Stability in fish feed and bioavailability to rainbow trout of two ascorbic acid forms

The stability in warm pelleted fish feed and the bioavailability to rainbow trout of crystalline ascorbic acid (AA) and a synthetic polymer-coated AA product (PCAA) were compared. The AA loss during pelleting was 29% for crystalline AA and 19% for PCAA. After 6 weeks at room temperature 73% of PCAA was still retained whereas less than 10% of crystalline AA was left. Rainbow trout were deprived of AA for 8 weeks, and then supplemented with AA from crystalline AA and PCAA, respectively. At the same time, two groups of trout were supplemented with a constant level of AA from either crystalline AA or PCAA. The concentration of free AA in the liver was used as an index of the AA status of the fish. The bioavailability of PCAA turned out to be the same as that of crystalline AA. After 8 weeks some of the fish given an AA-free diet showed dilated swim bladders, and/or inflammation, and haemorrhage in the gastrointestinal system. This was interpreted as vitamin C deficiency symptoms. We did not observe any difference in growth between fish given a diet containing AA and those given an AA-free diet.