FREE AND TOTAL ION CONCENTRATIONS IN PIG DIGESTA

Mineral bioavailability is related to the activity of the free ion or small-ligand metal ion complexes present in gastrointestinal (GI) trac t digesta. Therefore, it is necessary to distinguish between total ion and free-ion/small-ligand complexes (referred to simply as ''free'') concentrations. Free and total cation concentration in pig digesta fro m various GI locations were determined. Free ions were operationally d efined as those that passed through a 1,000 molecular weight cutoff fi lter. To test the effect of dietary supplementation on free ion concen trations, pigs were fed either basal diets of corn bran, corn grits, a nd soybean meal (10, 67, and 20 weight percent, respectively) or basal diets containing added Ca, Zn, Fe, and Cu. In addition, the Ca and K content of corn bran fragments retrieved from digesta was determined b y energy dispersive x-ray analysis to examine whether this dietary fib er preferentially absorbed minerals, thus reducing mineral bioavailabi lity. Free cation concentrations, expressed as a percentage of the tot al, averaged over all locations for both diets were: Na, 86%; K, 96%;, Ca, 11%; Mg, 40%; Zn, 5%; Fe, 4%; and Cu, 11%. For Ca, Mg, Zn, and Cu , the free:total cation concentration ratios differed (P < .05) betwee n upper and lower GI tract. Mineral supplementation did not alter free :total ratios of any ion in the GI tract. For supplemented diets, mine ral concentrations generally were higher throughout the GI tract, as w ere concentrations of free Ca. Free concentrations of Zn and Cu in the jejunum and ileum were higher (P < .01) with supplemented diets. The calcium content of retrieved corn bran was higher ( P < .01) than the initial content. This study suggests that corn bran fed at levels high er than 10% could act directly as a significant mineral sink if fed wi th pig diets deficient in minerals.