Vernon Young and the development of current knowledge in protein and amino acid nutritionVernon Young 1937–2004

Post-prandial tracer studies of protein and amino acid utilisation: what can they tell us about human amino acid and protein requirements?

Nitrogen balance (NB), the principal methodology used to derive recommendations for human protein and amino acid requirements, has been widely criticised, and calls for increased protein and amino acid requirement recommendations have been made, often on the basis of post-prandial amino acid tracer kinetic studies of muscle protein synthesis, or of amino acid oxidation. This narrative review considers our knowledge of the homeostatic regulation of the FFM throughout the diurnal cycle of feeding and fasting and what can and has been learnt from post-prandial amino acid tracer studies, about amino acid and protein requirements. Within the FFM, muscle mass in well fed weight-stable adults with healthy lifestyles appears fixed at a phenotypic level within a wide range of habitual protein intakes. However homoeostatic regulation occurs in response to variation in habitual protein intake, with adaptive changes in amino acid oxidation which influence the magnitude of diurnal losses and gains of body protein. Post-prandial indicator amino acid oxidation (IAAO) studies have been introduced as an alternative to NB and to the logistically complex 24 h [13C-1] amino acid balance studies, for assessment of protein and amino acid requirements. However, a detailed examination of IAAO studies shows both a lack of concern for homeostatic regulation of amino acid oxidation and  major flaws in their design and analytical interpretation, which seriously constrain their ability to provide reliable values. New ideas and a much more critical approach to existing work is needed if real progress is to be made in the area.

The association between dietary factors and body weight and composition in boys with Duchenne muscular dystrophy

BACKGROUND

Duchenne muscular dystrophy (DMD) is a X-linked neuromuscular disorder. Boys with DMD have high rates of obesity, but little is known about dietary factors that may contribute to weight gain in this population. This study aimedto explore the relationship between dietary factors, body mass index (BMI) z-score, body composition and motor function and to describe dietary intake in boys with DMD.

METHODOLOGY

A cross-sectional analysis of 3-day food diaries from ambulant and steroid treated boys with DMD aged 5-13 years was conducted. Correlation analysis explored the relationship between dietary factors, BMI z-score, fat mass % (FM%) and lean mass (LM%).

RESULTS

Themedian agewas 8.5 years [interquartile range (IQR) 7.2, 10.5]. Median energy/kg/day in those within a healthy weight range (n=11) was316 kJ/kg/day [IQR 276, 355] and greater than estimated requirements; and forthose above a healthy weight (n=26) energy intake was 185kJ/kg/day [IQR 143, 214] and lower than estimated requirements. Energy/kg/day was negatively associatedwith BMI z-score (r=-0. 650) and FM% (r=-0.817)but positively associated with LM% (r=0.805, all analyses p =<0.01). Younger age was associated (r=-0.609 p=<0.01) with a higher energy/kg/day. For all participants vegetable, grains, meat/alternatives and dairy intake was sub-optimal.

PRINCIPAL CONCLUSIONS

Younger boys with DMD within a healthy weight range are overconsuming energy dense nutrient poor foods. A focus on improving diet quality during early childhood may prove a useful strategy to reduce excess weight gain and support healthier eating habits in this vulnerable clinical population. This article is protected by copyright. All rights reserved.

Amino Acids in Beef Cattle Nutrition and Production

Proteins have been recognized for a long time as an important dietary nutritional component for all animals. Most amino acids were isolated and characterized in the late nineteenth and early twentieth century. Initially dietary proteins were ranked high to low quality by growth and N balance studies. By the 1950s interest had shifted to studying the roles of individual amino acids in amino acid requirements by feeding studies with non-ruminants as rodents, poultry and pigs. The direct protein feeding approaches followed by measurements of nutritional outcomes were not possible however in ruminants (cattle and sheep). The development of measuring free amino acids by ion exchange chromatography enabled plasma amino acid analysis. It was thought that plasma amino acid profiles were useful in nutritional studies on proteins and amino acids. With non-ruminants, nutritional interpretations of plasma amino acid studies were possible. Unfortunately with beef cattle, protein/amino acid nutritional adequacy or requirements could not be routinely determined with plasma amino acid studies. In dairy cows, however, much valuable understanding was gained from amino acid studies. Concurrently, others studied amino acid transport in ruminant small intestines, the role of peptides in ruminant N metabolism, amino acid catabolism (in the animal) with emphasis on branched-chain amino acid catabolism. In addition, workable methodologies for studying protein turnover in ruminants were developed. By the 1990s, nutritionists could still not determine amino acid requirements with empirical experimental studies in beef cattle. Instead, computer software (expert systems) based on the accumulated knowledge in animal and ruminal amino acids, energy metabolism and protein production were realized and revised frequently. With these tools, the amino acid requirements, daily energy needs, ruminal and total gastrointestinal tract digestion and performance of growing beef cattle could be predicted.

Identifying recommended dietary allowances for protein and amino acids: a critique of the 2007 WHO/FAO/UNU report

The WHO/FAO/UNU (2007) report examines dietary protein and amino acid requirements for all age groups, protein requirements during pregnancy, lactation and catch-up growth in children, the implications of these requirements for developing countries and protein quality evaluation. Requirements were defined as the minimum dietary intake which satisfies the metabolic demand and achieves nitrogen equilibrium and maintenance of the body protein mass, plus the needs for growth in children and pregnancy and lactation in healthy women. Insufficient evidence was identified to enable recommendations for specific health outcomes. A meta analysis of nitrogen balance studies identifies protein requirements for adults 10 % higher than previous values with no influence of gender or age, consistent with a subsequently published comprehensive study. A new factorial model for infants and children, validated on the basis of the adequacy of breast milk protein intakes and involving a lower maintenance requirement value, no provision for saltatory growth and new estimates of protein deposition identifies lower protein requirements than in previous reports. Higher values for adult amino acid requirements, derived from a re-evaluation of nitrogen balance studies and new stable isotope studies, identify some cereal-based diets as being inadequate for lysine. The main outstanding issues relate to the biological implausibility of the very low efficiencies of protein utilisation used in the factorial models for protein requirements for all population groups especially pregnancy when requirements may be overestimated. Also considerable uncertainty remains about the design and interpretation of most of the studies used to identify amino acid requirement values.

Optimal Quantity and Composition of Protein for Growing Children

Children have distinct nutritional needs relative to growth. Adequate intakes of energy and essential amino acids are necessary for optimal deposition of lean body mass and normal growth in young children. However, there are limited data concerning protein needs of children. Most recommendations for children represent an interpolation of data derived from infants and adults. Indeed, current protein requirements for young children, while scientifically based, are estimates at best. Historically, protein status in children was evaluated using classic nitrogen balance protocols. This work indicates that a wide range of protein intakes (0.6-2.9 g/kg) can be considered adequate for young, growing children. The ability of nitrogen balance studies to accurately reflect protein utilization has been examined and it appears that further investigations of protein utilization in children using stable isotope methodology, as well as traditional nitrogen balance protocols, are necessary to better evaluate protein needs of growing children. In addition, protein source may be an important factor in optimal diet design for growing children.