Effects of skim milk, whole milk and light cream on serum tryptophan binding and brain tryptophan concentrations in rats.

One hour after rats fasted for 13 hours consume a single meal of skim milk, whole milk, or light cream, serum nonesterified fatty acid (NEFA) concentrations changed in direct proportion to the fat content of the diet: serum NEFA levels decreased in rats ingesting skim milk (0.09% fat) to 34% of fasting control values; in animals consuming whole milk (3.59% fat), levels dropped to 50% of fasting levels; these levels did not change significantly, however, in rats consuming light cream (18.26% fat). The percentages of total serum tryptophan not associated with albumin in rats ingesting skim milk, whole milk, or light cream were 22.2, 26.0, and 42.5%, respectively. These variations in serum free tryptophan were not accompanied by significant differences in brain tryptophan among the three treatment groups. These results thus confirm that, with a natural food source, (a) postprandial serum NEFA levels reflect the total fat content of the diet; (b) serum free tryptophan concentration shift proportionately with serum NEFA; and (c) serum free tryptophan concentrations do not reliably predict brain tryptophan levels.