Strong Ion Difference: Questionable Stewardship.

A follow-up to a recent discussion of urinary acidity focused on Stewarts's theory of 'strong ion difference' (SID). The comments of Vasileiadis et al. warrant a reevaluation of Stewart's approach. Here, it will be discussed (i) that the SID is unlikely to be an independent variable as claimed by Stewart, (ii) that the calculation of [H+ ] based on SID described by Stewart is hardly doable, even in simple electrolyte solutions, (iii) that human metabolism provides an ongoing acid-base disturbance as an independent variable (normally excess H+ accompanied by chloride), (iv) that specific renal regulatory mechanisms indispensable to Stewart's concept, remain unknown, and (v) that studies of biological transmembrane cation transport have progressed to a level making serious opposition to the concept of active transport of H+ meaningless.