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.