Updated: Jul 22, 2018
From increased memory to reduced heart rates and improved mental wellbeing, the health benefits of getting outside are now being recognized by doctors and scientists worldwide.
People are spending, on average, an incredible 95 percent of their time indoors. Researchers have varying recommendations about how often people should be outside, but scientists recommend at least five hours outside a month.
Outside exercise in green outdoor surroundings may be a useful natural medicine (vis medicatrix naturae) to address health challenges facing developed countries. 
You’ll burn more calories
Outdoor natural environments may provide some of the best all-round health benefits by increasing physical activity levels with lower levels of perceived exertion. Performing the same exercise outdoors requires your body to work harder than doing the same activity indoors. Training in a climate-controlled environment does not supply the same resistance to your body as doing exercises in an environment that has high (or cold) temperatures and changing terrain that affects you pace. 
Research from the University of Exeter has found that road runners burn more calories when running at the same speed than treadmill runners, mainly because of the wind resistance they encounter. 
Increased mental well-being
Compared with indoor exercise, training in natural environments is associated with greater feelings of revitalisation , increased energy and positive engagement, together with decreases in negative mood sub-scales such as tension, confusion, anger and depression [5,6]. Enjoyment and satisfaction are greater with outdoor activity and it is stated that people were more likely to repeat the activity at a later date.
Alongside the social aspect of training in group, outdoor exercise may also increase enjoyment and adherence as it brings about positive behaviour changes in a large proportion of the population.
Free shot of vitamin D
Sunshine, not food, is where most of your vitamin D comes from. So even a healthy, well balanced diet, that provides all the other vitamins and goodness you need is unlikely to provide you with enough vitamin D .
You make vitamin D under your skin when you are outside in daylight, which is the reason vitamin D is sometimes called the ‘sunshine vitamin’. A vitamin is something that helps your body to function – a ‘nutrient’ – that we cannot make ourselves in our bodies. Vitamin D is different because, even though we call it a vitamin, it is actually a hormone and we can make it in our body.
It is the sun’s ultraviolet rays with a wavelength of 290 - 320nm that allows vitamin D to be made in the body. In human language this means that you do not have to sunbathe hours after hours to make vitamin D.
In Belgium, ultraviolet light is only strong enough--read: 'has the correct wavelength'--to make vitamin D on exposed skin (on the hands, face and arms or legs) during the months of April to September.
Clouds reduce the intensity of the UV-rays and limit therefore the vitamin D synthesis. However, strong sun also burns skin so we need to balance making vitamin D with being safe in the sun--take care to cover up or protect your skin with sunscreen before you get grilled.
During autumn and winter, we get vitamin D from our body’s stores, as vitamin D can be stored in our fat reserves, and from food sources but these are insufficient to keep up vitamin D levels, even for people with healthy lifestyles. Therefore the only way to ensure a healthy vitamin D status is to take a supplement.
You'll workout longer & harder
When you decide to exercise outdoors, the green scenery generally makes you enjoy your time spent sweating even more. That means you are “nudged” to work out longer. The oxygen in the air, the space & the natural ventilation caused by the wind enable you to work harder on the same movements which are beneficial for your fitness level and general health. The outside cold or hot temperatures, the changing terrain, and the wind are all factors that cause more resistance and ensure you have to work harder compared to indoor controlled conditions.
The great outdoors, therefore, should not be just considered a playground for those who seek the thrills of extreme sports, but emphasis should be placed on access for all.
You feel more energetic and enthusiastic, so you work out longer and harder while you experience a decrease in tension levels or frustrations.
Maintenance & development of nature
Parks and open recreational domains should be maintained to produce interesting areas of high biodiversity, where more sports should be promoted to be played, so as to increase opportunities for exercise. Not only may a well maintained surrounding elicit greater health benefits, it could also better protect the natural environment and preserve species.
The management of outdoor terrains, forests and more savage environments also needs careful consideration, including ensuring access for all. At the same time without pressing for too many people to visit these areas, as this would potentially destroy the natural environment that elicits these health benefits.
The challenge for researchers in this field is not only to determine whether knowledge of nature’s health benefits can act as a motivator for behaviour change, but also to ensure that the increased use of ‘nature as a therapy’ is accompanied by a conservationist approach that preserves the environment. It is wished for that by more individuals partaking in green exercise and enjoying the great outdoors, they will retain their evolutionary connection with nature and act to become more protective of it.
The challenge for researchers in this field is not only to determine whether knowledge of nature’s health benefits can act as a motivator for behaviour change, but also to ensure that the increased use of ‘nature as a therapy’ is accompanied by a conservationist approach that preserves the environment. It is wished for that by more individuals partaking in green exercise and enjoying the great outdoors, they will retain their evolutionary connection with nature and act more protective to preserve it.
 The Peninsula College of Medicine and Dentistry. "Benefits of outdoor exercise confirmed." ScienceDaily. ScienceDaily, 5 February 2011. <www.sciencedaily.com/releases/2011/02/110204130607.htm>.
 Jones AM, Doust JH."A 1% treadmill grade most accurately reflects the energetic cost of outdoor running." J Sports Sci. (1996 Aug)
 Fellin, Rebecca E., Kurt Manal, and Irene S. Davis. “Comparison of Lower Extremity Kinematic Curves During Overground and Treadmill Running.” Journal of Applied Biomechanics 26.4 (2010)
 Thompson Coon J, Boddy K, Stein K, Whear R, Barton J, Depledge MH. Does participating in physical activity in outdoor natural environments have a greater effect on physical and mental wellbeing than physical activity indoors? A systematic review. Environ Sci Technol. 2011;45(5):1761–1772. [PubMed]
 Barton J, Hine R, Pretty J. The health benefits of walking in greenspaces of high natural and heritage value. J Integr Environ Sci. 2009;6(4):261–278.
 Pretty J, Peacock J, Hine R, Sellens M, South M, Griffin M. Green exercise in the UK countryside: effects on health and psychological well-being and implications for policy and planning. J Environ Plan Manag. 2007;50(2):211–231