From an age of 30 days to 150 days, male rats were given ad libitum feed with a physiological content of nutrients (adequate to their age) and with different sources of fat: sunflower oil, pork lard, butter. The net protein ratio (NPR), body fat content, and the specific activity of phosphoenolpyruvate carboxykinase PEPCK were determined every 15 days. For all the three fat sources, NPR had the same high values in the intensive growth period (age 45 and 60 days: increased protein demand for building the organism); lower and the same again are the NPR values at an age of 75 to 105 days. From an age of 120 days to 150 days (the end of maturation), the long-continued intake of lard and butter had an adverse effect on the availability of protein for the animals, though their food had a physiological structure and an optimum proportion of the saccharide components. The result correlates with the growth of PEPCK activity, as indicated by exploitation of protein as substrate for gluconeogenesis, not for proteosynthesis. Pork lard and butter with a 3.5 - to 5-fold prevalence of saturated fatty acids, compared with sunflower oil, implies that short-chain fatty acids were provided as effectors of gluconeogenesis and that, under the conditions of physiological intake of nutrients, substrates were also provided for mitochondrial synthesis of fatty acids.(ABSTRACT TRUNCATED AT 250 WORDS)

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