Relative contributions of urine sulfate, titratable urine anion, and GI anion to net acid load and effects of age

• Published in: Physiological reports Vol. 9; no. 10; pp. e14870 - n/a
• Main Authors: Huo, Jenny, Li, Daniel, McKay, Charles, Hoke, Madeleine, Worcester, Elaine, Coe, Fredric
• Format: Journal Article
• Language: English
• Published: United States John Wiley & Sons, Inc 01.05.2021; John Wiley and Sons Inc; Wiley
• Subjects: Acid production; Acid-Base Equilibrium - physiology; Adult; Age; Aging - metabolism; Ammonia; Anions; Anions - urine; Calcium phosphates; Creatinine; Excretion; Female; Humans; Ions; Kidney - metabolism; Kidney Calculi - diagnosis; Kidney Calculi - urine; Load; Male; Metabolism; Middle Aged; net acid excretion; Original; Oxidation; Patients; Physiology; Potassium; Sulfates; Sulfates - urine; Sulfur; Titration; Uric acid; urinary acidification; Urine; urine pH; ammonia; acid production; urinary acidification; net acid excretion; urine pH
• ISSN: 2051-817X; 2051-817X
• DOI: 10.14814/phy2.14870

Models of acid–base balance include acid production from (1) oxidation of sulfur atoms on amino acids and (2) metabolically produced organic acid anions. Acid load is balanced by alkali from metabolism of GI anions; thus, net acid production is equivalent to the sum of urine sulfate and organic anion (measured by titration in urine), minus GI anion. However, the relative contributions of these three sources of acid production in people eating free choice diets, and presumably in acid–base balance, have not been well studied. We collected 26 urines from 18 normal subjects (10 male) and 43 urine samples from 34 stone formers (17 male) and measured sulfate, organic anion, and components of GI anion and acid excretion in each; values were expressed as mEq/mmol creatinine. Mean values of the urine components, except creatinine and pH, did not differ between the sexes or groups. Urine organic acid and acid production varied directly with age (p ≤ 0.03). In a general linear model of acid excretion, the coefficients for sulfate, organic anion, and GI anion were 0.34 ± 0.09, 0.49 ± 0.12, and −0.51 ± 0.06, respectively, p ≤ 0.005, and the model accounted for 54% of the variance. A model for urine ammonia gave similar results. Urine organic anion is a significant contributor to total acid production and may be responsible for an increase in acid production with age. The kidney must maintain acid‐base balance and acid load from food accounts for only part of daily acid excretion. An equal share comes from metabolic production, which appears to increase with age, while gut alkali absorption neutralizes a variable portion of the acid load. Total acid excretion is best represented by a function that includes all three components.