The sites of absorption of ascorbic acid (AA) in the rainbow trout (Oncorhynchus mykiss) gut were located in the stomach (20.7%), pyloric caecae region (23.4%), middle intestine (21.9%) and posterior intestine (20.1%). Ascorbic sulfate (AS) were found to be absorbed in the stomach (27.7%) although it is likely that the figure obtained is an overestimate. This conclusion is based on the concentration factor of the intestinal contents (faeces/food ratio) being much higher for ascorbic sulfate than for an external dietary marker, chromic oxide. The apparent protein absorption rates from the three diets used were not significantly different (P < 0.01), ranging from 86.9 to 91.5%. Three subsequent samplings, one week apart, did not reveal changes in AA or AS absorption in trout, suggesting that fish were neither "saturated" with AA nor had they developed the means to utilize the dietary AS. In most cases, the mineral contents in faeces were similar for all diets, although the concentration factors (faeces/food level), varied greatly during the course of the trial (1.9–7.1 for Zn, 1.8–4.9 for Fe, and 2.4–3.7 for Cu). The concentrations of Fe and Zn in the contents of the posterior intestine were lower in the fish fed an AA-supplemented diet than in fish fed on AS and deficient diets, which may suggest an increased absorption of these elements.
Fingerlings and marketable size rainbow trout (Oncorhynchus mykiss): Proximate composition, cholesterol content and fatty acid profile in fillets
