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Discipline: nutrition/feeding; Key words: milk protein, proteomics, bioactive, low abundance protein.
Research which investigates the milk proteins (milk proteome) as a source of bioactive compounds that have physiological importance instead of only supplying amino acids is not often seen. Many of the bioactive peptides are released after digestion of the high-abundance milk protein fraction, which includes all casein forms as well as some whey proteins. The low-abundance milk proteins, which include all other whey proteins, are known to have functionality as either peptide fragments or as entire intact proteins that may withstand gastric action. These bioactive proteins and peptides, derived from both the low- and high-abundance protein fraction, have a variety of activities and amongst others play a role in human health. The health properties associated with them include antimicrobial, antihypertensive, immune-modulatory, mineral binding and antioxidative activities. The question is whether the milk proteome can be modulated by the diet of the cow to increase the functional benefits. This was the basis of a study by Dr R. Tacoma and colleagues which was published in the Journal of Dairy Science, Volume 100 of 2017, page 7246 to 7261, with the title: Ratio of dietary rumen degradable protein to rumen undegradable protein affects nitrogen partitioning but does not affect the bovine milk proteome produced by mid-lactation Holstein dairy cows.
The objective of the study was to determine if the dietary rumen degradable protein (RDP) to rumen undegradable protein (RUP) ratio could alter the milk proteome. Six Holstein cows of on average 2.5 lactations were blocked by days in milk (about 80 days) and milk yield (about 58 kg per day) and randomly assigned to treatment groups. The experiment was conducted as a double-crossover design consisting of three 21-day periods. Within each period, treatment groups received diets with either (1) a high RDP:RUP ratio (62.4:37.6% of crude protein) or (2) a low RDP:RUP ratio (51.3:48.7% of crude protein). Both diets were otherwise iso-nitrogenous and iso-energetic. To confirm the nitrogen (N) and energy status of the cows, dry matter intake (DMI) was determined daily, rumen fluid samples were collected for volatile fatty acid analysis, blood samples were collected for plasma glucose, β-hydroxybutyrate, urea nitrogen, and fatty acid analysis, and total 24-hour urine and faecal samples were collected for N analysis. Milk samples were collected to determine the general milk composition and the protein profile.
No treatment differences were observed in DMI, milk yield, general milk composition, plasma parameters, or rumen volatile fatty acid concentrations, indicating no shift in total energy or protein available. Milk urea N and plasma urea N concentrations were higher in the high RDP group, indicating some shift in N partitioning due to diet. A total of 595 milk proteins were identified, with 83% of these proteins known to be involved in cellular processes. Although none of the low-abundance proteins were affected by diet, feeding the diet high in RUP decreased the β-casein, κ-casein and total milk casein concentration. The results confirm that diet can alter the milk proteome but more investigations of the interactions between diet and the milk protein profile are needed to quantify responses.
Thus, nutritional manipulation of the diet of the cow to alter milk composition offers a promising approach to enhance the milk protein profile and could provide the development of functional foods which can support increased healthfulness of milk. Although the alteration of the RDP:RUP ratio of the diet did not induce any differences in the low abundance milk protein profile, the effect on the casein profile which is produced by metabolism, demonstrates the potential to influence specific udder-derived milk proteins.