Increasing habitual protein intake results in reduced postprandial efficiency of peripheral, anabolic wheat protein nitrogen use in humans

Dietary Requirements for Proteins and Amino Acids in Human Nutrition

Background:

An optimal dietary Amino Acid (AA) intake is necessary for the growth of body proteins. A new method for protein quality evaluation called Digestible Indispensable AA Score (DIAAS) was established. Despite that the information about dietary AA requirements are available for different stages of life, in practical conditions, most people deal only with the dietary proteins, if ever.

Objective:

The aim of this mini-review was to analyze the protein quality of protein sources by DIAAS and to find AA and protein dietary requirements in human nutrition.

Methods:

A literature research was performed using the keyword “amino acid”, “diet”, “nutrition”, “human”, “muscle” and “requirement” individually or all together, in Scopus, Web of Science and Pubmed.

Results:

The optimum amount of dietary AA is necessary for AA utilization in the body and is limited by AA which is present in the lowest amount; the surplus of other AA is not utilized in the body. Food and Agriculture Organization reported requirements for dietary protein and AA intake for infants, children, however for adults (over 18) for maintenance. Most of the studies which are dealing with the dietary AA requirements in sports nutrition are aimed at the blends of AA and for branched-chain AA. Concerning the protein quality, at this time, there is little information about the protein quality evaluated by DIAAS method.

Conclusion:

Dietary intake of high-quality protein or a blend of different proteins will provide all AA to the body. However, studies on protein quality evaluation by DIAAS method are necessary to perform.

Dietary proteins contribute little to glucose production, even under optimal gluconeogenic conditions in healthy humans

Dietary proteins are believed to participate significantly in maintaining blood glucose levels, but their contribution to endogenous glucose production (EGP) remains unclear. We investigated this question using multiple stable isotopes. After overnight fasting, eight healthy volunteers received an intravenous infusion of [6,6-²H₂]-glucose. Two hours later, they ingested four eggs containing 23 g of intrinsically, uniformly, and doubly [¹⁵N]-[¹³C]-labeled proteins. Gas exchanges, expired CO₂, blood, and urine were collected over the 8 h following egg ingestion. The cumulative amount of dietary amino acids (AAs) deaminated over this 8-h period was 18.1 ± 3.5%, 17.5% of them being oxidized. The EGP remained stable for 6 h but fell thereafter, concomitantly with blood glucose levels. During the 8 h after egg ingestion, 50.4 ± 7.7 g of glucose was produced, but only 3.9 ± 0.7 g originated from dietary AA. Our results show that the total postprandial contribution of dietary AA to EGP was small in humans habituated to a diet medium-rich in proteins, even after an overnight fast and in the absence of carbohydrates from the meal. These findings question the respective roles of dietary proteins and endogenous sources in generating significant amounts of glucose in order to maintain blood glucose levels in healthy subjects.

Effect of proteins from different sources on body composition

High-protein diets have beneficial effects on body fat regulation, but the difference in effect of various types of protein is not known. Thus, this review examines whether proteins from different sources have similar effects on body composition and energy balance. Animal proteins, especially those from dairy, seem to support better muscle protein synthesis than plant proteins. This could potentially enhance energy expenditure, but no conclusion can be drawn from the scant evidence. Some studies, but not all, demonstrate the higher satiating effect of whey and fish proteins than other protein sources. The evidence from intervention studies comparing the effects of different protein sources on body weight is inconclusive. However, body composition was not evaluated precisely in these studies and the literature is still incomplete (e.g. comparative data are missing for legumes and nuts). Protein intake enhances energy expenditure, satiety and fat loss, but there is no clear evidence to indicate whether there is a difference in the effect dependent on the source of the protein, i.e. from animal or plant-based foods.

Monosodium glutamate raises antral distension and plasma amino acid after a standard meal in humans

The consumption of monosodium glutamate (MSG) is advocated to elicit physiological and metabolic effects, yet these effects have been poorly investigated directly in humans and in particular in the postprandial phase. Thirteen healthy adults were supplemented for 6 days with a nutritional dose of MSG (2 g) or sodium chloride (NaCl) as control, following a crossover design. On the 7th day, they underwent a complete postprandial examination for the 6 h following the ingestion of the same liquid standard meal (700 kcal, 20% of energy as [(15)N]protein, 50% as carbohydrate, and 30% as fat) supplemented with MSG or NaCl. Real-ultrasound measures of antral area indicated a significant increased distension for the 2 h following the meal supplemented with MSG vs. NaCl. This early postprandial phase was also associated with significantly increased levels of circulating leucine, isoleucine, valine, lysine, cysteine, alanine, tyrosine, and tryptophan after MSG compared with NaCl. No changes to the postprandial glucose, insulin, glucagon-like peptide (GLP)-1, and ghrelin were noted between MSG- and NaCl-supplemented meals. Subjective assessments of hunger and fullness were neither affected by MSG supplementation. Finally, the postprandial fate of dietary N was identical between dietary conditions. Our findings indicate that nutritional dose of MSG promoted greater postprandial elevations of several indispensable amino acids in plasma and induced gastric distension. Further work to elucidate the possible sparing effect of MSG on indispensable amino acid first-pass uptake in humans is warranted. This trial was registered at clinicaltrials.gov as NCT00862017.

Absorption kinetics are a key factor regulating postprandial protein metabolism in response to qualitative and quantitative variations in protein intake

We have previously demonstrated that increasing the habitual protein intake widened the gap in nutritional quality between proteins through mechanisms that are not yet fully understood. We hypothesized that the differences in gastrointestinal kinetics between dietary proteins were an important factor affecting their differential response to an increased protein intake. To test this hypothesis, we built a 13-compartment model providing integrative insight into the sequential dynamics of meal nitrogen (Nm) absorption, splanchnic uptake, and metabolism, and subsequent peripheral transfer and deposition. The model was developed from data on postprandial Nm kinetics in certain accessible pools, obtained from subjects having ingested a (15)N-labeled milk or soy protein meal, after adaptation to normal (NP) or high (HP) protein diets. The faster absorption of Nm after soy vs. milk caused its earlier and stronger splanchnic delivery, which favored its local catabolic utilization (up to +30%) and limited its peripheral accretion (down to -20%). Nm absorption was also accelerated after HP vs. NP adaptation, and this kinetic effect accounted for most of the HP-induced increase (up to +20%) in splanchnic Nm catabolic use, and the decrease (down to -25%) in peripheral Nm anabolic utilization. The HP-induced acceleration in Nm absorption was more pronounced with soy than with milk, as were the HP effects on Nm regional metabolism. Our integrative approach identified Nm absorption kinetics, which exert a direct and lasting impact on Nm splanchnic catabolic use and peripheral delivery, as being critical in adaptation to both qualitative and quantitative changes in protein intake.

Increased protein intake on controlled oxalate diets does not increase urinary oxalate excretion

High animal protein intake is a risk factor for calcium oxalate stone disease. The effect of dietary protein on the urinary excretion of calcium, acid and citrate is well established. However, its effect on oxalate excretion is unclear, due in part to an inadequate control of dietary oxalate intake in previous studies. This relationship warrants clarification due to the proposed important role of the metabolism of amino acids in endogenous oxalate synthesis. In this study, 11 normal subjects consumed controlled oxalate diets containing 0.6, 1.2 and 1.8 g protein/kg body weight/day. The analysis of 24 h urine collections confirmed that as protein intake increased, urinary calcium and glycolate increased and urinary pH and citrate decreased. The increased glycolate excretion was due in part to an increased hydroxyproline, but not glycolate consumption. Total daily urinary oxalate excretion did not change. When indexed to creatinine there was a small but significant decrease in oxalate excretion. This is most likely due to hyperfiltration. These results indicate that as dietary protein intake increases, the catabolism of diet-derived amino acids is not associated with an increased endogenous oxalate synthesis in normal subjects.

Urea-nitrogen production and salvage are modulated by protein intake in fed humans: results of an oral stable-isotope-tracer protocol and compartmental modeling

BACKGROUND

The influence of protein source on postprandial urea kinetics is poorly understood, despite its nutritional significance with respect to nitrogen homeostasis. Furthermore, traditional tracer infusion studies underestimate acute postprandial change in urea kinetics.

OBJECTIVE

We investigated postprandial, non-steady state urea kinetics and their modulation by qualitative and quantitative factors of protein intake by the combined use of robust clinical data on nitrogen postprandial distribution and mathematical modeling.

DESIGN

In healthy subjects standardized to a normal protein intake for 7 d, dietary and total nitrogen kinetics were measured for 8 h in plasma proteins, body, and urinary urea after the ingestion of a (15)N-labeled milk (n = 8), soy (n = 8), or wheat (n = 8) protein meal. In subjects who received the soy protein meal, these postprandial measurements were repeated after a further 7-d adaptation to a high protein intake. A 4-compartment model was developed to calculate from these data the postprandial kinetics of production, urinary excretion, and intestinal hydrolysis of urea nitrogen from both dietary and endogenous sources.

RESULTS

Urinary urea excretion was not influenced by the protein source in the meal but was influenced by the protein level in the diet. By contrast, urea production and hydrolysis were higher when ingesting plant versus animal protein, together with a higher efficiency of urea hydrolysis (50-60% versus 25% of the urea produced being hydrolyzed, respectively).

CONCLUSIONS

We conclude that urea hydrolysis is an acute nitrogen-sparing mechanism that can counterbalance a postprandial higher urea production, and the efficiency of this recycling is higher when the usual protein intake is lower.