In long-term PN patients, which statement about zinc status is true?

• A. Zinc deficiency is the most commonly suspected trace element abnormality
• B. Serum zinc is a reliable indicator of zinc status
• C. Parenteral doses of 50 mg/day have been proven safe
• D. Parenteral zinc interferes with copper bioavailability

Answer: (D)

In long-term PN, trace element management must account for how elements affect each other. Zinc given parenterally in high amounts can drive copper deficiency by a mechanism involving metallothionein: zinc stimulates metallothionein production in enterocytes, and metallothionein binds copper tightly, reducing its absorption. As enterocytes are shed, bound copper is lost with it, lowering systemic copper bioavailability. This interaction—zinc interfering with copper absorption and status—is the reason this statement is true.

Serum zinc levels aren’t a reliable read on body zinc status because they vary with inflammation, infection, and timing of the draw, so a normal value doesn’t guarantee adequate zinc stores or function.

Doses of 50 mg/day of parenteral zinc have not been proven safe, so assuming safety at that level isn’t supported.

Zinc deficiency can occur, but the clinically important and testable point here is how zinc can compromise copper bioavailability in PN.