The phrase "natural sources" refers to things that come from nature without human intervention. It includes things like air, water, plants, animals, and minerals that are not made or changed by people. Full definition
Obviously you should be trying to get lots of natural sources of protein but with a busy lifestyle, I find that some supplements and snacks can be helpful. (breathedeeplyandsmile.com)
There are also plenty of natural sources for key vitamins and minerals as well as probiotics and healthy extracts. (pawster.com)
Your body just doesn't use fake sources of vitamins as readily as natural sources from real food. (truthaboutabs.com)