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Contribution of dietary protein and inorganic sulfur to urinary sulfate: toward a biomarker of inorganic sulfur intake^1,2,3

¹ From the Department of Molecular and Cellular Pathology, Ninewells Hospital and Medical School, Dundee, United Kingdom.

Background: Sulfiting agents are widely used as food additives. Limits are set on their use in foods because they may adversely affect health. Sulfiting agents are excreted in urine as sulfate, which is indistinguishable from sulfate derived from sulfur amino acids.

Objective: The objective was to assess the contribution of inorganic sulfur to urinary sulfate excretion and of dietary protein to urinary sulfate and nitrogen excretion with the aim of developing a urinary biomarker of inorganic sulfur intake.

Design: Nine healthy men were fed a sequence of 3 diets for 15 d (n = 7), 5 diets for 10 d (n = 6), or both. The diets contained 51–212 g protein/d (0.43–1.71 g S/d) and 0.17–0.27 g inorganic S/d; p-aminobenzoic acid-validated 24-h urine samples (n = 47) were analyzed for sulfate and nitrogen.

Results: Dietary inorganic sulfur was efficiently excreted as sulfate in urine. Urinary sulfate derived from protein correlated strongly (r² = 0.86) with urinary nitrogen. Urinary recovery of protein sulfur and nitrogen decreased from 84% at average protein intakes (72 g/d) to 70% at high protein intakes (212 g/d). The nitrogen:sulfur ratio (in g) of the protein in the study diets was 18.9, which was maintained in urine (18.4 ± 0.1) after dietary inorganic sulfur intake was subtracted from urinary sulfate. Therefore, inorganic sulfur intake (g/d) = urinary sulfur (g/d) – 0.054 x urinary nitrogen (g/d). For typical UK intakes of inorganic sulfur (0.25 g/d), this biomarker should produce mean (± SD) values of 0.24 ± 0.10 g S/d.

Conclusion: Twenty-four-hour urinary nitrogen and sulfate values can be used to predict inorganic sulfur intake.

Key Words: Sulfate • sulfiting agents • inorganic sulfur • urine • biomarker • nitrogen • protein