Editorial on "Measurement of protein digestibility in humans by a dual tracer method"-a key limiting factor of protein quality

Pinto Bean Amino Acid Digestibility and Score in a Mexican Dish with Corn Tortilla and Guacamole, Evaluated in Adults Using a Dual-Tracer Isotopic Method

BACKGROUND

Ultra-processed foodstuffs have been replacing traditional beans with tortillas in the Mexican diet in the last decades. Therefore, scientific support is needed to promote a return to good-quality traditional dishes.

OBJECTIVES

This study aims to evaluate the amino acid digestibility and score of pinto beans (Phaseolus vulgaris) consumed with corn tortillas and guacamole in adults using the dual-tracer method.

METHODS

The pinto beans were intrinsically labeled using 250 mL of 2H2O (99.8%) per 19 L pot with 3 plants. A paste of cooked beans on toasted corn tortillas and guacamole topping were administered to 3 male and 3 female adults (21-25 years old; BMI, 19-23.5 kg/m2). The protocol was plateau feeding given along with U-[13C]-spirulina protein to evaluate indispensable amino acid (IAA) digestibility using the dual-tracer method. Blood samples were taken in the plateau state. The digestibility of each IAA of the bean protein was calculated by the ratio of its enrichment in the beans to the spirulina in the meal and its appearance in plasma collected in the plateau state, as a percentage corrected by spirulina digestibility. Additionally, the digestible IAA score (DIAAS) was calculated.

RESULTS

The 2H enrichment of IAA in the pinto beans was 471 parts per million excess. The isotopic enrichment of 2H and 13C in IAA at 5-8 hours presented plateau states with mean CVs of 12.2% and 13.3%, respectively. The mean digestibility of IAA from pinto beans was 77% ± 1.6%, with the lowest value for threonine. The DIAAS calculated with respect to the pattern requirement for children older than 3 years, adolescents, and adults was 83%, with methionine and cysteine being the limiting amino acids.

CONCLUSIONS

A Mexican dish of pinto beans, tortillas, and guacamole is a good source of protein as evaluated in adults and could be promoted as a nutritious snack. The assay is registered with the Ethical Committee of the Centro de Investigación en Alimentación y Desarrollo, A.C. as CE/015/2019.

Blood 15N:13C Enrichment Ratios Are Proportional to the Ingested Quantity of Protein with the Dual-Tracer Approach for Determining Amino Acid Bioavailability in Humans

BACKGROUND

Assessment of amino acid bioavailability is of key importance for the evaluation of protein quality; however, measuring ileal digestibility of dietary proteins in humans is challenging. Therefore, a less-invasive dual stable isotope tracer approach was developed.

OBJECTIVE

We aimed to test the assumption that the 15N:13C enrichment ratio in the blood increases proportionally to the quantity ingested by applying different quantities of 15N test protein.

METHODS

In a crossover design, 10 healthy adults were given a semi-liquid mixed meal containing 25 g (low protein) or 50 g (high protein) of 15N-labeled milk protein concentrate simultaneous with 0.4 g of highly 13C-enriched spirulina. The meal was distributed over multiple small portions, frequently provided every 20 min during a period of 160 min. For several amino acids, the blood 15N- related to 13C-isotopic enrichment ratio was determined at t = 0, 30, 60, 90, 120, 180, 240, 300, and 360 min and differences between the 2 meals were compared using paired analyses.

RESULTS

No differences in 13C AUC for each of the measured amino acids in serum was observed when ingesting a low- or high-protein meal, whereas 15N AUC of amino acids was ∼2 times larger on the high-protein meal (P < 0.001). Doubling the intake of 15N-labeled amino acids increased the 15N:13C ratio by a factor of 2.04 ± 0.445 for lysine and a factor between 1.8 and 2.2 for other analyzed amino acids, with only phenylalanine (2.26), methionine (2.48), and tryptophan (3.02) outside this range.

CONCLUSIONS

The amino acid 15N:13C enrichment ratio in the peripheral circulation increased proportionally to the quantity of 15N-labeled milk protein ingested, especially for lysine, in healthy adults. However, when using 15N-labeled protein, correction for, e.g., α-carbon 15N atom transamination is advised for determination of bioavailability of individual amino acids. This trial was registered at www.clinicaltrials.gov as NCT02966704.

New advances in stable tracer methods to assess whole-body protein and amino acid metabolism

PURPOSE OF REVIEW

Stable isotope methods have been used for many years to assess whole-body protein and amino acid kinetics in healthy conditions and in response to aging, exercise and (clinically stable) disease states.

RECENT FINDINGS

In recent years, tracer research expanded to the anabolic response to feeding in critical illness and its use during acute metabolic stressors. Furthermore, new isotope approaches and tracer insights have been obtained. In the postabsorptive state, the novel tracer pulse approach has several advantages above the established continuous tracer approach because of the metabolic information that can be obtained, easy applicability, and low tracer costs. The use of bolus versus sip-feeding approaches to assess the anabolic response to a meal is dependent on the research question and its feasibility. Promising new tracer approaches have been developed to measure the anabolic capacity, and protein digestibility and absorption. Advances have been made in the field of mass spectrometry in low enrichment analysis.

SUMMARY

Novel tracer approaches are available that can more readily be used in critical illness and during acute metabolic stressors. Besides the use of tracer application in various clinical conditions, more research is needed on how to incorporate isotopes on an individual level.