Which form of zinc is best to take?
If you are choosing a zinc supplement, glycinate and gluconate are probably better options than zinc oxide, although that evidence comes from a commercially involved source. In any case, avoid taking zinc together with a meal high in grains or legumes, as the phytates in them strongly inhibit absorption.
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Zinc glycinate and zinc gluconate are better absorbed than cheaper variants such as zinc oxide or zinc sulfate. That is the finding of a narrative review of clinical studies. Bear in mind, however, that the review was written by employees of a supplement manufacturer (Pharmavite), which colours the conclusions somewhat. The finding is plausible, but is best read with a degree of caution.
If you want to know whether the form used to fortify food makes a difference, the answer is: no. There is no measurable difference in how much zinc the body absorbs from zinc oxide versus zinc sulfate in fortified foods. Both are also recognised as safe for consumption.
A greater influence on zinc absorption than the supplement form is what you eat alongside it. Phytate, a compound found in grains, legumes and seeds, binds zinc in the gut and makes it virtually unabsorbable. This effect is well established and is the reason why people who eat a largely plant-based diet need a higher zinc intake. Anyone wanting to supplement with zinc would be wise not to take it together with a meal rich in wholegrains or beans.
Soluble fibres such as pectin or inulin may slightly improve zinc absorption, but study results are contradictory. No firm recommendations can be based on this yet. The same applies to experimental nanoparticle forms of zinc: in the laboratory these are promising, but no studies in humans have been conducted, so they are not yet relevant in practice.
Findings on glycinate/gluconate come from a single narrative review with a commercial interest. Zinc oxide vs. zinc sulfate in food fortification has been studied using isotope tracers. The phytate effect is robustly supported and recognised by EFSA. Other claims are experimental or inconsistent.