Effects of dietary NaCl on the in vivo apparent absorption of dietary nutrients determined in rainbow trout (Oncorhynchus mykiss)

Abstract

Rainbow trout (mean body weight 52.3g) were reared in freshwater and fed a semi-purified low-sodium (Na) diet (Na content 0.027%) or NaCl-supplemented diets (Na content 0.35% and 1.75%). The chyme of the proximal and distal intestines of fish were collected by dissection. The diets and chyme were analyzed, and apparent digestibility (net absorption) and indigestibility of dietary nutrients was estimated based on yttrium as a reference. Na concentration of the intestinal chyme, both proximal and distal regions, was similar regardless of dietary Na levels. A major Na secretion into the proximal lumen was noted in fish fed the low-Na diet. The net phosphorus (P) absorption was already ~90% at the proximal intestine, but dietary Na had no detectable effect on net P absorption, both in the proximal and distal intestine. The Ca content was more than two times higher in the distal than the proximal chyme. Dietary Na had no effect on chyme Ca content, but tended to increase Mg content in the proximal and distal chyme. There was no apparent effect of dietary Na on net Fe and Zn absorption in the proximal and distal intestine. Dietary Na decreased net absorption of Mn. Dietary Na tended to increase dry matter and organic carbon digestibilities in the distal intestine. Digestibility of crude protein (CP) was increased by dietary Na in the proximal, but not distal, region. These results collectively indicate that dietary Na may slightly increase digestibilities of macronutrients in rainbow trout reared in freshwater. Statement of relevanceSodium (Na) is an important ion for dietary nutrient absorption in the intestine. However, the effect of dietary Na on an in vivo nutrient absorption efficiency has been little studied in fish. Hence, we evaluated the effect of dietary NaCl on nutrient digestibility (absorption) in rainbow trout reared in freshwater.