Certainly one can get magnesium from foods:

Here is a list.
http://ods.od.nih.gov/factsheets/magnesium

Testing blood serum for magnesium will only show very low or very high results that are accurate. The middle range is not reflective of what is happening in the cells themselves, as magnesium floating around is not working for specific purpose.

Some people get intracelluar tests which may be more accurate, but do cost more money.

When supplementing magnesium start at 1/2 the RDA and assume you are getting some from food. 3 ounces of almonds daily will do that for you, or a serving of Edamame beans, etc.

Magnesium is lost, thru the urine when diuretics are used. Caffeine and alcohol included.

This is my magnesium thread:
http://neurotalk.psychcentral.com/thread1138.html

There are doctors who have written books about magnesium's importance:
Dr. Jay Cohen MD is one... who reversed his erythromelalgia with magnesium:
http://www.medicationsense.com/about_cohen.html

Some of his book is available online thru Google.