I take supplements, and for a short period early in my career I worked at the FDA analyzing nutrition policy. Part of that was analyzing policy related to the D

I take supplements, and for a short period early in my career I worked at the FDA analyzing nutrition policy. Part of that was analyzing policy related to the Dietary Supplement Health and Education Act, which loosely governs dietary supplements. I am not a medical professional, but after talking with a lot of scientists about this topic, the most often repeated advice was that a person using dietary supplements should get a blood test of the nutrients and minerals in their body. This can be done through your doctor or through a wide variety of retail lab companies.