Being outside in the sunlight is one of the easiest ways to obtain enough vitamin D (something that studies estimate 70% of Americans are deficient in). Running outdoors in nature is one of the best ways to receive this crucial vitamin which helps regulate inflammation in the body. Those who run indoors often have lower levels of vitamin D in their system, and may have increased levels of inflammation throughout the body. When you acquire your Vitamin D from safe, moderate sun exposure, it is impossible to overdose on Vitamin D, as is the case when consuming a supplement form.