Decreased whole-body and splanchnic glutamate metabolism in healthy elderly men and patients with chronic obstructive pulmonary disease in the postabsorptive state and in response to feeding

Glutamate: A Safe Nutrient, Not Just a Simple Additive

BACKGROUND

In 2017, an European Food Safety Authority (EFSA) opinion on the use of glutamate and its salts as food additives led to an ADI of 30mg/kg body weight/day. Then, in 2021, an EFSA statement presented a proposal for harmonising the establishment of Health-Based Guidance Values for nutrients that are also regulated substances (including food additives). The present review argues that the 2017 glutamate ADI is unsuitable because safety of glutamate should firstly consider its status as a nutrient and not only as an additive.

SUMMARY

Glutamate is a non-essential amino acid playing a key role in nitrogen homeostasis. The dietary exposure to glutamate in adults is extensive, due to its ubiquitous presence in foods, under three forms: bound to proteins, naturally free and free form added as an additive. Glutamate naturally included into proteins is the major source of dietary glutamate. Thus, since it plays a role in nitrogen homeostasis, it is a nutrient before being an additive. Its pharmacokinetics is largely impacted by concomitant food intake but the extent to which plasma glutamate concentration must rise to have deleterious effects is never encountered in humans consuming glutamate in their daily diets. This is due to the fact that glutamate is highly metabolized in the splanchnic area. Key Messages: Glutamate should be considered as a safe nutrient before as an additive by risk assessor.

Comprehensive metabolic flux analysis to explain skeletal muscle weakness in COPD

BACKGROUND & AIMS

Metabolic characterization of a well-defined group of patients could be a powerful tool in revealing metabolic signatures to explain limb muscle weakness in chronic diseases. Studies are currently limited in Chronic Obstructive Pulmonary Disease (COPD) to the identification of differential amino acid concentrations but lack comprehensive analysis of the flux through relevant muscle function related metabolic pathways.

METHODS

In 23 stable patients with moderate to very severe COPD and 19 healthy controls, a comprehensive metabolic flux analysis was conducted by administering an intravenous pulse and primed constant infusion of multiple stable tracers of amino acids known to play a role in muscle health. Blood samples were obtained to calculate production (WBP) and interconversion rates, and plasma concentrations of these amino acids. Lower and upper limb muscle strength, muscle mass, lung function, physical activity level, and disease history and characteristics were assessed.

RESULTS

The COPD group was characterized by lower and upper limb muscle weakness (P < 0.01) despite preserved muscle mass. Higher values were found in COPD for plasma glutamine, WBP of leucine (P < 0.001), 3-methylhistidine (P < 0.01) (marker of enhanced myofibrillar protein breakdown), citrulline (P < 0.05), and arginine to citrulline conversion (P < 0.05) (reflecting enhanced nitric oxide synthesis). Plasma concentration of β-hydroxy β-methylbutyrate (HMB with anticatabolic, anabolic and contractile properties), WBP of glycine (precursor of creatine and glutathione), and transcutaneous O₂ saturation explained up to 79% and 65% of the variation in strength of the lower and upper limb muscles, respectively, in COPD.

CONCLUSIONS

Comprehensive metabolic flux analysis revealed a homogenous metabolic signature in stable patients with COPD and a specific metabolic profile in those with skeletal muscle weakness.

CLINICAL TRIAL REGISTRY

ClinicalTrials.gov; No. NCT01787682; URL: www.clinicaltrials.gov.

Metabolic phenotyping using kinetic measurements in young and older healthy adults

BACKGROUND

The aging process is often associated with the presence of sarcopenia. Although changes in the plasma concentration of several amino acids have been observed in older adults, it remains unclear whether these changes are related to disturbances in whole body production and/or interconversions.

METHODS

We studied 10 healthy young (~22.7y) and 17 older adults (~64.8y) by administering a mixture of stable amino acid tracers in a pulse and in a primed constant infusion. We calculated whole body production (WBP) and metabolite to metabolite interconversions. In addition, we measured body composition, muscle function, and provided questionnaires to assess daily dietary intake, physical activity, mood (anxiety, depression) and markers of cognitive function. Plasma enrichments and metabolite concentrations were measured by GC- and LC-MS/MS and statistics were performed by student t-test.

RESULTS

Older adults had a 11% higher body mass index (p=0.04) and 27% reduced peak leg extension force (p=0.02) than the younger group, but comparable values for muscle mass, mood and cognitive function. Although small differences in several plasma amino acid concentrations were observed, we found older adults had about 40% higher values of WBP for glutamine (221±27 vs. 305±21μmol/kgffm/h, p=0.03) and tau-methylhistidine (0.15±0.01 vs. 0.21±0.02μmol/kgffm/h, p=0.04), 26% lower WBP value for arginine (59±4 vs. 44±4μmol/kgffm/h, p=0.02) and a reduction in WBP (50%; 1.23±0.15 vs. 0.69±0.06μmol/kgffm/h, p=0.001) and concentration (25%; 3.5±0.3μmol/l vs. 2.6±0.2μmol/l, p=0.01) for β-Hydroxy β-Methylbutyrate. No differences were observed in protein catabolism. Clearance of arginine was decreased (27%, p=0.03) and clearance of glutamine (58%, p=0.01), leucine (67%, p=0.001) and KIC (76%, p=0.004) were increased in older adults.

CONCLUSIONS

Specific differences exist between young and older adults in amino acid metabolism.

Digestion and Postprandial Metabolism in the Elderly

The elderly are an increasing segment of the population. Despite the rapid gains in medical knowledge and treatments, older adults are more likely to experience chronic illnesses that decrease quality of life and accelerate mortality. Nutrition is a key modifiable lifestyle factor which greatly impacts chronic disease risk. Yet despite the importance of nutrition, relatively little is known of the impact of advancing age on the gastrointestinal function, the digestive responses, and the post-meal metabolic adaptations that occur in response to ingested food. Knowledge of the age-related differences in digestion and metabolism in the elderly is essential to the development of appropriate nutritional recommendations for the maintenance of optimal health and prevention of disease.

Older Adults Have Delayed Amino Acid Absorption after a High Protein Mixed Breakfast Meal

OBJECTIVES

To measure the postprandial plasma amino acid appearance in younger and older adults following a high protein mixed meal.

DESIGN

Cross-sectional study.

SETTING

Clinical research setting.

PARTICIPANTS

Healthy men and women aged 60-75 (n=15) years, and young controls aged 20-25 years (n=15) matched for body mass index and insulin sensitivity based on the homeostatic model assessment of insulin resistance.

INTERVENTION

High protein mixed meal of complete food products.

MEASUREMENTS

Circulating amino acid concentrations were determined hourly before and for 5 hours after meal ingestion.

RESULTS

There was no difference between cohorts in postprandial appearance of non-essential amino acids, or area under the curve of any individual amino acid or amino acid class. However, older adults had higher baseline concentrations of aspartic acid, glutamic acid, glycine, ornithine, threonine and tyrosine and lower baseline concentrations of hydroxyproline, isoleucine, leucine, methionine and valine compared to younger adults. Younger adults showed peak essential (EAA) and branched-chain amino acid (BCAA) concentrations at 1 hour post meal while older adults' peak EAA and BCAA concentration was at 3 hours. Similarly, peak total amino acid concentrations were at 3 hours in older adults.

CONCLUSION

Older adults digested and absorbed the protein within a mixed meal more slowly than younger adults. Delayed absorption of AA following a mixed meal of complete food products may suppress or delay protein synthesis in senescent muscle.

Metabolic and functional effects of glutamate intake in patients with chronic obstructive pulmonary disease (COPD)

BACKGROUND & AIMS

Patients with chronic obstructive pulmonary disease (COPD) often suffer from skeletal muscle weakness due to muscle wasting and altered muscle metabolism. Decreased muscle glutamate concentration in COPD is consistently reported and is associated with decreased muscle glutathione concentration and early lactic acidosis. We hypothesized that an increased availability of glutamate via glutamate ingestion increases muscle glutamate concentration leading to acute improvements in skeletal muscle energy metabolism and function.

METHODS

Two experiments were conducted. In experiment 1, in two groups of 6 male COPD patients (FEV(1): 44.8+/-3.4%pred) and 6 healthy controls, blood samples and muscle biopsies were taken at 0 and 80 min after repeated glutamate (30 mg/kg BW) or control ingestion (1.25 ml/kg BW), and after 20 min cycling at 50% peak workload. In experiment 2, in 10 COPD patients (FEV(1): 36.1+/-2.5%pred), the effect of the two drinks was tested on cycle endurance time and contractile quadriceps fatigue measured by magnetic stimulation before and after cycling at 75% peak workload.

RESULTS

Glutamate ingestion increased plasma (p<0.01) but not muscle glutamate concentration. Muscle total and reduced glutathione and plasma lactate concentration were not affected by glutamate ingestion. Glutamate ingestion did not influence contractile muscle fatigue and endurance time.

CONCLUSION

Continuous oral glutamate ingestion for 80 min did not lead to an acute effect on skeletal muscle substrate metabolism and muscle performance in COPD patients and in age-matched healthy controls.

Effect of glutamate ingestion on whole-body glutamate turnover in healthy elderly and patients with chronic obstructive pulmonary disease

OBJECTIVE

Decreased whole-body glutamate turnover is found in healthy elderly and in patients with chronic obstructive pulmonary disease (COPD). Glutamate supplementation as an option to increase whole-body glutamate turnover and, hence, glutamate availability has never been investigated. In the present study, we developed a protocol based on repeated glutamate ingestion to increase plasma glutamate concentration to a steady-state level without inducing toxic side effects and to evaluate the effect of repeated glutamate ingestion on whole-body glutamate turnover in patients with COPD and healthy elderly.

METHODS

In part 1, the response of plasma glutamate concentration was determined in young healthy volunteers who repeatedly ingested a glutamate solution. The tolerance of the glutamate drink was evaluated in 26 healthy volunteers by a food tolerance questionnaire. In part 2, eight male patients with COPD and eight healthy elderly ingested the glutamate drink, an isomolar amount of a glutamine drink, or only water to test the effect on plasma glutamate concentration and whole-body glutamate turnover.

RESULTS

In part 1, repeated ingestion of 30 mg of glutamate per kilogram of body weight every 20 min increased plasma glutamate concentration five-fold to steady-state level within 80 min and without any side effects. In part 2, repeated ingestion of glutamate significantly increased whole-body glutamate turnover in healthy controls and patients with COPD, although the increase was smaller in patients with COPD than in controls.

CONCLUSION

We found that repeated ingestion of 30 mg of glutamate per kilogram of body weight every 20 min can increase glutamate availability in healthy elderly and patients with COPD, who are likely more dependent on external glutamate ingestion than are young adults.