Table 2.

Potential biomarkers of high intake or toxicity for iron, zinc, folic acid, and vitamin A

Micronutrient	Potential biomarkers of excess or toxicity (Reference)	Comments on use
Vitamin A	Retinyl esters10	No consensus on cutoffs; affected by fasting status, hypertriglyceridemia, and liver damage
	Retinol isotope dilution10	Quantifies liver vitamin A stores, though no consensus on total body stores or hepatic concentrations indicating toxicity; unknown if affected by inflammation
	Breast milk vitamin A10	Not homeostatically controlled, so may indicate excessive exposure
Folic acid	Unmetabolized folic acid11	No clear relationship between biomarker and clinical adverse effects
Iron	Ferritin16	Can indicate high iron stores in the absence of inflammation
	Serum iron, transferrin saturation, and total iron binding capacity (TIBC)16	Can indicate high iron stores in the absence of inflammation
	Non‐transferrin‐bound iron (NTBI)106	Represents the form of circulating iron likely to cause oxidative damage
Zinc	Plasma copper; ceruloplasmin activity26	No clear relationship between biomarker and clinical adverse effects