Health+Benefits+of+Green+Exercise

=**Introduction**=

= = Over the past few years, physicians have started writing prescriptions for outdoor exercise. toc These ‘prescriptions’ range from walking in a local park to takinga white-water rafting trip 1 . As researchers uncover additional links between time spent in natural environments and psychological and physical health, a greater number of healthcare professionals are jumping on the ‘green exercise’ bandwagon. Green exercise is a term that refers to exercising in nature, rather than indoors or in a crowded downtown area. Studies have linked these activities to increased immune system functioning 2 , improved short term memory 2 , and reduced PTSD symptoms 1 . After discussing the declining health in America, this review will examine the four main biochemical pathways encompassing green exercise research. A short discussion on the possible social implications of the research will follow.

=**Declining Health in America **=

= = Over one-third of Americans (36.5%) are obese 3 , and 31.1% of adults worldwide are inactive 4 . Furthermore, the number of Americans experiencing chronic stress continues to increase. The 2016 //Stress in America// survey released by the American Psychological Association stated that “24 percent of adults reported experiencing extreme stress 5 .” The direct negative effects of chronic stress on health, specifically the immune system, have been well documented 6 . Chronic stress can cause “headaches, insomnia, weight gain, anxiety, pain, and even high blood pressure 5 .” Finding an inexpensive, simple cure to these health issues could help reduce healthcare costs and increase patients’ quality of life.

=**Biochemical Pathways **=

= = Most green exercise research can fall into one of four biological pathways: environmental factors, behavioral conditions, and physiological and psychological states 2 . These pathways may be connected to one another through a central biochemical mechanism. The most likely option is through immune system functioning. The immune system's complex network of cells, tissues, and organs work together to fight disease. When this system becomes damaged, various health complication can occur. <span style="font-family: 'Arial',sans-serif;">If exposure to nature helps to repair this damage throughout the body, green exercise may prove to be the miracle cure society has been looking for.

**<span style="font-family: 'Arial',sans-serif;">Environmental Factors **
<span style="font-family: 'Arial',sans-serif;">Environmental factors consist of specific biological and chemical components found in natural areas. In 2008, a study showed that a “forest environment enhanced human natural killer (NK) cell activity, the number of NK cells, and intracellular anti-cancer proteins in lymphocytes 7 <span style="font-family: 'Arial',sans-serif;">.” Natural killer cells are a type of white blood cell that serve as an important part of the immune system; they are able to quickly identify and destroy infected cells before the infection can spread throughout the body. The study was inconclusive as to which variables were contributing to NK cell activity, but researchers hypothesized that the natural phytoncides, which are aromatic volatile compounds given off by trees, were responsible for the changes 7 <span style="font-family: 'Arial',sans-serif;">. Researchers later isolated the phytoncides and tested their effects on natural killer cells in a controlled environment. Test subjects were exposed to vaporized //Chamaecyparis obtuse// stem oil in a hotel room over a three-day period. Daily blood and urine testing showed that “phytoncide exposure significantly increased human NK cell activity and the percentage of CD16+ NK cells 7 <span style="font-family: 'Arial',sans-serif;">.” The exposure, however, did not affect white blood cell counts.

**<span style="font-family: 'Arial',sans-serif;">Behavioral Conditions **
<span style="font-family: 'Arial',sans-serif;">Behavioral conditions, such as sleep duration, social ties, and levels of physical activity, are positively linked to green exercise as well 2 <span style="font-family: 'Arial',sans-serif;">. Research has shown that although people who hike outside exert more energy than those who complete the same distance and elevation changes indoors, they rate their exertion as less and the exercise as more enjoyable 8 <span style="font-family: 'Arial',sans-serif;">. In fact, green exercise improves participants’ self-esteem and mood after only five minutes of exposure 9 <span style="font-family: 'Arial',sans-serif;">. Additionally, the restorative qualities of green exercise indicate that people will exercise more frequently if they do so in natural environments 10 <span style="font-family: 'Arial',sans-serif;">.

**<span style="font-family: 'Arial',sans-serif;">Physiological States: Biological Markers **
<span style="font-family: 'Arial',sans-serif;">A large portion of green exercise research focuses on measuring how exposure to nature affects levels of biological markers. In 2011, walking in the woods was shown to have a therapeutic effect on blood pressure 11 <span style="font-family: 'Arial',sans-serif;">. Hypertensive participants were split into two groups. One group took leisurely walks through the forest, while the other walked through an urban area. All other environmental conditions were controlled; the participants stayed in hotels, ate the same meals, and were monitored over a seven-day period. Although blood pressure stayed the same for those in the urban control group, the nature participants had a clinically significant decrease in both systolic and diastolic blood pressures (SBP and DBP) 11 <span style="font-family: 'Arial',sans-serif;">. Blood samples before and after the study to determine which cardiovascular disease-associated factors accounted for the changes 11 <span style="font-family: 'Arial',sans-serif;">. The blood was analyzed for levels of <span class="st" style="font-family: 'Arial',sans-serif;">homocysteine (Hcy), <span style="font-family: 'Arial',sans-serif;">endothelin-1 (ET-1), and constituents of the renin-angiotensin system (RAS), including renin, angiotensinogen (AGT), angiotensin II (ANGII), and ANGII receptor type 1 or 2 (AT1 or AT2), “as these factors have been reported having close linkages to essential hypertension 11 <span style="font-family: 'Arial',sans-serif;">.” Although blood levels in the forest group revealed a mild reduction in the renin and angiotensin II factors, the changes were not clinically significant. There was, however, “a significant association between SBP and Ang II (//<span style="font-family: 'Arial',sans-serif;">p // < 0.001), ET-1 (//<span style="font-family: 'Arial',sans-serif;">p // < 0.01), and Hcy (//<span style="font-family: 'Arial',sans-serif;">p // < 0.01) was observed. DBP was significantly associated with Ang II (//<span style="font-family: 'Arial',sans-serif;">p // < 0.05) and ET-1 (//<span style="font-family: 'Arial',sans-serif;">p // < 0.05) 11 <span style="font-family: 'Arial',sans-serif;">.”

<span style="font-family: 'Arial',sans-serif;">A similar study was later conducted examining the effects of nature on patients with chronic heart failure (CHF) 12 <span style="font-family: 'Arial',sans-serif;">. An almost identical methodology to the hypertension study was used and blood tests analyzed the levels of the same cardiovascular disease-associated factors. Levels of B-type natriuretic peptide (BNP) were also analyzed as they are “directly associated with cardiac hemodynamics and symptom severity in patients with HF and, therefore, [serve] as a marker of cardiac functional status 12 <span style="font-family: 'Arial',sans-serif;">.” Participants in the forest group showed significantly lower levels of BNP compared to those in the city group, thus suggesting that exposure to nature has a favorable effect on heart function. Furthermore, the forest group showed significantly lower levels of ET-1 and increased levels of AT2, which plays an important role in preventing the onset of chronic heart failure 12 <span style="font-family: 'Arial',sans-serif;">.

**<span style="font-family: 'Arial',sans-serif;">Psychological States: Mental Health and Stress **
<span style="font-family: 'Arial',sans-serif;">Many studies examine how green exercise effects both perceived and measured levels of stress and overall mental health. A 2016 study examined salivary amylase activity as an indicator of an individual's stress level after exposure to natural environments 13 <span style="font-family: 'Arial',sans-serif;">. Amylase is an enzyme that breaks down starch into sugars that the body can use for energy. Levels of salivary amylase have shown to be strong indicators of stress in humans. These levels were measured before and after walks in both urban and forest environments using hand-held monitors. Walks in the natural environment reduced amylase activity, thus showing a reduction in stress levels, as compared to walks in urban environments 13 <span style="font-family: 'Arial',sans-serif;">.

<span style="font-family: 'Arial',sans-serif;">Stress levels have also been measured via surveys and perceived wellness questionnaires. Researchers in Germany found that participants of an eight-day wilderness program experienced a positive impact on their subjective well-being and perceived stress levels 14 <span style="font-family: 'Arial',sans-serif;">. Participants reported “less stress and increased mindfulness, self-efficacy, happiness, and overall life satisfaction” after the trip’s completion 14 <span style="font-family: 'Arial',sans-serif;">. The study’s randomized controlled group allowed for these perceived health changes to be deemed clinically significant. The wilderness and control groups were both instructed to fill out questionnaires before and after the class trip. The questionnaire included questions such as “Your problems seem to be piling up” and “I am confident that I could deal efficiently with unexpected events 14 <span style="font-family: 'Arial',sans-serif;">.” Although the responses remained constant for those in the control group, students who participated in the excursion reported “less stress and increased mindfulness, self-efficacy, happiness, and overall life satisfaction” after the trip’s completion 14 <span style="font-family: 'Arial',sans-serif;">. Overall happiness, for example, increased from 7.73 to a 9.07 in the outdoor participants on a scale from 1 to 10 14 <span style="font-family: 'Arial',sans-serif;">.

<span style="font-family: 'Arial',sans-serif;">Dacher Keltner, a psychologist at the University of California at Berkeley, is currently studying the psychological and physiological responses of nature on those suffering from post-traumatic stress disorder (PTSD) 1 <span style="font-family: 'Arial',sans-serif;">. Although the results of his studies are promising thus far, the next step will be to create control groups and narrow down the biochemical signaling mechanisms responsible for improving symptoms.

=**<span style="font-family: 'Arial',sans-serif;">Conclusion **=

= = <span style="font-family: 'Arial',sans-serif;">In this review, the four main biological mechanisms encompassing green exercise research were identified and discussed. These pathways include environmental factors, behavioral conditions, and physiological and psychological states. A central biochemical mechanism, specifically immune system functioning, may reveal the overreaching explanations to the positive effect of green exercise on health. Given that the direct negative effects of chronic stress on the immune system have been well documented, it is not a stretch to connect these studies to immune system functioning as well. In addition to being perceived as more enjoyable, green exercise has been shown to increase self-esteem, decrease biological markers associated with hypertension and chronic heart failure, and reduce both perceived and measured levels of stress. Although results have been encouraging thus far, the entire field needs additional research conducted to support its findings. More research is still needed to determine whether a central biochemical pathway exists and, if so, whether immune system functioning is the answer.

=**<span style="font-family: 'Arial',sans-serif;">Discussion **=

<span style="font-family: 'Arial',sans-serif;">If research indicates that access to natural areas is essential to public health, green space and natural elements should be accounted for during city planning and development. Additionally, mental health researchers note that “youths from less privileged social groups may have greater difﬁculties to take part in structured outdoor adventures 14 <span style="font-family: 'Arial',sans-serif;">.” Improving access to wilderness areas, whether by integrating more natural areas in urban settings or by adding outdoor programs to school curriculums, can help guarantee all adolescents and young adults are able to build resiliency and strengthen their overall mental health.

<span style="font-family: 'Arial',sans-serif;">From the healthcare perspective, insurance providers are already taking note of the positive associations between green exercise and health. Tyler Norris, vice president of total-health partnerships at Kaiser Permanente, remarked, “The quality of the science and what we are learning is encouraging. If we could package the outdoors and call it a pharmaceutical, it would be sold widely 1 <span style="font-family: 'Arial',sans-serif;">.” It may seem unbelievable that insurance could one day reimburse people for hiking gear or white-water rafting trips, but some health professionals believe that it might not be so far-fetched. Stephen Lockhart, chief medical officer at Sutter Health, explains, “Insurance companies are all about managing risk and reducing costs. So if outdoor adventure is seen as a way to do that, they’ll adopt it 1 <span style="font-family: 'Arial',sans-serif;">.”

=**<span style="font-family: 'Arial',sans-serif;">Works Cited **=

= = 1 <span style="font-family: 'Arial',sans-serif;"> Reimers, Frederick. It’s time for doctors to prescribe outdoor therapy [Internet]. Outside; 2016 Oct 24 [cited 2017 June 15]. Available from https://www.outsideonline.com/2127271/time-for-doctors-prescribe-outdoor-therapy

2 <span style="font-family: 'Arial',sans-serif;"> Kuo, Ming. How might contact with nature promote human health? Promising mechanisms and a possible central pathway. Frontiers in Psychology [Internet]. 2015 Aug 25 [cited 2017 June 15]; 6(1093). Available from http://journal.frontiersin.org/article/10.3389/fpsyg.2015.01093/full

3 <span style="font-family: 'Arial',sans-serif;"> Adult obesity facts [Internet]. Centers for Disease Control and Prevention; 2016 Sept 1 [cited 2017 June 14]. Available from https://www.cdc.gov/obesity/data/adult.html

4 <span style="font-family: 'Arial',sans-serif;">Gladwell V, Brown D, Wood C, Sandercock G, and Barton J. The great outdoors: how a green exercise environment can benefit all [Internet]. Extrem Physiol Med. 2013 Jan 3 [cited 2017 July 3]; 2(3). Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3710158/

5 <span style="font-family: 'Arial',sans-serif;"> Kelly J. What are the leading causes of stress for Americans? [Internet]. Advocate Health Care; 2017 Jan 26 [cited 2017 July 3]. Available from: http://www.ahchealthenews.com/2017/01/26/leading-causes-stress-americans-2/

6 <span style="font-family: 'Arial',sans-serif;"> Pruett S. Stress and the immune system [Internet]. Pathophysiology. 2003 May [cited 2017 July 2]; 9(2): 133-153. Available from: http://www.pathophysiologyjournal.com/article/S0928-4680(03)00003-8/fulltext

7 <span style="font-family: 'Arial',sans-serif;"> Li Q et al. Effect of phytoncide from trees on human natural killer cell function [Internet]. Int J Immunopathol Pharmacol. 2009 Dec [cited 2017 June 27]; 22(4): 951-959. Available from: http://journals.sagepub.com/doi/pdf/10.1177/039463200902200410

8 <span style="font-family: 'Arial',sans-serif;"> Niedermeier M, Einwanger J, Hartl A, Kopp M. Affective responses in mountain hiking—A randomized crossover trial focusing on differences between indoor and outdoor activity [Internet]. PLOS. 2017 May 16 [cited 2017 June 27]. Available from: https://doi.org/10.1371/journal.pone.0177719

9 <span style="font-family: 'Arial',sans-serif;"> American Chemical Society. In the green of health: just 5 minutes of ‘green exercise’ optimal for good mental health. Science Daily. 2010 May 10 [cited July 9]. Available from: https://www.sciencedaily.com/releases/2010/05/100502080414.htm

10 <span style="font-family: 'Arial',sans-serif;"> Hug et al. Restorative qualities of indoor and outdoor exercise settings as predictors of exercise frequency [Internet]. Heath & Place. 2009 Dec [cited 2017 July 9]; 15(4): 971-980. Available from: http://www.sciencedirect.com/science/article/pii/S135382920900029X

11 <span style="font-family: 'Arial',sans-serif;"> Mao et al. Therapeutic effect of forest bathing on human hypertension in the elderly [Internet]. Journal of Cardiology. 2012 Dec [cited 2017 June 27]; 60(6):495-502.

12 <span style="font-family: 'Arial',sans-serif;"> Mao G et al. The salutary influence of forest bathing on elderly patients with chronic heart failure [Internet]. Int J Environ Res Public Health. 2017 March 31 [cited 2017 July 9]; 14(4): 368. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5409569/

13 <span style="font-family: 'Arial',sans-serif;"> Yamaguchi M, Deguchi M, Miyazaki Y. The effects of exercise in forest and urban environments on sympathetic nervous activity of normal young adults [Internet]. Journal of International Medical Research. 2016 June 25 [cited 2017 June 27]; 34(2): 152-159. Available from: http://journals.sagepub.com/doi/abs/10.1177/147323000603400204

14 <span style="font-family: 'Arial',sans-serif;">Mutz, M and Muller, J. Mental health benefits of outdoor adventures: results from two pilot studies. Journal of Adolescence [Internet]. 2016 June [cited 2017 June 21]; 49(105-114).