Identifying mechanisms for how exercise impacts cancer risk

The mechanisms linking physical activity to cancer risk and control were first proposed by Dr. Anne McTiernan (AICR/WCRF CUP Panel member), almost a decade ago. AICR/WCRF Continuing Update Project reports show that physical activity reduces risk for breast, colon and endometrial cancers.

Now, a review from a research group in Denmark sums up the current understanding of underlying mechanism and the molecular basis of such connections. Their review concludes that exercise reduces cancer risk by inhibiting tumor growth, improves quality of life by alleviating symptoms and enhances treatment by improving immune response and tumor perfusion with chemotherapy.

Understanding the mechanisms of how exercise plays a role in cancer risk and control may help providers personalize exercise prescription or dose to enhance prevention and maximize benefits in patients and survivors.

Controls growth of tumors

Exercise has consistently been shown to reduce the growth rate of tumors in animal models. This is due to both physical and hormonal effects within tumors. Physical activity increases blood flow which helps mobilize immune factors to attack and slow the growth of tumor cells. Exercise also stimulates signaling that can change cell growth, reduce the chance of the cancer spreading and make tumor cells more visible to the immune system.  

Although exercise in itself is not capable of directly eradicating tumors, it may contribute to slowing or stopping metastatic potential. In lab studies, reduction in tumor growth by exercise training has been as high as 67 percent. Adding serum from mice that exercised, to cell studies, inhibits cell growth by 10-15% compared to adding serum from mice that have not exercised. 

Alleviation of symptoms

Some cancer patients experience substantial weight loss (cachexia) and muscle loss (sarcopenia) caused either by treatment or, for certain cancers, through factors released by their tumors. In one animal study, exercise eliminated muscle and weight loss. However, in human studies, the weight loss and gain responses, even among those with a specific cancer type undergoing the same treatment, are highly variable and the mechanisms are currently unknown.

Cancer patients often experience depression, anxiety, and cognitive impairment during treatment. Many exercise intervention studies have shown that exercise training, in particular endurance training, can help reduce such symptoms. A molecular link between exercise and depression in mice shows exercise increasing the metabolism of a tryptophan metabolite (kynurnine) that can induce depression; if kynurnine is metabolized in the muscles, the levels in the brain is reduced. More studies are needed to determine how these results might apply to cancer patients.

Improves treatment efficacy

Exercise training strongly affects blood circulation and facilitates oxygen delivery to peripheral tissues. The elevated blood pressure during vigorous exercise may increase   the ability of chemotherapy agents and immune cells to pass through and into the tumor. 

Most of the studies examining mechanisms have been in animal models of cancer. However, they enable researchers to focus on the most likely candidate mechanisms in humans.

The authors suggest it is important to incorporate exercise training into standard treatments for cancer patients, from surgery to radiotherapy, chemotherapy, immunotherapy, and other treatment approaches. This is recommended not only to improve their overall health but also as a targeted approach to regulate cancer progression and formation and to improve cancer-associated adverse events, and anti-cancer treatment efficacy.

The awareness of beneficial effects of exercise in cancer patients needs to be promoted, the study says. The authors believe that with a growing knowledge into the link between physical activity and cancer control, exercise training in cancer patients can move from a “one fits all” approach to a more individualized approaches where one can determine the “dose” of exercise.

 

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    Published on January 24, 2018

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