A discussion on the 'dispensable' amino acids

Beyond the Bowl: Understanding Amino Acid Requirements and Digestibility to Improve Protein Quality Metrics for Dog and Cat Foods

The determination of amino acid (AA) requirements for mammals has traditionally been done through nitrogen (N) balance studies, but this technique underestimates AA requirements in adult animals. There has been a shift toward researchers using the indicator amino acid oxidation (IAAO) technique for the determination of AA requirements in humans, and recently in dogs. However, the determination of AA requirements specific to adult dogs and cats at maintenance is lacking and the current requirements outlined by the National Research Council are based on a dearth of data and are likely underreporting the requirements of indispensable AA (IAA) for the population. To ensure the physiological requirements of our cats and dogs are met, we need methods to accurately and precisely measure digestibility. In vivo methods, such as ileal cannulation, are most commonly used, however, due to ethical considerations, we are moving away from animal models and toward in vitro methods. Harmonized static digestion models have the potential to replace in vivo methods but work needs to be done to have these methods more accurately represent the gastrointestinal tract (GIT) of cats and dogs. The Digestible IAA Score (DIAAS) is one metric that can help define protein quality for individual ingredients or mixed diets that uses AA SID estimates and ideally those can be replaced with in vitro AA digestibility estimates. Finally, we need accurate and reliable laboratory AA analyses to measure the AA present in complete diets, especially those used to quantify methionine (Met) and cysteine (Cys), both often limiting AAs in cat and dog diets. Together, this will guide accurate feed formulation for our companion animals to satisfy requirements while avoiding over-supplying protein, which inevitably contributes to excess N excretion, affecting both the environment and feed sustainability.

Tolerable Upper Intake Level for Individual Amino Acids in Humans: A Narrative Review of Recent Clinical Studies

Individual amino acids are widely popular as supplements because of various perceived and real health benefits. However, currently, there are no recommendations set by national health agencies for tolerable upper intake levels (UL) for amino acids because of a lack of well-conducted human dose-response trials. In the past decade, under the initiative of the International Council on Amino Acid Science, a nonprofit organization, a series of UL human clinical studies were conducted. The goal of this narrative review is to summarize the studies on 6 essential amino acids (leucine, tryptophan, methionine, lysine, histidine, and phenylalanine), 2 nonessential amino acids (arginine and serine), and 2 nonproteinogenic amino acids (ornithine and citrulline) and provide the first set of ULs. A brief background of the concept of the DRI framework of UL, the concept of UL for amino acids, and a perspective of the results are also provided. The data suggest that in relatively healthy adult individuals, the tested amino acids are well tolerated, and ULs, or the no-observed-adverse-effect-level (NOAEL), lowest-observed-adverse-effect-level (LOAEL), can be determined. The ULs were for leucine-young (35 g/d), tryptophan (4.5 g/d), and leucine-elderly (30 g/d); NOAEL and LOAEL for methionine at 3.2 and 6.4 g/d, respectively; NOAEL for arginine (30 g/d); NOAEL and LOAEL for lysine at 6 and 7.5 g/d, respectively; NOAEL and LOAEL for histidine at 8 and 12 g/d, respectively; and NOAEL for phenylalanine (12 g/d), serine (12 g/d), ornithine (12 g/d) and citrulline (24 g/d). This first set of human UL data are hoped to help national and international agencies set safety standards for supplemental amino acids.

Extending the Range of SLIM-Labeling Applications: From Human Cell Lines in Culture to Caenorhabditis elegans Whole-Organism Labeling

The simple light isotope metabolic-labeling technique relies on the in vivo biosynthesis of amino acids from U-[¹²C]-labeled molecules provided as the sole carbon source. The incorporation of the resulting U-[¹²C]-amino acids into proteins presents several key advantages for mass-spectrometry-based proteomics analysis, as it results in more intense monoisotopic ions, with a better signal-to-noise ratio in bottom-up analysis. In our initial studies, we developed the simple light isotope metabolic (SLIM)-labeling strategy using prototrophic eukaryotic microorganisms, the yeasts Candida albicans and Saccharomyces cerevisiae, as well as strains with genetic markers that lead to amino-acid auxotrophy. To extend the range of SLIM-labeling applications, we evaluated (i) the incorporation of U-[¹²C]-glucose into proteins of human cells grown in a complex RPMI-based medium containing the labeled molecule, considering that human cell lines require a large number of essential amino-acids to support their growth, and (ii) an indirect labeling strategy in which the nematode Caenorhabditis elegans grown on plates was fed U-[¹²C]-labeled bacteria (Escherichia coli) and the worm proteome analyzed for ¹²C incorporation into proteins. In both cases, we were able to demonstrate efficient incorporation of ¹²C into the newly synthesized proteins, opening the way for original approaches in quantitative proteomics.

Is the amino acid pattern in medical nutrition therapy crucial for successfully attenuating muscle mass loss in adult ICU patients? Secondary analysis of a RCT

BACKGROUND AND AIMS

We hypothesized that in long-term immobilized intensive care unit (ICU) patients, both the quantity and quality of protein nutrition are vital in supporting muscle mass maintenance. Hence, the aim of this secondary analysis of our recently performed RCT was to calculate the intake of individual amino acids and to evaluate the potential associations of amino acid patterns with muscle mass loss during the ICU stay.

METHODS

Clinical and nutritional data were collected from a recent RCT conducted in long-term immobilized, critically ill patients receiving medical nutrition therapy with either 1.8 g (interventional group) or 1.2 g (standard group) of protein/amino acids per kg body weight per day over 4 weeks. Intake of the individual amino acids as well as the sum scores of the indispensable, conditionally indispensable, and dispensable amino acids were calculated for all patients, both group specific (n = 21 in each group) and in total (n = 42), based on the detailed nutrition protocols; inter-group differences were analyzed by t-tests. Linear regression models were used to test the effects of individual amino acids and the sum scores on the extent of skeletal muscle loss by measuring the quadriceps muscle layer thickness during the study period. The significance level was adjusted for multiple testing according to the Bonferroni procedure (α = 0.002).

RESULTS

In both groups, the proportion of indispensable amino acids was approximately 41% of the total exogenous protein supply, with the proportion of enteral administration slightly over 50%. The intake of conditionally indispensable amino acids (glutamine, tyrosine, cysteine, histidine, and arginine) accounted for 17% and 18% of the total amino acids in the interventional and standard groups, respectively; glutamine (5% of total amino acids) was exclusively administered enterally. The intake of dispensable amino acid varied widely, with glutamic acid, proline, and asparagine/aspartic acid representing the highest proportions (10%, 8%, and 8% of total amino acids, respectively). For all amino acids, no statistically significant association was observed between the quantitative intake and the skeletal muscle changes after terminating the intervention phase.

CONCLUSION

This secondary analysis of the RCT conducted in routine clinical practice did not support our working hypothesis that the amino acid patterns of medical nutrition therapy have a statistically significant impact on the skeletal muscle loss in long-term immobilized ICU patients. Due to the limited variety of enteral/parenteral products used in this single-center study, the calculated amino acid patterns showed only small differences. Larger multi-center trials with adequate power are needed to evaluate the potential effects of the individual amino acids or defined amino acid patterns on the muscle protein metabolism in further detail.

TRIAL REGISTRATION

German Clinical Trials Register (http://www.drks.de); DRKS-ID: DRKS00013594.