Maximum Tolerable Level, Digestion, and Metabolism of D-Glucose (Cerelose) in Rainbow Trout (Salmo gairdneri) Reared on a Practical Trout Diet

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Abstract
The maximum tolerable dietary level, digestion, and metabolism of D-glucose (cerelose) were investigated in rainbow trout (Salmo gairdneri) reared on practical diets containing optimum protein and lipid levels at two different water temperatures for 12 wk. Liver glycogen levels (LG) and liver:body weight ratios (LB) initially increased with increasing dietary glucose, but leveled off above 10–15% glucose. The plateauing of LB and LG was consistent with the plateauing of liver glucose-6-phosphate dehydrogenase activity (G6PD) and probably overall hexose monophosphate shunt (HMS) activity at high dietary levels of glucose. This indicates that there may be an optimum ratio of digestible carbohydrate to lipid in salmonid diets. LG and LB tended to be higher in trout reared upon the same dietary glucose level in fish reared at 11 °C as compared with those at 15 °C. This may have been caused by increased activity of the HMS as indicated by increased activity of liver G6PD in fish acclimated to 11 °C as compared to trout reared at 15 °C on the same dietary glucose level (when assayed at 15 °C). The digestion coefficient of glucose was uniformly high (96–99%) and not affected by either dietary glucose level (up to 25% of the diet) or water temperature (11 or 15 °C). The maximum tolerable level of glucose in salmonid diets appears to be dependent upon the protein, lipid, and overall energy content of the diet.Key words: trout, glucose digestion, glucose metabolism, maximum tolerable levels, water temperature, dietary energy