The relative effects of a low-protein-high-carbohydrate diet on the free amino acid composition of liver and muscle.

1. Free amino acid concentrations in the plasma have been compared with those in liver and quadriceps muscle, in rats fed on diets containing 209 (control) and 31 (low-protein) g protein/kg. The effects of the low-protein diet on diurnal variations in these values were also measured. 2. In the plasma, the total amino acid concentration was significantly lower in animals given the low-protein diet, at all times of day except 12.00 hours. In the liver, and to a lesser extent the muscle, total amino acid concentration was maintained. 3. In the control animals, diurnal variation in the concentrations of both essential and nonessential amino acids was very similar in plasma, liver and muscle. In animals given the low-protein diet, although the same diurnal pattern was maintained for non-essential amino acids, that occurring among the essential amino acids had virtually disappeared 4. In plasma, the mean 24 h concentration of essential amino acids decreased from 2-43 mmol/l in control animals to only 1-29 mmol/l in the low-protein-fed animals. Concentrations in muscle and liver were reduced by a similar proportion (from 8-60 to 5-56 mumol/g and from 8-67 to 5-05 mumol/g respectively). Conversely the concentrations of non-essential amino acids in animals given the low-protein diet were increased in plasma (from 1-53 to 2-00 mmol/l), muscle (from 12-5 to 14-3 mumol/g), and liver (from 16-8 to 20-5 mumol/g), muscle showing the lowest increase. 5. With the exceptions of lysine, threonine, cystine and tyrosine, the concentrations of all other essential amino acids were reduced more in liver than in muscle. The relationship between this and the failure to maintain plasma albumin concentations is discussed.