I’m biased. I run on both roads and trails, but the latter is where my heart is. Personally, I believe everyone should be out running on dirt, rocks and roots. Scientifically, evidence also suggests that runners can benefit from time spent on the trails for three reasons: you’ll move better, be happier and get sick less often.

Road running takes place on a mostly flat, featureless surface. Save for a few cracks and pebbles, road running lacks novel stimulation for the ankles, knees, hips and muscles. Lack of variability is neurologically boring, so having “movement variablity” is key. The term describes the ongoing, unconscious reactions an athlete must make to a changing environment. It’s been called “repetition without repetition.”

In trail running, every foot fall demands numerous reactions both big and small as your joints and muscles negotiate rocks, roots and other terrain. Everything from the toes, ankles, knees, hips and all the way up the spine is involved. This process builds your movement vocabulary and improves your running skills. The variation in movement and impact strengthens your tissues in ways that help you avoid overuse injuries.

Further research from Stanford University indicates that walking in nature, away from an urban setting, also has a positive effect on mood and may help reduce depression. Researchers had two groups of subjects walk for 90 minutes; one walked in grassland, the other along a four-lane road. Both groups tracked heart rate, respiration and brain activity before and after the walks.

This showed signficant changes in the brain. According to the study, “Neural activity in the subgenual prefrontal cortex, a brain region active during rumination—repetitive thought focused on negative emotions—decreased among participants who walked in nature versus those who walked in an urban environment.”

Along with its mood benefits, spending time in nature provides a host of physiological benefits according to research from the University of East Anglia. Similar findings were reported in a literature review from the University of Illinois College of Agricultural, Consumer and Environmental Sciences. The study examined over 140 studies involving more than 290 million people. Researchers found that spending time in green space came with several health benefits including reduced risk of type II diabetes, cardiovascular disease, early death, preterm birth and increased sleep duration.

They found that people who lived close to nature had reduced diastolic blood pressure, heart rate and physiological markers of stress. “In fact, one of the really interesting things we found is that exposure to greenspace significantly reduces people’s levels of salivary cortisol, a physiological marker of stress,” the study revealed.

Research from the University of Illinois found similar effects of time spent in nature. While they weren’t entirely certain how nature exerts it’s positive health effects, they found that living near green space allowed for more opportunities to exercise and therefore exposed subjects to organic compounds in vegetation and soil that enhanced the immune system.

“The realization that there are so many pathways helps explain not only how nature promotes health, but also why nature has such huge, broad effects on health. Nature doesn’t just have one or two active ingredients,” writes University of Illinois environment and behavior researcher Ming Kuo. It’s more like a multivitamin that provides us with all sorts of the nutrients we need.

That’s how nature can protect us from all these different kinds of diseases—cardiovascular, respiratory, mental health, musculoskeletal, etc. Kuo suggests that exposure to nature helps switch us away from “fight or flight” mode and into “rest and digest” mode. “When the body is in ’fight or flight’ mode, it shuts down everything that is immediately nonessential, including the immune system,” she says.