My son has been taking magnesium supplements for 6 years now and I truly believe it is just as important as the GSH. It makes such a difference in his energy level and overall well being.
And Kim is right about the doctors being absolutely ignorant on this subject. My son was 4 or 5 years old, and complained almost daily of leg pain and the posts by Kim and others led me to believe he was suffering from Mg deficiency. But, when I discussed this with our CF doc, he stood there with his hands on his hips and said magnesium deficiency does not exist in this country. I started giving my son Mg supplements anyway and within weeks the pain in his legs went away.
Kim Payne recommends Solgar Magnesium Glycinate, but those pills are rather large. We use buffered magnesium glycinate by Kirkman Labs. The capsules are small and they are buffered so they are less likely to irritate the digestive tract.