Background: The digestive hydrolysis of dietary proteins leads to the release of peptides in the intestinal tract, where they may exert a variety of functions, but their characterization and quantification are difficult.

Soy proteins have been shown to result in lower postprandial nitrogen retention than milk proteins, but the mechanisms underlying these differences have not been elucidated. To investigate this question, we measured the postprandial kinetics of the appearance of individual ¹⁵N-amino acids in the serum of healthy adults after the ingestion of either ¹⁵N-soy (n = 8) or ¹⁵N-milk proteins (n = 8) in a mixed single meal (46 kJ/kg). The kinetics of total and dietary amino acids (AA) in the peripheral circulation were characterized by an earlier and higher peak after soy protein ingestion. Dietary AA levels peaked at 2.5 h in the soy group vs. 3.9 h in the milk group (P < 0.02). This time interval difference between groups was associated with a faster transfer of dietary N into urea in the soy group (peak at 3 vs. 4.75 h in the milk group, P < 0.005) and a higher level of incorporation into the serum protein pool from 3 to 8 h after the soy meal. The dietary AA pattern in the peripheral blood closely reflected the dietary protein AA pattern. Postprandial glucose, insulin, and glucagon levels and profiles did not differ between groups. Soy AA were digested more rapidly and were directed toward both deamination pathways and liver protein synthesis more than milk AA. We conclude that differences in the metabolic postprandial fates of soy and milk proteins are due mainly to differences in digestion kinetics; however, the AA composition of dietary proteins may also play a role.

Background: The in vivo quality of milk protein fractions has seldom been studied in humans. Objective: Our objective was to compare the postprandial utilization of dietary nitrogen from 3 [¹⁵N]-labeled milk products: micellar caseins (MC), milk soluble protein isolate (MSPI), and total milk protein (TMP). Design: The macronutrient intakes of 23 healthy volunteers were standardized for 1 wk, after which time the subjects ingested a meal containing MC (n = 8), MSPI (n = 7), or TMP (n = 8). [¹⁵N] was measured for an 8-h period in plasma amino acids, proteins, and urea and in urinary urea. Results: The transfer of dietary nitrogen to urea occurred earlier after MSPI ingestion than after MC and TMP ingestion, and concentrations remained high for 8 h, concomitantly with higher but transient hyperaminoacidemia and a higher incorporation of dietary nitrogen into plasma amino acids. In contrast, deamination, postprandial hyperaminoacidemia, and the incorporation of dietary nitrogen into plasma amino acids were lower in the MC and TMP groups. Finally, total postprandial deamination values were 18.5 ± 2.9%, 21.1 ± 2.8%, and 28.2 ± 2.9% of ingested nitrogen in the TMP, MC, and MSPI groups, respectively. Conclusions: Our results confirm the major role of kinetics in dietary nitrogen postprandial utilization and highlight the paradox of MSPI, which, despite its high Protein Digestibility Corrected Amino Acid Score, ensures a rate of amino acid delivery that is too rapid to sustain the anabolic requirement during the postprandial period. Milk proteins had the best nutritional quality, which suggested a synergistic effect between soluble proteins and caseins.

Although there is a considerable interest of high-protein, low-carbohydrate diets to manage weight control, their safety is still the subject of considerable debate. They are suspected to be detrimental to the renal and hepatic functions, calcium balance, and insulin sensitivity. However, the long-term effects of a high-protein diet on a broad range of parameters have not been investigated. We studied the effects of a high-protein diet in rats over a period of 6 mo. Forty-eight Wistar male rats received either a normal-protein (NP: 14% protein) or high-protein (HP: 50% protein) diet. Detailed body composition, plasma hormones and nutrients, liver and kidney histopathology, hepatic markers of oxidative stress and detoxification, and the calcium balance were investigated. No major alterations of the liver and kidneys were found in HP rats, whereas NP rats exhibited massive hepatic steatosis. The calcium balance was unchanged, and detoxification markers (GSH and GST) were enhanced moderately in the HP group. In contrast, HP rats showed a sharp reduction in white adipose tissue and lower basal concentrations of triglycerides, glucose, leptin, and insulin. Our study suggests that the long-term consumption of an HP diet in male rats has no deleterious effects and could prevent metabolic syndrome.