MEASUREMENTS OF DIGESTIBILITIES OF CHOLESTEROL AND FATTY-ACIDS USING 5-ALPHA-CHOLESTANE AS AN INERT MARKER IN THE TILAPIA, OREOCHROMIS-NILOTICUS, AND THE FRESH-WATER PRAWN, MACROBRACHIUM-ROSENBERGII

An attempt was made to measure the apparent digestibility (AD) of chol esterol and fatty acids using Sa-cholestane as a marker (cholestane-me thod) using tilapia, Oreochromis nilotests, and the freshwater prawn, Macrobrachium rosenbergii, as test animals. The results were compared with those obtained by the Cr2O3-method. Casein-based test diets conta ining 0.3% levels of cholestane and chromic oxide as markers were prep ared for determination of AD of dietary cholesterol and fatty acids. A fter 2 weeks of acclimation, the test animals, tilapia (average 3.0g w et weight) and freshwater prawn (average 0.5 g wet weight) were fed th e test diets for I week, and faecal samples were collected over 6 subs equent days. Lipids in the diets and faeces were extracted with chloro form-methanol and saponified with 10% KOH in methanol. Gas-liquid chro matography (GLC) on 1.5% OV-17 of the unsaponifiable matters afforded the quantities of 5 alpha-cholestane and cholesterol, whereas that on 5% Shinchrom E-71 of the saponifiable matters provided the quantities of fatty acids. Chromic oxide was determined by a wet-digestion method . In tilapia, the AD of cholesterol determined by cholestane- and Cr2O 3-methods were 78.1% and 73.5%, respectively. The AD of individual fat ty acids differed with the types of fatty acids, but those of fatty ac ids determined by the two methods in this study were similar, except f or a slightly higher value of several fatty acids such as 16:1 in the cholestane-method than in the Cr2O3-method. This indicates that 5 alph a-cholestane as well as Cr2O3 can be conventionally used as a marker f or determining AD of fatty acids and cholesterol. The cholestane-metho d is advantageous as it permits parallel analysis of cholesterol (or f atty acids) and Sol-cholestane by GLC using small amounts of faecal sa mple. The cholestane-method was also useful for analysing digestibilit ies of cholesterol and total lipids in freshwater prawn. However, slig ht differences between the cholestane- and Cr2O3-methods were observed in the AD of several fatty acids such as 14:0 and 16:1.