The bioavailability of various selenium compounds to a marine wading bird

The uptake of dietary selenium (about 3.5 mg/kg AF dry wt) as selenomethionine, selenocystine, selenite, selenate, and fish selenium in the plasma and red blood cells (RBC) of the oystercatcher has been investigated. The birds received the various selenium compounds subsequently, for at least 9 wk. After dietary supplementation of selenocystine, selenite, and selenate, plasma selenium was about 350 micrograms/L and RBC selenium 2.1 mg/kg dry wt. After supplementation of selenomethionine, the plasma concentration increased to 630 micrograms/L, and the RBC concentration to 4.1 mg/kg dry wt. When the fodder contained 3.1 mg/kg fish Se, an average plasma and RBC concentration of 415 micrograms/L and 14.4 mg/kg dry wt, respectively, was measured. The maximal increase of the selenium concentration in the plasma was attained at first sampling, 14 d after a change in dietary selenium (selenomethionine or fish Se); the uptake seemed to be a concentration-regulated process. RBC concentrations (Y in mg/kg drug wt) increased with time (X in d) according to Y = a - b e-cX. Fifty percent of the total increase was attained within 17 d, suggesting that diffusion into the RBC played a role. The selenium concentration in the plasma was positively correlated with the (fish)Se concentration in the fodder; the RBC concentration (60 d after the change in diet) was positively correlated with the plasma concentration. When the diet contained fish Se, the blood selenium concentrations of the captive birds were similar to the concentrations measured in field birds. Fish Se is a yet undetermined selenium compound. The present experiment showed that fish Se differed from selenomethionine, selenocystine, selenite, or selenate in uptake from the food and uptake in the RBC.