The preponderance of dietary potassium (approximately 77 to 90 percent) is excreted in urine, while the remainder is excreted mainly in feces, with much smaller amounts being lost in sweat (Agarwal et al., 1994; Holbrook et al., 1984; Pietinen, 1982). The correlation between dietary potassium intake and urinary potassium content is high (r = 0.82) (Holbrook et al., 1984). The great majority of potassium that is filtered by the glomerulus of the kidney is reabsorbed (70 to 80 percent) in the proximal tubule such that only a small amount of filtered potassium reaches the distal tubule. The majority of potassium in urine results from secretion of potassium into the cortical collecting duct, a secretion regulated by a number of factors, including the hormone aldosterone. An elevated plasma concentration of potassium stimulates the adrenal cortex to release aldosterone, which in turn increases secretion of potassium in the cortical collecting duct and hence into urine.
A diet rich in potassium from fruits and vegetables favorably affects acid-base metabolism because these foods are rich in precursors of bicarbonate, which neutralizes diet-induced acid in vivo (Sebastian et al., 1994, 2002). The net quantitative outcome of this acid-base interaction is termed “the net endogenous acid production” (NEAP). Because most endogenous noncarbonic acid is derived from protein, and because most endogenous bicarbonate (base) is derived from organic anions present in potassium-rich fruits and vegetables, the dietary protein-to-potassium ratio closely estimates NEAP and thus predicts urinary net acid excretion, which in turn predicts calcium excretion. For many years it has been hypothesized that the modern Western diet could induce a low-grade metabolic acidosis that in turn could induce bone demineralization, osteoporosis, and kidney stones (Barzel, 1995; Barzel and Jowsey, 1969; Lemann et al., 1966; Wachman and Bernstein, 1968). The results of several recent epidemiological (New et al., 1997, 2000; Tucker et al., 1999) and metabolic (Maurer et al., 2003; Morris RC et al., 2001; Sebastian et al., 1994) studies support this hypothesis.
Noncarbonic acids are generated from metabolism of both plant and animal proteins (e.g., in both, sulfuric acid is generated from the metabolism of sulfur-containing amino acids found in meats, fish, dairy products, grains, and to a lesser extent, in fruits and vegetables). Unlike fruits and vegetables, meats and other animal foods contain few precursors of bicarbonate. The only plant food group that consistently yields noncarbonic acid precursors in excess