Update: dietary protein and microbial protein contribution

Factors influencing microbial protein synthesis in the rumen have been reviewed several times in recent years. Original publications in the past 3 yr have reported microbial and feed protein nitrogen contribution postruminally when feeding a variety of dietary proteins. Ammonia is a satisfactory source of nitrogen for growth of the majority of rumen species, but substitution of intact protein for urea (source of ammonia) usually stimulates microbial protein production. Protein sources such as soybean meal appear to possess properties (perhaps rate of degradability) that optimize microbial growth in vivo. Protein sources more undegradable than soybean meal, when fed as the major nitrogen source, sometimes reduce microbial growth. However, nondegradable proteins may compensate for less microbial protein by supplying intact dietary protein postruminally, so the amino acids potentially available may be equal to or greater than those available when readily degradable protein is fed. Soybean meal may reduce microbial growth in diets containing grass silage at protein exceeding 16.8%. Various measurements of microbial and intact dietary protein postruminally show that the contribution of each can be manipulated. Accuracy of quantitative predictions of postruminal contribution depends on several factors that require more research.     

Aging and protein metabolism

During aging, there are qualitative and quantitative modifications of proteins in various tissues. In muscle, myofibrillar and mitochondrial proteins are affected, resulting in a loss of strength and, to a lesser degree, endurance. Mechanisms of sarcopenia remain not well known and probably involve loss of motoneurons, muscle disuse and hormonal alterations. Partial prevention of muscle loss is possible by resistance training. In all tissues, and particularly in the brain, oxidative changes in proteins are likely to alter various functions of proteins.     

Relationship between protein intake and hepatic protein synthesis in rats.

Protein synthesis has been measured in vitro in postmitochondrial extracts from livers of rats fed levels of casein ranging from 0 to 40% by weight. The maximal capacity for protein synthesis per milligram of RNA, measured with each amino acid added at 250 mumol/L, was 40-60% higher in rats fed a protein-free diet than in those fed 6 or 15% casein. Our results suggest that the livers of rats fed a protein-free diet are primed for the synthesis of tissue proteins and, given an adequate supply of amino acids, the rate of protein synthesis would be as high as or higher than the rate in protein-replete animals. When amino acids were added to the in vitro system at concentrations found in plasma of rats fed 0, 6, 15 or 40% casein the rate of protein synthesis increased by three- to fourfold over this range, with the highest rate observed for the 15% dietary casein level. We conclude that when protein intake is below the requirement level, the rate of liver protein synthesis may be limited by amino acid supply, by the capacity of the system for protein synthesis or by both.     

Dietary protein, endurance exercise, and human skeletal-muscle protein turnover

PURPOSE OF REVIEW: Whereas diet and exercise have been shown to influence whole-body protein utilization, little is known about the impact of these factors on skeletal-muscle protein turnover. We highlight the role of dietary protein in modulating skeletal-muscle protein turnover in response to endurance exercise. Effects of endurance exercise on skeletal-muscle protein metabolism are presented and the influence of habitual protein intake on exercise-related protein responses is discussed. RECENT FINDINGS: Skeletal-muscle protein turnover increases in response to endurance exercise training and following a single endurance exercise bout. Nutritional supplementation postexercise favorably affects skeletal-muscle protein synthesis and demonstrates amino acid availability as pivotal to the skeletal-muscle synthetic response following exercise. The level of habitual protein intake influences postexercise skeletal-muscle protein turnover. SUMMARY: Dietary protein and exercise are powerful stimuli affecting protein turnover. Since variation in habitual protein intake influences skeletal-muscle protein turnover postexercise, investigations are needed to determine what role protein intake has in regulating skeletal-muscle protein metabolism. Long-term, well controlled diet and exercise intervention studies are essential for clarification of the relation between protein intake, endurance exercise, and skeletal-muscle protein turnover. Studies designed to characterize this relationship should be attentive to habitual macronutrient and energy intakes.     

Soy protein in relation to human protein and amino acid nutrition

The nutritional value of processed soy protein (isolated soy proteins and soy-protein concentrates) in human protein and amino acid nutrition is evaluated on the basis of a review of studies of growth and nitrogen balance in infants, children, adolescents, and adults. Findings show that well-processed soy-protein isolates and soy-protein concentrates can serve as the major, or even sole, source of protein intake and that their protein value is essentially equivalent to that of food proteins of animal origin. The importance of the sulfur amino acid content of soy protein for practical human nutrition is also examined from nitrogen-balance data. Under conditions of an anticipated normal usage of soy protein, methionine supplementation is not only unnecessary but may even be undesirable for young children and adults. However, for newborns, the data suggest that modest supplementation of soy-based formulas with methionine may be beneficial. Soy proteins have also been found to be of good quality to include in hypocaloric diets for weight reduction in obese subjects. Finally, the data indicate that soy proteins are well-tolerated and of good acceptability.     

High-sensitivity C-reactive protein and cancer

Background

High-sensitivity C-reactive protein (hs-CRP) is a commonly used inflammatory marker. The association between hs-CRP and cancer is less consistent than that between hs-CRP and cardiovascular diseases. This study explored the association between hs-CRP and cancer, using a large database of Korean health examination records.

Methods

A total of 80 781 Koreans who visited the health promotion center of a general hospital were included. There were 729 cases of cancer of any primary site during a 3-year period. Subjects with a known cancer or a condition capable of affecting hs-CRP were excluded.

Results

Serum hs-CRP was significantly higher in cancer cases (2.9 mg/L) than in non-cases (1.4 mg/L; P < 0.0001). With the lowest hs-CRP category (<1 mg/L) as reference, the crude odds ratios (ORs) for cancer were 1.36 (95% confidence interval [CI] = 1.16–1.62) for the second highest category (1–3 mg/L) and 2.49 (95% CI = 2.02–3.07) for the highest category (>3 mg/L), and the adjusted ORs for cancer were 1.16 (95% CI = 0.95–1.42) for the second highest category and 1.94 (95% CI = 1.51–2.51) for the highest category. After excluding cancer cases detected within 1 year after the check-up, the associations remained, although the reduced number of cancer cases (n = 88) attenuated the significance of the associations.

Conclusions

Serum hs-CRP was positively associated with the risk of cancer, although causality cannot be inferred in this cross-sectional study. The results support the hypothesis that chronic inflammation plays a role in cancer.Key words: high-sensitivity C-reactive protein, cancer, inflammation     

Changes in protein C and free protein S during pregnancy and following hysterectomy.

Two longitudinal studies were undertaken to measure the naturally occurring coagulation inhibitors, protein C and protein S, in females who were either pregnant or who were undergoing hysterectomy. Functional and immunological protein C and free protein S were assayed. During pregnancy, protein C levels remained unchanged except for a small increase in protein C antigen at 28-32 weeks gestation. The free protein S fell significantly and progressively during pregnancy, although only in about one-third of patients did the level fall below the normal range. In the hysterectomy study, a significant fall in protein C occurred on days one and three after surgery but had returned to normal by the time of discharge (days 7-10). A small rise in free protein S was observed at time of discharge, but this factor was otherwise unchanged. We conclude that the changes observed may contribute to the hypercoagulable state which is associated with pregnancy or major surgery.     

Effect of protein level and protein source on zinc absorption in humans

The effect of increasing levels of various protein sources on zinc absorption from a legume-based meal was studied in humans with the use of a radionuclide technique. The meals were extrinsically labelled with 65Zn and absorption was determined from measurements of the whole-body retention of the isotope. The mean fractional zinc absorption for the 13 meals was 24.7 +/- 6.9% and was only influenced by the protein content of the meal to a limited extent (r = 0.45). However, the amount of zinc absorbed from the meals was strongly correlated with both the protein (r = 0.85) and zinc content (r = 0.86): 5.9 +/- 1.7 mumol of zinc was absorbed from the basal bean meal which had the lowest protein content; the addition of low zinc chicken doubled the protein content and increased zinc absorption to 10.3 +/- 2.0 mumol; the addition of zinc-rich beef also doubled the protein content, however, zinc absorption was increased to 15.9 +/- 4.7 mumol. It is concluded that the zinc content of the main protein source of the diet determines the amount of zinc absorbed to a large extent. However, relatively small amounts of animal protein can significantly improve the value of a legume-based meal as a source of zinc.     

IgE responses in mice fed moderate protein deficient and high protein diets

While severe protein energy malnutrition (PEM) has been known to depress several immune functions, allergies are suppressed by decreasing IgE and impairing vascular permeability and mast cell functions. To address the effect of moderate protein malnutrition without growth arrest and protein hypernutrition on type I allergy, we examined the effect of various levels of protein nutrition on allergy at humoral immunity and the regulation of Th cell function levels. Mice fed 100 g/kg (moderate protein malnutrition; MPM), 200 g/kg (normal protein nutrition; PN) and 400 g/kg (protein hypernutrition; PH) protein diets were intraperitoneally sensitized to ovalbumin (OVA) in aluminum hydroxide. Higher elevations of OVA-specific IgE and total IgE in the serum were observed in the PH group as compared to the PN group. However, OVA-specific IgE in the MPM group was not significantly different from that in the PN group, although the former appeared higher than the latter. While CD3, CD4, CD8 and B220 expressions in the splenic lymphocytes were decreased in the MPM group, B220 expressions were increased in the PH group. Splenic lymphocyte proliferative responses to OVA were augmented in the PH group and depressed in the MPM group. IFN-gamma production from splenic lymphocytes was significantly decreased; however, IL-4 production was not affected significantly in the MPM group, and increased in the PH group. These findings suggest that immune functions to specific antigens in the MPM state are depressed at the cytokine level but not in terms of IgE responses. They also suggest that immune functions become Th2-predominant in the PH state, resulting in an increased risk of type I allergy.