Question: "Why would a male be low in calcium?"

You either have something wrong with parathyroid hormone governing your calcium levels, in which case you would want to see a doctor about that, or you have a long-going deficiency of related nutrients. 

Not enough calcium and not enough vitamin D should not cause low serum calcium — unless the deficiency has been going on for a very long time and is very bad.

Then again, I don't know what measurement you're referring to. So, maybe the calcium was a tiny bit low, and you remeasure it, and it's not low anymore; it was a fluke. 

But if you're talking about confirmed low serum calcium, then nutritionally, I would look at long-standing severe deficiencies of calcium and vitamin D. I'd follow it up with measurements of PTH and calcitriol to better assess the situation.

This Q&A can also be found as part of a much longer episode, here:https://chrismasterjohnphd.com/podcast/2019/02/09/ask-anything-nutrition-feb-1-2019/ 

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Access the show notes, transcript, and comments here.