Question: What to do when serum magnesium is high but RBC magnesium is low?

The magnesium in the blood and the hair is high. When you say blood, I'm assuming this is serum or plasma because the RBC magnesium is low. I'm hoping that's not whole blood magnesium in which case it would be hard to separate from the RBC magnesium. But I mean even for whole blood, if the RBC magnesium is low and the blood magnesium is high, then the magnesium that's in the blood that's high is in the serum or plasma, not in the RBCs obviously.

Clearly this means that you're deficient in magnesium transport. You're not deficient in magnesium. So, the last thing that you should do is start blasting high-dose magnesium at that. Because not only is it not going to help, but you basically have two or three times the risk of harm from supplementing high-dose magnesium, because the harm of high-dose magnesium comes when your serum levels go to double the upper limit of the reference range. If your serum level is high, and your RBC is low, and you start blasting.

B6, even if it's adequate, maybe try 10 milligrams. Work your way slowly up to 100 milligrams of P5P. See if that helps. If it doesn't, you probably have a more serious issue with magnesium transport. You might have a rare genetic defect in a magnesium transporter. Off the top of my head, I'm not sure how to manage that. There's probably things you can't do. It might come down to just maximizing all the different possible ways that you can get magnesium into your system and cells. That might mean that you want a modest hypermagnesemia.

In other words, you want your serum magnesium to be a little over the top of the upper reference range in order to try to drive magnesium into the red blood cells. But you still need to measure it regularly so that you know that you're not anywhere near twice the top of the upper reference range. Then just do what you can to maximize the other factors. Insulin, salt, and B6 is what I think there.

This Q&A can also be found as part of a much longer episode, here: https://chrismasterjohnphd.com/podcast/2019/03/30/ask-anything-nutrition-march-4-2019

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