Protein turnover

Nitrogen balance and ileal amino acid digestibility in growing pigs fed diets reduced in protein concentration

Two studies were conducted to assess the effect of dietary protein reduction on N utilization, N excretion, and AA digestibility in growing pigs. The objective was to determine whether pigs fed diets with a reduced CP concentration could maintain the same N retention as pigs fed an adequate diet. The second objective was to test whether reducing dietary CP concentration decreases AA digestibility. In each study, six barrows were allotted to one of six dietary treatments in a Latin square design. Treatments consisted of four corn-soybean meal-based diets containing 15, 12, 9, and 6% CP, a casein-based diet containing 15% CP, and a protein-free diet. Crystalline AA were included in the 12, 9, and 6% CP diets. The indispensable:dispensable AA ratio was maintained at 45:55 with the addition of L-glutamic acid to the 9 and 6% CP diets. The casein-based and protein-free diets were used to determine endogenous total tract N and ileal AA losses. In the first study, total N losses and N absorbed decreased linearly (P < 0.001) as dietary CP concentration decreased from 15 to 6%. Both a linear (P < 0.001) and a quadratic (P < 0.05) decrease in N retention were found with decreasing dietary CP concentration. Nitrogen retained as a percentage of intake and absorbed increased (P < 0.001) as dietary CP concentration was reduced from 15 to 6%. In the second study, six barrows were surgically fitted with a T-cannula at the terminal ileum to determine ileal AA digestibility. For all dispensable and most indispensable AA, apparent and standardized ileal digestibility increased linearly (P 0.01, and for arginine, P < 0.05) as dietary CP concentration decreased. These results indicate that dietary CP concentration can be decreased from 15 to 12% with crystalline AA supplementation to meet an ideal AA profile without adversely affecting N retention, and that decreasing dietary CP concentration from 15 to 6% increases both dispensable and indispensable AA ileal digestibility.

Nutrition, ageing and ill health

There are physical, mental, social and environmental changes which take place with ageing; for example, decreased physical activity, increase in body fat, decrease in lean body mass and consequently decreased energy intake may be associated with physiological functions that affect metabolism, nutrient intake, physical activity and risk of disease. There are now many studies which have found that undernutrition is prevalent and often unrecognized in patients admitted to hospitals and institutions. There is also evidence which links protein-energy undernutrition or its markers with clinical outcomes in acute and non-acute hospital settings and that nutritional supplements can improve outcomes in some of these settings. However, most clinically-available nutrition screening instruments lack sensitivity and specificity, and abnormal nutritional indicators may simply reflect effects of age, functional disability, or severe underlying disease. Thus, causal relationship cannot be assumed without a sufficiently powerful intervention study which adequately adjusts for the effects of non-nutritional factors, such as the number and severity of co-morbid conditions on clinical outcome.

Protein synthesis in vivo in rats fed on lipid-rich liquid diets

Changes in tissue composition and protein synthesis ratio were studied in the major tissues of the body in young rats fed on lipid-rich, isonitrogenous purified liquid diets, a convenient method for inducing voluntary overfeeding under controlled nutritional conditions. Overfed rats showed faster growth induced by the energy excess. Analysis of tissue composition (protein, DNA and RNA contents) revealed that growth was due mainly to tissue hyperplasia in which protein and DNA contents increased in parallel. Fractional protein synthesis ratio measured in vivo by the flooding-dose method of phenylalanine showed a marked increase in all tissues. This change could be attributed to an increase in the ribosomal activity for protein synthesis in most tissues. Therefore, our results indicate that addition of a supplementary energy source (as lipids) to a well-balanced diet improves growth and protein synthesis in growing rats.

A conceptual approach to optimal nutrition of brood mares

Cumulative nutritional demands on good brood mares are unrivalled among domestic livestock, but little attention has been given to the influence of nutrition on their reproductive performance and efficiency. We suggest that part of the decline in racing performance of progeny of mares over 10 years of age may be caused by suboptimal nutrition. This article contrasts the concepts of minimal and optimal nutrient requirements, revives Hammond's concept of nutrient partitioning to the conceptus and mammary gland, and discusses energy and nutrient requirements during gestation and lactation. Further consideration is given to the aged brood mare.

Utilization of prealbumin as a nutritional parameter

The response of prealbumin was compared to that of albumin and transferrin in 16 patients following 7 days of metabolic/nutritional support. Baseline values were compared to day 7 results to assess the degree of change. Prealbumin demonstrated a significant increase in the mean serum concentration (13.0 vs 19.6 mg/100 ml) in the presence of a positive nitrogen balance. Transferrin exhibited a similar significant response (168.8 vs 223.7 mg/100 ml). Albumin, body weight, and serum iron concentration did not change significantly during the 7-day period. Prealbumin effectively demonstrated an anabolic response in the study sample and could possibly be used as an early indicator of visceral protein anabolism in patients receiving metabolic/nutritional support.

Tumour-host metabolic interaction and cachexia

Cachexia is a terminal metabolic problem observed in a wide variety of tumours. In this article I propose that the syndrome is a direct consequence of the common feature to all malignant tumours: growth. I suggest that the requirement for essential amino acids can be used as the unifying principle that links the tumour to the two main components of cachexia: muscle wastage and anorexia. This underlying factor is usually clouded by the overlapping of individual tumour characteristics.

Parenteral nutrition of injured patients: Effect of manipulation of aminoacid infusion (increasing branched chain while decreasing aromatic and sulphurated aminoacids)

Sixteen critically ill injured patients received parenteral nutrition providing nitrogen (0.34 g kg(-1) day(-1)) and glucose (32 kcal kg(-1) day(-1)) for 5 days. They were randomly divided into two groups with respect to aminoacid supply: an essential aminoacid solution vs the same solution enriched in branched chain amino acid (BCAA) content and decreased in phenylalanine and methionine content (mean BCAA intake, 0.55 vs 0.69 g kg(-1) day(-1)). Basal values of nitrogen metabolism without treatment showed no difference between the two groups. Nitrogen losses and 3 methylhistidine (3-MEH) excretion were elevated; the plasma aminoacid pattern was altered by the trauma and except for phenylalanine, aspartate and glutamate, plasma aminoacid concentrations were decreased below normal values. Net muscular aminoacid output was demonstrated by femoral arterio-venous (av) differences that were all negative except for glutamate and citrulline TPN with both solutions improved the nitrogen balance and reduced the negative aminoacid balance across the leg. Adjusting a TPN regimen to increase the BCAA content without altering the total nitrogen infused, had no effect on overall nitrogen balance, but exerted a beneficial effect on body protein catabolism, as assessed by the urinary 3-MEH excretion rate, and a transient improvement in the aminoacid balance across the leg at the peak of the infusion. The short-lived effect of BCAA suggest a metabolic effect of these aminoacids which deserves further study.

Enteral feeding in burn hypermetabolism: nutritional and metabolic effects of different levels of calorie and protein intake

Enteral nutrition was provided by continuous pump-controlled gastrostomy tube feeding for 14 days in 97 guinea pigs bearing a 30% full thickness burn. Seven defined combinations of caloric and protein intake were studied. With a caloric intake of 175 kcal/kg/day, equaling the measured energy expenditure, the animals receiving 10% of calories as protein had a significantly greater postburn weight loss (p less than 0.05) and muscle mass depletion (p less than 0.05), and a significantly lower muscle nitrogen concentration (p less than 0.05), serum albumin level (p less than 0.01) and liver nitrogen content (p less than 0.01). With the same caloric intake but with more than 20% of calories as protein, the weight loss and the muscle wasting were reduced, but not abolished, and the serum albumin level and liver nitrogen content were normalized. Also with the diets containing 200 kcal/kg/day the muscle tissue depletion could not be abolished. However, with this caloric intake, the animals given 20% of calories as protein had a lower weight loss and a higher serum albumin level (p less than 0.01), but also a greater fat infiltration of the liver (p less than 0.01). At both levels of caloric intake, the nitrogen balance correlated significantly with the level of nitrogen intake but did not correlate with the changes of body weight. The incidence of diarrhea was lowest in animals fed 20% protein calories at a caloric intake of 175 kcal/kg/day. All things considered, the best metabolic and nutritional results were obtained with diets containing 20 to 30% of calories as protein and providing a caloric intake that paralleled the measured energy expenditure.

Influence of total parenteral nutrition on protein metabolism following acute injury: assessment by urinary 3-methylhistidine excretion and nitrogen balance

The use of total parenteral nutrition after acute injury, either surgical or accidental, is widely accepted for its important benefits, although it is not yet completely understood whether a reduction of body protein catabolism can be effectively achieved. We applied total parenteral nutrition to 14 critically ill patients after either trauma or major surgery. Their daily nitrogen balance, urinary 3-methylhistidine and creatinine excretion, and molar 3-methylhistidine/creatinine ratio, during initial 24-hr fasting period, were respectively -0.19 +/- 0.01 (SEM) g kg-1, 5.46 +/- 0.47 mumol kg-1, 27 +/- 4 mg kg-1, and 0.030 +/- 0.005. Daily nonprotein calorie intake of 31.11 +/- 0.58 kcal kg-1, as glucose, and administration of nitrogen 0.350 +/- 0.004 g kg-1, as 10% crystalline L-aminoacids solution, and insulin 1 IU every 5.03 +/- 0.14 g of glucose, resulted in progressive decline of urinary 3-methylhistidine (4.21 +/- 0.43 mumol kg-1, p less than 0.001), creatinine (22 +/- 2 mg kg-1, NS), and their molar ratio (0.022 +/- 0.002, NS). Mean nitrogen balance resulted in 0.032 +/- 0.008 g kg-1. Since urinary 3-methylhistidine role as a marker of protein catabolism is well established, its decrease under total parenteral nutrition together with greatly improved nitrogen balance, demonstrates that our treatment can effectively quench protein catabolism, meanwhile enhancing protein synthesis.