Bioavailability of vitamin C from two ascorbyl monophosphate esters in rainbow trout, Oncorhynchus mykiss (Walbaum)

Abstract

The relative bioavailability of ascorbic acid (AA) from several sources was compared in juvenile rainbow trout, Oncorhynchus mykiss (Walbaum). Fingerling trout were offered AA, and equimolar amounts of ascorbyl monophosphate Ca salt (CaAMP) or ascorbyl monophosphate-K, Mg salt (K/MgAMP) at graded levels of 20, 40, 80 and 320 mg kg−1. A control group was fed the same formula diet devoid of AA. During the 12 week long experiment, dietary concentrations of CaAMP and K/MgAMP remained unchanged while AA concentration decreased by 62.5%. No significant growth differences were found over a 12 week growout period, during which body weights increased 8–9 fold. No mortality or acute scurvy signs were observed in fish. Lysozyme activities tended to decrease in fingerlings supplemented with the highest AA level from all sources, although the difference was significant only in the case of the CaAMP diets. Blood plasma alkaline phosphatase showed no significant differences between treatments. However, phosphatase activity toward ascorbyl monophosphate (AMP) extracted from the intestine showed a gradual increase as the dietary AMP source was increased from 20 to 80 mg kg−1. Significant differences were found in AA concentration in the liver after 3 weeks of feeding graded levels of different sources of ascorbic acid. The differences in tissue AA concentrations among groups fed different dietary supplements increased with the duration of feeding. The bioavailability of AA and AA esters as measured by AA concentrations in the liver, kidney, intestine and blood plasma were equal. Despite the decrease in AA concentration in feeds supplemented with unprotected ascorbic acid after several weeks of storage, relationships between concentrations of ascorbic acid in the liver and those actually measured in feeds were not significantly different between treatments.