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Exercise as it relates to stress is complex. On the one hand, exercise itself is a stressor; on the other, it has been shown to both strengthen the body's response to stress in some cases and weaken it in others. With cancer, this complexity may lead to the avoidance of exercise altogether, the fear being that any extra stress could worsen a weakened condition. Yoga may have special applicability here. It seems to have several mechanisms that set it apart from more intense types of exercise. Yoga can lower stress acutely and may adjust stress mechanisms in the long run, possibly conferring survival benefit to cancer patients.

Stress, Aging, and Cancer
The release of stress hormones, in particular the glucocorticoid cortisol, is a key component of homeostasis in mammals. When it comes to aging and cancer, there are several overlapping mechanisms. The chaperone proteins seem to be of special note here. The word chaperone means a person who looks after or accompanies another person acting as his director or supervisor. Chaperone molecules in the body "look after" proteins and assure that they can fold themselves into the proper configuration to function optimally. Most notable in cancer and aging is chaperone molecules' abilities to stabilize DNA, thus reducing damage during aging and decreasing mutation risk. 
 
Kroll et al. (2010) and Kuningas et al. (2008) published reviews showing that chaperone proteins are powerful players in longevity through their ability to stabilize genes and hormone receptors.^1,2 Interestingly, cortisol has been shown to increase the expression of chaperone proteins, especially those working to protect DNA. Kroll et al. suggest that cortisol may be increasing longevity by reducing the risk of cancer and gene damage during aging. In animals of similar metabolic rates; that is, mice and bats, the higher cortisol levels in bats correlate with a 30-year increase in life expectancy.¹ In humans, high levels of cortisol are often found in centenarians.³ Cortisol also seems to confer survival to differentiated cells in the lab.^1-3

This is where things get confusing. If indeed higher levels of cortisol have an advantage to aging and cancer risk, why the negative research on cortisol and immune function? The issue may not be absolute amounts of cortisol at all, but rather the cortisol rhythm, its balance with other hormones, and cortisol receptor sensitivity. Cushing's disease and chronic stress, where cortisol levels rise unopposed by other hormones like DHEA, show decreased longevity, unfavorable changes related to chaperone proteins, and a blunting of the normal circadian cortisol curve.¹ It is this rhythm and balance that may confer a survival benefit given cortisol's impact on chaperone molecules. Kroll et al. and others believe that this chaperone stabilization is the reason that cortisol has direct tumor-suppressing capabilities; thus maintaining healthy cortiol rhythm and function would seem beneficial.⁴

Yoga, Cortisol, and Cancer
Yoga seems to have special action in relation to stress, cortisol, and cancer. Two recent reviews by Silver et al. (2009) and Desaive et al. (2008) discuss the role of stress in cancer and yoga's potential action against it.^5,6 While the role of stress in causing cancer was not discussed, the role of stress in relation to cancer was. A diagnosis of cancer represents an extreme of emotion and uncertainty and can lead to a deranged stress response in patients. Yoga has shown promise in addressing this, and while all studies are not in agreement, there are some positive trends.⁶ Yoga therapy has been shown to predict survivability in breast cancer, is correlated with better outcomes in cancers of the head and neck, and allays anxiety and stress-related dysfunction in cancer treatment.^7,12-14 The research on yoga and its use in cancer is gaining ground. A recent report by Brauer et al. in 2010 looked at the leading cancer-treatment hospitals in America. This study showed that 56% of these centers incorporate yoga into their clinic offerings.

The mechanism by which yoga seems to work is both genetic and hormonal. However, since hormones like cortisol affect gene expression, it is difficult to know which comes first. Balkrishna et al. in 2009 looked at the expression of 28,000 genes in leukemia patients undergoing yoga therapy or no yoga therapy.⁸ Of the 28,000 genes analyzed, just under 5000 genes were shown to be upregulated in the yoga practitioners vs. the control group. The vast majority of these genes were related to balancing stress-signaling pathways, raising antioxidant status, and increasing survival of white blood cells. Several other studies show that this effect may be due to a healthier modulation and adjustment of cortisol rhythms in the body.^12,15 Yoga seems to do more than simply lower cortisol levels, but rather improves the quality of the circadian rhythm by restoring the "flatter" cortisol curves to a more normal undulating pattern. This may be a key mechanism in yoga's beneficial effect on outcomes in cancer patients.^{5,6,9,11,12,14,15}

Final Thoughts
When analyzing yoga for therapeutic purposes, it is important to be familiar with the different types of yoga. Some yoga can be quite vigorous and stimulating, while others more restorative and relaxing. However, all yoga regardless of intensity focuses practitioners on breathing and encourages a meditative awareness of breath and body. Based on research with walking and meditation, it would seem that more restorative yoga sessions would be most beneficial for those weakened by treatment or the disease process.^9,16 Although more research needs to be done, it appears that yoga has several different mechanisms to aid cancer treatment, most notably cortisol regulation.

Notes

1. Kroll et al. Correlations of plasma cortisol levels, chaperone expression and mammalian longevity: a review of published data. Biogerontology. Epub 2010 Jan 29.
2. Kuningas et al. Genes encoding longevity: from model organisms to humans. Aging Cell. 2008;7:270–280.
3. Genedani et al. Influence of f-MLP, ACTH and CRH on in vitro chemotaxis of monocytes from centenarians. Neuroimmunomodulation. 2008;15:285–289.
4. Chebotaev et al. The mechanisms of tumor suppressor effect of glucocorticoid receptor in skin. Mol Carcinog. 2007;46:732–740
5. Smith et al. An evidence-based review of yoga as a complementary intervention for patients with cancer. Psychooncology. 2009;18(5):465–475.
6. Desaive et al. Stress spectrum disorders in oncology. Curr Opin Oncol. 2008;20(4):378–385
7. Silver et al. Predictors of functional decline in locally advanced head and neck cancer patients from South Brazil. Head Neck. Epub 2010 Jan 6.
8. Balkrishna. To study the effect of the sequence of seven pranayama by Swami Ramdev on gene expression in leukemia patients and rapid interpretation of gene expression. J Clin Pathol. 2009;62(11):1052–1053.
9. Ando et al. A qualitative study of mindfulness-based meditation therapy in Japanese cancer patients. Support Care Cancer. Epub 2010 May 16.
10. Brauer et al. Complementary and alternative medicine and supportive care at leading cancer centers: a systematic analysis of websites. J Altern Complement Med. 2010;16(2):183–186.
11. Ulger et al. Effects of yoga on the quality of life in cancer patients. Complement Ther Clin Pract. 2010;16(2):60–63.
12. Vadiraja et al. Effects of a yoga program on cortisol rhythm and mood states in early breast cancer patients undergoing adjuvant radiotherapy: a randomized controlled trial. Integr Cancer Ther. 2009;8(1):37–46.
13. Rao et al. Anxiolytic effects of a yoga program in early breast cancer patients undergoing conventional treatment: a randomized controlled trial. Complement Ther Med. 2009;7(1):1–8
14. Danhauer et al. Restorative yoga for women with breast cancer: findings from a randomized pilot study. Psychooncology. 2009;18(4):360–368.
15. Sephton et al. Diurnal cortisol rhythm as a predictor of breast cancer survival. J Natl Cancer Inst. 2000;92(12):994–1000.
16. Tsunetsugu et al. Physiological effects of Shinrin-yoku (taking in the atmosphere of the forest) in an old-growth broadleaf forest in Yamagata Prefecture, Japan. J Physiol Anthropol. 2007;25(2):135–142.