Effect of postpartum propylene glycol allocation to over-conditioned Holstein cows on concentrations of milk metabolites.

The objective of the study was to investigate the effect of propylene glycol (PG) allocation on concentrations of milk metabolites with potential use as indicators of glucogenic status in high yielding postpartum dairy cows. At time of calving, nine ruminally cannulated Holstein cows were randomly assigned to ruminal dosing of 500 g/d tap water (CON, n = 4) or 500 g/d PG (PPG, n = 5). The PG was given with the morning feeding week 1-4 postpartum (treatment period) and cows were further followed during week 5-8 postpartum (follow-up period). All cows were fed the same postpartum diet. Milk samples were obtained at each milking (3 times/d) in the treatment period, and at morning milking during the follow-up period. Weekly blood samples were obtained from -4 to +8 weeks relative to calving and daily blood samples from -7 until +7 d relative to calving. The main effect of PG allocation was an increased glucogenic status, e.g. visualised by a prompt marked increase in blood fructosamine. During the treatment period, milk concentration of free glucose tended to be greater, whereas milk concentrations of isocitrate and BHBA were lower for PPG compared with CON. It is proposed that the ratio between free glucose and isocitrate in milk may be a potential biomarker for glucogenic status in the vulnerable early postpartum period. We will pursue this issue in the future.

Phenylalanine stable isotope tracer labeling of cow milk and meat and human experimental applications to study dietary protein-derived amino acid availability.

BACKGROUND & AIMS: Availability of dietary protein-derived amino acids (AA) is an important determinant for their utilization in metabolism and for protein synthesis. Intrinsic labeling of protein is the only method to directly trace availability and utilization. The purpose of the present study was to produce labeled milk and meat proteins and investigate how dietary protein-derived AA availability is affected by the protein-meal matrix. METHODS: Four lactating cows were infused with L-[ring-d5]phenylalanine and one with L-[15N]phenylalanine for 72 h. Milk was collected, and three of the [d5]phenylalanine cows were subsequently slaughtered. Two human studies were performed to explore plasma AA availability properties utilizing the labeled proteins. One study compared the intake of whey protein either alone or together with carbohydrates-fat food-matrix. The other study compared the intake of meat hydrolysate with minced beef. Cow blood, milk, meat and human blood samples were collected and analyzed by mass spectrometry. RESULTS: Whey and caseinate acquired label to 15-20 mol percent excess (MPE), and the meat proteins reached 0.41-0.73 MPE. The [d5]phenylalanine appeared fast in plasma and peaked 30 min after whey protein alone and meat hydrolysate intake, whereas whey protein with a food-matrix and the meat minced beef postponed the [d5]phenylalanine peak until 2 and 1 h, respectively. CONCLUSIONS: Phenylalanine stable isotope-labeled milk and meat were produced and proved a valuable tool to investigate AA absorption characteristics. Dietary protein in food-matrices showed delayed postprandial plasma AA availability as compared to whey protein alone and meat hydrolysate.

Effect of degree of hydrolysis of whey protein on in vivo plasma amino acid appearance in humans.

Whey protein is generally found to be faster digested and to promote faster and higher increases in plasma amino acid concentrations during the immediate ~60 min following protein ingestion compared to casein. The aim of the present study was to compare three different whey protein hydrolysates with varying degrees of hydrolysis (DH, % cleaved peptide bonds) to evaluate if the degree of whey protein hydrolysis influences the rate of amino acid plasma appearance in humans. A casein protein was included as reference. The three differentially hydrolysed whey proteins investigated were: High degree of hydrolysis (DH, DH = 48 %), Medium DH (DH = 27 %), and Low DH (DH = 23 %). The casein protein was intact. Additionally, since manufacturing of protein products may render some amino acids unavailable for utilisation in the body the digestibility and the biological value of all four protein fractions were evaluated in a rat study. A two-compartment model for the description of the postprandial plasma amino acid kinetics was applied to investigate the rate of postprandial total amino acid plasma appearance of the four protein products. The plasma amino acid appearance rates of the three whey protein hydrolysates (WPH) were all significantly higher than for the casein protein, however, the degree of hydrolysis of the WPH products did not influence plasma total amino acid appearance rate (estimates of DH and 95 % confidence intervals [CI] (mol L(-1) min(-1)): High DH 0.0585 [0.0454, 0.0754], Medium DH 0.0594 [0.0495, 0.0768], Low DH 0.0560 [0.0429, 0.0732], Casein 0.0194 [0.0129, 0.0291]). The four protein products were all highly digestible, while the biological value decreased with increasing degree of hydrolysis. In conclusion, the current study does not provide evidence that the degree of whey protein hydrolysis is a strong determinant for plasma amino acid appearance rate within the studied range of hydrolysis and protein dose.