Impact of habituated dietary protein intake on fasting and postprandial whole-body protein turnover and splanchnic amino acid metabolism in elderly men: a randomized, controlled, crossover trial

Quantification and interpretation of postprandial whole-body protein metabolism using stable isotope methodology: a narrative review

Stable isotopes are routinely applied to determine the impact of factors such as aging, disease, exercise, and feeding on whole-body protein metabolism. The most common approaches to quantify whole-body protein synthesis, breakdown, and oxidation rates and net protein balance are based on the quantification of plasma amino acid kinetics. In the postabsorptive state, plasma amino acid kinetics can easily be assessed using a constant infusion of one or more stable isotope labeled amino acid tracers. In the postprandial state, there is an exogenous, dietary protein-derived amino acid flux that needs to be accounted for. To accurately quantify both endogenous as well as exogenous (protein-derived) amino acid release in the circulation, the continuous tracer infusion method should be accompanied by the ingestion of intrinsically labeled protein. However, the production of labeled protein is too expensive and labor intensive for use in more routine research studies. Alternative approaches have either assumed that 100% of exogenous amino acids are released in the circulation or applied an estimated percentage based on protein digestibility. However, such estimations can introduce large artifacts in the assessment of whole-body protein metabolism. The preferred estimation approach is based on the extrapolation of intrinsically labeled protein-derived plasma bioavailability data obtained in a similar experimental design setting. Here, we provide reference data on exogenous plasma amino acid release that can be applied to allow a more accurate routine assessment of postprandial protein metabolism. More work in this area is needed to provide a more extensive reference data set.

Dietary protein recommendations to support healthy muscle ageing in the 21st century and beyond: considerations and future directions

This review explores the evolution of dietary protein intake requirements and recommendations, with a focus on skeletal muscle remodelling to support healthy ageing based on presentations at the 2023 Nutrition Society summer conference. In this review, we describe the role of dietary protein for metabolic health and ageing muscle, explain the origins of protein and amino acid (AA) requirements and discuss current recommendations for dietary protein intake, which currently sits at about 0⋅8 g/kg/d. We also critique existing (e.g. nitrogen balance) and contemporary (e.g. indicator AA oxidation) methods to determine protein/AA intake requirements and suggest that existing methods may underestimate requirements, with more contemporary assessments indicating protein recommendations may need to be increased to >1⋅0 g/kg/d. One example of evolution in dietary protein guidance is the transition from protein requirements to recommendations. Hence, we discuss the refinement of protein/AA requirements for skeletal muscle maintenance with advanced age beyond simply the dose (e.g. source, type, quality, timing, pattern, nutrient co-ingestion) and explore the efficacy and sustainability of alternative protein sources beyond animal-based proteins to facilitate skeletal muscle remodelling in older age. We conclude that, whilst a growing body of research has demonstrated that animal-free protein sources can effectively stimulate and support muscle remodelling in a manner that is comparable to animal-based proteins, food systems need to sustainably provide a diversity of both plant and animal source foods, not least for their protein content but other vital nutrients. Finally, we propose some priority research directions for the field of protein nutrition and healthy ageing.

Effect of 1-year daily protein supplementation and physical exercise on muscle protein synthesis rate and muscle metabolome in healthy older Danes: a randomized controlled trial

BACKGROUND

The skeletal muscle mass decreases with age and the responsiveness of aging muscles' protein synthesis rate (MPS) to protein intake seems to deteriorate.

OBJECTIVE

This study investigated the impact of 12 months of protein supplementation with or without physical exercise training on the basal and postprandial MPS and the skeletal muscle metabolome of healthy older Danes (> 65 years, 29 females/37 males).

METHODS

Subjects were randomized to follow one of five intervention groups: (1) carbohydrate, (2) collagen protein, (3) whey protein, (4) home-based light resistance training with whey protein, and (5) center-based heavy-load resistance training with whey protein. Before and after the intervention, a tracer infusion trial was conducted to measure basal and postprandial MPS in response to intake of a cocktail consisting of 20 g whey hydrolysate + 10 g glucose. In addition, the skeletal muscle metabolome was measured using gas chromatography-mass spectrometry (GC-MS) at basal state and 4 h after the intake of the cocktail.

RESULTS

One year of daily protein or carbohydrate supplementation did not alter the basal and protein-stimulated postprandial muscle protein synthesis rate or the muscle metabolome of healthy older Danes. Basal MPS (%/h) at baseline for all subjects were 0.0034 ± 0,011 (mean ± SD). In contrast to previous studies, no difference was observed in basal MPS between males and females (p = 0.75). With the developed untargeted GC-MS methodology, it was possible to detect and tentatively annotate > 70 metabolites from the human skeletal muscle samples.

CONCLUSION

One year of protein supplementation in comparison to an isocaloric-control supplement seems to affect neither the MPS at basal or postprandial state nor the skeletal muscle metabolome.

CLINICAL TRIAL REGISTRY

Number: NCT02115698, clinicaltrials.gov/ct2/show/NCT02115698.

Dietary protein requirements and recommendations for healthy older adults: a critical narrative review of the scientific evidence

Adequate protein intake is essential for the maintenance of whole-body protein mass. Different methodological approaches are used to substantiate the evidence for the current protein recommendations, and it is continuously debated whether older adults require more protein to counteract the age-dependent loss of muscle mass, sarcopenia. Thus, the purpose of this critical narrative review is to outline and discuss differences in the approaches and methodologies assessing the protein requirements and, hence, resulting in controversies in current protein recommendations for healthy older adults. Through a literature search, this narrative review first summarises the historical development of the Food and Agriculture Organization/World Health Organization/United Nations University setting of protein requirements and recommendations for healthy older adults. Hereafter, we describe the various types of studies (epidemiological studies and protein turnover kinetic measurements) and applied methodological approaches founding the basis and the different recommendations with focus on healthy older adults. Finally, we discuss important factors to be considered in future studies to obtain evidence for international agreement on protein requirements and recommendations for healthy older adults. We conclude by proposing future directions to determine 'true' protein requirements and recommendations for healthy older adults.

Even or skewed dietary protein distribution is reflected in the whole-body protein net-balance in healthy older adults: A randomized controlled trial

BACKGROUND & AIM

For older adults, the dietary protein intake has shown to be skewed towards the evening meal. Resultingly, the vital source of essential amino acids could be insufficient after some meals, while after the evening meal the dietary protein could be suboptimally utilized for protein synthesis. The present study explored if an even distribution of the protein intake could improve the dietary amino acid absorption and whole-body protein net-balance.

METHODS

Twenty-four healthy elderly males and females were included in a randomized controlled trial. Ten days of habituation to either an EVEN (n = 12) or SKEWED (n = 12) protein intake, was followed by a trial day. The total protein intake was controlled at 1.5 g/kg LBM, divided into 30% at each main meal in EVEN, and into 15% at breakfast and lunch and 60% at dinner in SKEWED. Snacks with 5% of the protein intake were served between meals. Energy intake in the meals and snacks were equal in both groups. Intrinsically labelled ²H₅-phenylalanine minced meat was served as the dietary protein to assess the amino acid absorption. On the trial day, infusion of ²H₈-phenylalanine and ²H₂-tyrosine, and blood samples taken over 11 h were used to measure whole-body protein turnover. Vastus lateralis muscle biopsies were taken to measure 9 h muscle protein FSR.

RESULTS

Amino acid absorption rates and concentrations were greater in EVEN compared to SKEWED protein intake. Whole-body protein breakdown rates were lower with similar protein synthesis rates, and consequently the net-balance was greater in EVEN after breakfast and lunch compared to SKEWED and were the same in both groups after dinner. Muscle protein FSR were not different between EVEN and SKEWED.

CONCLUSIONS

The whole-body protein net-balance was more positive in EVEN compared to SKEWED for an extended time of the measured period, driven by a lower whole-body protein breakdown in EVEN.

CLINICAL TRIALS REGISTRATION

NCT03870425, https://clinicaltrials.gov/ct2/show/NCT03870425.

Changes in 24-h energy expenditure, substrate oxidation, and body composition following resistance exercise and a high protein diet via whey protein supplementation in healthy older men

PURPOSE

To investigate changes in 24-h energy expenditure (EE), substrate oxidation, and body composition following resistance exercise (RE) and a high protein diet via whey protein supplementation (alone and combined) in healthy older men.

METHODS

In a pooled groups analysis, 33 healthy older men [(mean ± SE) age: 67 ± 1 years; BMI: 25.4 ± 0.4 kg/m2] were randomized to either RE (2×/week; n = 17) or non-exercise (n = 16) and either a high protein diet via whey protein supplementation (PRO, 2 × 25 g whey protein isolate/d; n = 17) or control (CON, 2 × 23.75 g maltodextrin/d; n = 16). An exploratory sub-analysis was also conducted between RE+CON (n = 8) and RE+PRO (n = 9). At baseline and 12 weeks, participants resided in respiration chambers for measurement of 24-h EE and substrate oxidation and wore an accelerometer for 7 days for estimation of free-living EE.

RESULTS

Resistance exercise resulted in greater increases in fat-free mass (1.0 ± 0.3 kg), resting metabolic rate [(RMR) 36 ± 14 kcal/d], sedentary EE (60 ± 33 kcal/d), and sleeping metabolic rate [(SMR) 45 ± 7 kcal/d] compared to non-exercise (p < 0.05); however, RE decreased activity energy expenditure in free-living (-90 ± 25 kcal/d; p = 0.049) and non-exercise activity inside the respiration chamber (-1.9 ± 1.1%; p = 0.049). PRO decreased fat mass [(FM) -0.5 ± 0.3 kg], increased overnight protein oxidation (30 ± 6 g/d), and decreased 24-h protein balance (-20 ± 4 g/d) greater than CON (p < 0.05). RE+PRO decreased FM (-1.0 ± 0.5 kg) greater than RE+CON (p = 0.04).

CONCLUSION

Resistance exercise significantly increased RMR, SMR, and sedentary EE in healthy older men, but not total EE. PRO alone and combined with RE decreased FM and aided body weight maintenance. This study was registered at clinicaltrials.gov as NCT03299972.

A low postabsorptive whole body protein balance is associated with markers of poor daily physical functioning in Chronic Obstructive Pulmonary Disease

BACKGROUND & AIMS

Postabsorptive whole body protein kinetics are related to age, gender, body mass index (BMI), and habitual protein intake level. It is unclear how protein synthesis, breakdown, and postabsorptive protein balance rates are affected in Chronic Obstructive Pulmonary Disease (COPD)) and whether these relate to disease severity, lifestyle characteristics and poor daily functioning.

METHODS

We studied 91 COPD (GOLD 1-4) and 56 age matched control subjects without COPD or other chronic or acute health disease/condition in the postabsorptive state and measured body composition by Dual-energy X-ray Absorptiometry, and disease severity and comorbidities by medical screening, blood analysis and questionnaires. We assessed whole body production rates of phenylalanine and tyrosine by pulse stable isotope tracer infusion to calculate whole body protein breakdown (PB) and hydroxylation of phenylalanine to tyrosine, representative of postabsorptive protein balance. We measured muscle and cognitive function, and physical performance by isokinetic dynamometry, cognitive assessments, and 6-min walk test. We assessed physical activity level, mood and dietary protein intake by questionnaires. We measured plasma enrichments by LC-MS/MS and statistics by Fisher's exact test or analysis of covariance. Data are mean [95% CI].

RESULTS

The COPD patients had moderate to severe airflow obstruction, multiple comorbidities, and elevated values for plasma high sensitivity c-reactive protein (hs-CRP) and glucose. Although PB (3630 [3361, 3900] vs 3504 [3297, 3711] umol/h, p = 0.1649) was not different, postabsorptive protein balance was lower in COPD patients (274.2 [242.4, 306.1] vs 212.9 [194.7, 231.0] umol/h, p < 0.0001), both compared to control subjects. A lower postabsorptive protein balance was associated with age (p < 0.0001) and higher levels for systolic blood pressure (p = 0.0051) and hs-CRP (p = 0.0046) but not with lung function. Furthermore, a lower postabsorptive protein balance level was associated with a lower intake of total calories and protein (p < 0.0001) and lower muscle strength (p = 0.0248), while only in COPD with a lower physical performance (p = 0.0343). We found no association with cognitive function or mood. For all subjects, a cumulative model that included group, gender, age, BMI, systolic blood pressure, hs-CRP, caloric intake, protein intake, and leg strength was able to explain 55% of the variation in postabsorptive protein balance.

CONCLUSION

These data suggest that systemic inflammation, high blood pressure and low protein intake are risk factors of a lower postabsorptive protein balance in COPD patients. A lower postabsorptive protein balance is associated with markers of poor daily physical functioning.

Effects of resistance exercise and whey protein supplementation on skeletal muscle strength, mass, physical function, and hormonal and inflammatory biomarkers in healthy active older men: a randomised, double-blind, placebo-controlled trial

PURPOSE

To determine the individual and combined effects of 12 weeks of resistance exercise (RE) and whey protein supplementation on skeletal muscle strength (primary outcome), mass and physical function, and hormonal and inflammatory biomarkers in older adults.

METHODS

Thirty-six healthy older men [(mean±SE) age: 67±1 y; BMI: 25.5±0.4 kg/m²] were randomised to either control (CON; n=9), whey protein (PRO; n=9), RE+control (EX+CON; n=9), or RE+whey protein (EX+PRO; n=9) in a double-blinded fashion. Whole-body RE (2 sets of 8 repetitions and 1 set to volitional failure at 80% 1RM) was performed twice weekly. Supplements (PRO, 25 g whey protein isolate; CON, 23.75 g maltodextrin) were consumed twice daily.

RESULTS

EX+CON and EX+PRO increased leg extension (+19±3 kg and +20±3 kg, respectively) and leg press 1RM (+27±3 kg and +39±2 kg, respectively) greater than the CON and PRO groups (P<0.001, Cohen's d=1.50-1.90). RE (EX+CON and EX+PRO groups pooled) also increased fat-free mass (FFM) (+0.9±0.3 kg) and 6-min walk test distance (+21±5 m) and decreased fat mass (-0.4±0.4 kg), and interleukin-6 (-1.0±0.4 pg/mL) and tumor necrosis factor-alpha concentration (-0.7±0.3 pg/mL) greater than non-exercise (CON and PRO groups pooled; P<0.05, Cohen's f=0.37-0.45). Whey protein supplementation (PRO and EX+PRO groups pooled) increased 4-m gait speed greater than control (CON and EX+CON groups pooled) (+0.08±0.03 m/s; P=0.007, f=0.51).

CONCLUSION

RE increased muscle strength, FFM and physical function, and decreased markers of systemic inflammation in healthy active older men. Whey protein supplementation alone increased gait speed. No synergistic effects were observed. This study was registered at clinicaltrials.gov as NCT03299972.

Postprandial muscle protein synthesis rate is unaffected by 20-day habituation to a high protein intake: a randomized controlled, crossover trial

PURPOSE

During the last decade more researchers have argued in favor of an increased protein intake for older adults. However, there is a lack of knowledge on the long-term effects of conforming to such a high protein intake with regards to the basal and postprandial muscle protein turnover. The purpose of this study was to compare the postprandial synthesis response in muscle proteins, and the abundance of directly incorporated food-derived amino acids following habituation to high vs. recommended level of protein intake.

METHODS

In a double blinded crossover intervention 11 older male participants (66.6 ± 1.7 years of age) were habituated for 20 days to a recommended protein (RP) intake (1.1 g protein/kg lean body mass (LBM)/day) and a high protein (HP) intake (> 2.1 g protein/kg LBM/day). Following each habituation period, intrinsically labelled proteins were ingested as part of a mixed meal to determine the incorporation of meal protein-derived amino acids into myofibrillar proteins. Furthermore, the myofibrillar fractional synthesis rate (FSR) and amino acid kinetics across the leg were determined using gold standard stable isotope tracer methodologies. RT qPCR was used to assess the expression of markers related to muscle proteinsynthesis and breakdown.

RESULTS

No impact of habituation was observed on skeletal muscle amino acid or protein kinetics. However, the shunting of amino acids directly from artery to vein was on average 2.9 [Formula: see text]mol/min higher following habituation to HP compared to RP.

CONCLUSIONS

In older males, habituation to a higher than the currently recommended protein intake did not demonstrate any adaptions in the muscle protein turnover or markers hereof when subjected to an intake of an identical mixed meal.

CLINICAL TRIAL REGISTRY

Journal number NCT02587156, Clinicaltrials.org. Date of registration: October 27th, 2015.

Comparison of basal whole-body protein kinetics and muscle protein synthesis between young and older adults

Significant research has been dedicated to counteracting age-related muscle loss, but underlying mechanisms have not been clearly established. Previous research examining differences in basal protein kinetics between young and older individuals has been limited by a lack of evaluation of protein breakdown and net balance. The aim of this study was to more comprehensively examine differences in basal protein kinetics between younger and older males and females. Basal whole-body protein kinetics and muscle fractional synthetic rate (FSR) from 91 younger (18-38 years; 52% female) and 66 older (51-81 years; 53% female) healthy adults were determined using stable isotope tracer techniques (L-[ring-² H₅ ]phenylalanine and L-[ring-² H₂ ]tyrosine). There were no group × sex interaction effects (p > .05). Older individuals had greater whole-body protein synthesis (mean difference old-young (Δ) ± SE: 28.54 ± 8.15 mg/kg LBM/hr; p = .001) and breakdown (Δ: 15.44 ± 7.33 mg/kgLBM/hr; p = .038), but a less negative net balance (Mean ± SD: Young: -31.22 ± 7.42 mg/kg LBM/hr; Old: -18.11 ± 21.60 mg/kg LBM/hr; p < .001) compared to young individuals. Basal FSR was not significantly different between young and older (Δ: 0.007 ± 0.003%/hr; p = .052). Across the age range, females had greater whole-body protein turnover (PSΔ: 19.10 ± 7.00 mg/kgLBM/hr; PBΔ: 19.22 ± 6.31 mg/kgLBM/hr; p < .01) compared to males. Results demonstrate a difference in basal whole-body protein kinetics between young and older adults, with older adults having a higher protein turnover rate and a less negative net balance. Across the age range, females were also found to have a higher turnover rate compared to males. Differences may represent a shift in older physiology toward mechanisms that increase the efficiency of amino acid reutilization, especially in women.

Daily Protein and Energy Intake Are Not Associated with Muscle Mass and Physical Function in Healthy Older Individuals-A Cross-Sectional Study

Dietary protein has a pivotal role in muscle mass maintenance with advancing age. However, an optimal dose and distribution of protein intake across the day as well as the interaction with energy intake for the maintenance of muscle mass and physical function in healthy older adults remain to be fully elucidated. The purpose of this study was to examine the association between muscle mass, strength, and physical function, and the total amount and distribution of protein and energy intake across the day in healthy older individuals. The research question was addressed in a cross-sectional study including 184 Danish men and woman (age: 70.2 ± 3.9 years, body mass: 74.9 ± 12.1 kg, Body Mass Index (BMI): 25.4 ± 3.7 kg/m²) where a 3-day dietary registration, muscle mass, strength, and functional measurements were collected. We found that neither daily total protein intake nor distribution throughout the day were associated with muscle mass, strength, or physical function. Consequently, we do not provide an incentive for healthy older Danish individuals who already adhere to the current internationally accepted recommended dietary protein intake (0.83 g/kg/day) to change dietary protein intake or its distribution pattern throughout the day.