Does zinc supplementation reduce absorption of calcium or magnesium?

In general, no.  While it’s true that zinc can reduce absorption of calcium it would have to be a high level to do so; studies showing such a relationship have primarily been on zinc sulfate supplements containing about 140 mg of elemental zinc.  This is far above the RDA/DV level of 15 mg. typically found in dietary supplements, or even the 30-50 mg. found in some immune or prostate formulas.  One typical study used 142 mg/day of zinc to achieve this; far higher than the amount in supplement formulas.  The study noted that the decrease in calcium absorption occurred when low calcium and high zinc intake was noted, but not with adequate calcium intake, and specifically tested lower doses of zinc along with low calcium intake and did not find a reduced absorption of calcium at commonly supplemented levels:  https://www.ncbi.nlm.nih.gov/pubmed/1452955

 

So the issue is limited to a very high zinc intake combined with a very low calcium intake, not the levels found in common supplements.  One will find zinc and calcium together in multiple vitamins or some mineral formulas, but at typically available levels they are not shown to cause the reduced absorption of calcium found in those studies cited above. 

 

The issue has also been studied for magnesium versus zinc intake.  Likewise, very high zinc intake interfered with low calcium intake (500 mg.) but less likely at higher levels of calcium intake.  Again, the levels of zinc in question (around 140 mg.) are not typically found in over-the-counter zinc supplements or multivitamins. https://www.ncbi.nlm.nih.gov/pubmed/7836627/ 

 

 It is more convenient for people to supplement with combination formulations instead of taking several single ingredient supplements, typically increasing compliance with label instructions and dosing.  Knowing that it's okay to take minerals in combinations that are commonly found in the marketplace is reassuring when some pundits claim that it is inappropriate based on the unusual circumstances of low mineral intake combined with abnormally high zinc intake.  For physicians prescribing the high strength zinc products, this is something to consider, but not something for consumers of dietary supplement formulas to be concerned about.