"Natural substances" refer to materials and compounds that occur in nature, such as plants, minerals, and animals. They are not man-made or created synthetically in a laboratory.
The works are characterized by everything from graffiti and chewing gum to investigations of natural substances such as grass and dirt. (afmuseet.no)
However, these will be overshadowed by «functional medicine» which its focus on restoring normal cell physiology with natural substances found in the human biochemical pathways. (bioidenticalhormones101.com)
Unfortunately sleep medications as a group tend to have unpleasant side effects, so many people prefer to use natural substance such as melatonin, and others, instead. (naturopathiccurrents.com)