Physical activity is also a useful adjunct to improve the deleterious sequelae experienced during cancer treatment. These deleterious sequelae may include fatigue, muscular weakness, deteriorated functional capacity, including many others.
How? This very new study (thank you, Shaun) in mice found that epinephrine released during exercise mobilizes natural killer (NK) cells which make their way into tumors and slow their growth:
Voluntary Running Suppresses Tumor Growth through Epinephrine- and IL-6-Dependent NK Cell Mobilization and Redistribution, Cell Metabolism, February 2016
In this study, tumor-bearing mice randomized to voluntary wheel running showed over 60% reduction in tumor incidence and growth across five different tumor models.
People with cancer often experience weight loss, cachexia it’s called, but:
Weight loss was completely prevented in the running tumor-bearing mice.
Natural killer cells are lymphocytes, part of our immune system. There are other anti-cancer immune cells that are activated when we exercise, e.g. T-cells. NK cells seem to hold a prominent position:
NK cells are the most responsive cells to the exercise-dependent mobilization.
Fanatic Cook 
You see that spiky brown blob in the picture? The cancer? A pharmaceutical company would salivate over what those little blue killer cells are doing … finding cancer and shrinking it, leaving all the healthy tissue next to it robust. Yet, our immune system does that by itself, every day, all the time. Doesn’t that get you? It gets me.
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