Glutamate Supplementation in Young Men
Condition: Healthy
Study type: Interventional

Summary: 
      Amino acids play an important role in human metabolism. In aging individuals and in some
      diseases, certain amino acids, such as glutamate, are at lower than normal levels. Glutamate
      appears to be involved in providing energy and maintaining normal blood sugar (glucose)
      levels, processes which both depend heavily on skeletal muscle. The maintenance of healthy
      blood sugar levels, in particular, is tightly related to overall muscle mass and quality. To
      better understand the link between glutamate and glucose metabolism in skeletal muscle, the
      investigators will be using a nutritional approach to raise the body's glutamate levels with
      monosodium glutamate (MSG) supplementation, and raise blood glucose levels with a sugary
      drink. By altering the normal levels of glutamate and glucose in the blood and muscle tissue,
      the investigators can gather more information about the role of glutamate in energy
      metabolism. This will help the design of future studies investigating the function of
      glutamate in aging and disease. During this study the investigators will increase the levels
      of glutamate and glucose in the bloodstream by asking participants to ingest MSG along with a
      sugary drink. The goal is to subsequently determine: A) the amount of glutamate and glucose
      that ends up in the muscle; and B) whether normal skeletal muscle glucose metabolism, and the
      behaviour of additional amino acids (other than glutamate) is altered. The hypothesis is that
      when MSG and sugar drink are ingested together glucose uptake and metabolism within the
      skeletal muscle will be elevated.
    

Detailed description: 
      Glutamate is involved in several aspects of glucose metabolism in both muscle and liver,
      however its role has not been fully defined. It is the primary amino acid taken up by resting
      and exercising skeletal muscle, where it interacts with pyruvate (derived from glycolysis) to
      produce the TCA cycle intermediate 2-oxoglutarate and the gluconeogenic precursor alanine.
      Glutamate is also required for the production of glutamine, another gluconeogenic precursor,
      via intramuscular reactions with ammonia. Given that skeletal muscle is responsible for 85%
      of whole body glucose disposal, investigating the interplay between glutamate, its related
      amino acids, and glucose homeostasis is of particular relevance. However, despite its many
      links to energy provision, the role of glutamate in skeletal muscle glucose metabolism
      remains poorly understood.

      Few studies have manipulated circulating concentrations of both glutamate and glucose in
      humans, and none have evaluated the effect of this unique manipulation in skeletal muscle
      tissue. In healthy young adults, a dose of roughly 10 g monosodium glutamate (MSG)
      transiently elevates plasma (700-800%) and intramuscular (30%) glutamate concentrations.
      Acute MSG supplementation also stimulates modest increases in plasma concentrations of
      aspartate, alanine, and glutamine (which are produced from glutamate within the muscle and
      subsequently released into circulation). Intriguingly, insulin is secreted in response to MSG
      supplementation, an effect that appears to be mediated by glutamate binding to an excitatory
      amino acid receptor on the pancreas. In addition to its action on the pancreas, there is some
      evidence to suggest that glutamate may function as a secondary messenger by enhancing
      glucose-stimulated insulin secretion during periods of increased carbohydrate (CHO)
      availability. However this secondary effect of glutamate on insulin is poorly understood.

      The ability of glutamate to independently stimulate insulin secretion provides further
      support for an association between glutamate and glucose metabolism, yet only two studies to
      date have administered MSG during an oral glucose challenge to directly examine this
      relationship. One study observed no effect of MSG on glucose tolerance, however peak plasma
      glutamate was dramatically blunted in this study compared to previous reports (~ 80 vs.
      400-500 µM), likely due to glutamate being retained in the gut as a result of co-ingestion
      with CHO. In a separate study, the authors developed a methodological approach to circumvent
      this issue: by staggering the administration of MSG and CHO by 30 min, plasma glutamate and
      glucose were both significantly and simultaneously elevated. Furthermore, MSG administration
      improved glucose tolerance. In support of these findings, improvements in glucose clearance
      following a high-fat meal combined with MSG has also been reported. Interestingly, not all
      studies have observed enhanced insulin secretion with higher glutamate availability. This
      suggests that the ability of carbohydrate to stimulate insulin secretion may overpower any
      effect of glutamate on this hormone, but the specific mechanisms remain to be fully
      elucidated.

      Recently, investigators have developed a cell culture model to show that glutamate stimulates
      glucose uptake in rat L6 myotubes in the absence of insulin. This suggests that glutamate is
      capable of acting directly on skeletal muscle, and supports previous findings of improved
      glucose tolerance with acute MSG supplementation despite no further increase in insulin
      secretion. Additionally, cell data demonstrates that glutamate-stimulated glucose uptake
      results from increased glucose transporter 4 (GLUT4) translocation to the sarcolemma, via the
      activation of AMP-activated protein kinase (AMPK) and p38 mitogen-activated protein kinase
      (MAPK). It is possible that these mechanisms underpin glutamate-mediated improvements in
      glucose tolerance in humans. However, this - as well as the fate of glucose upon being taken
      up by the muscle cell - has yet to be investigated.

      There is a high degree of interplay between glutamate and glucose metabolism, but it remains
      unclear whether elevated plasma concentrations of glutamate and glucose influence one
      another's uptake into human skeletal muscle, as well as their subsequent respective
      intramuscular metabolic reactions. Therefore, the overarching goal of this study is to
      uncover the effects of acute MSG+CHO supplementation on plasma and intramuscular amino acid
      concentrations, as well as aspects of skeletal muscle glucose metabolism, in healthy young
      men in comparison to the ingestion of MSG and CHO alone.

      The investigators will stagger the administration of MSG and CHO by 30 min to achieve
      simultaneous peak circulating concentrations of glutamate and glucose. Under these
      conditions, the aim is to:

      Primary:

        1. Quantify and compare changes in circulating glutamate and intramuscular glutamate
           following MSG+CHO versus MSG alone.

        2. Evaluate the acute effects of MSG+CHO on the intramuscular free amino acid pool
           (specifically aspartate, alanine, and glutamine) compared with MSG adminstration alone.

      Secondary:

        1. Confirm the findings of our previous study (7) which demonstrated that MSG+CHO
           attenuates the rise in plasma glucose (but does not affect insulin or C-peptide
           concentrations) compared with CHO alone.

        2. Determine if MSG+CHO is associated with a greater or lesser degree of signaling through
           the AMPK/p38 MAPK and/or insulin pathway in skeletal muscle compared to MSG and CHO
           alone.

        3. Explore whether aspects of skeletal muscle glucose uptake, storage, and/or utilization
           are altered during MSG+CHO, relative to CHO alone.

      The investigators hypothesize that when glutamate and glucose are simultaneously available in
      peak concentrations in circulation they will observe:

      Primary:

        1. Lower circulating and intramuscular glutamate concentrations following MSG+CHO versus
           MSG alone.

        2. Similar intramuscular concentrations of aspartate, alanine, and glutamine in response to
           MSG+CHO and MSG alone.

      Secondary:

        1. A blunted increase in plasma glucose (but no difference in insulin or C-peptide
           concentrations) following MSG+CHO compared with CHO alone.

        2. Elevated expression and activation of proteins related to both AMPK/p38 MAPK and insulin
           signaling following MSG+CHO. However, AMPK/p38 MAPK signaling will be greatest following
           MSG alone, with negligible activation of the insulin signaling pathway. Conversely,
           insulin signaling will be greatest following CHO alone, with negligible signaling
           through AMPK/p38 MAPK.

      Enhanced skeletal muscle glucose uptake (i.e. increased GLUT4 translocation to the plasma
      membrane) and utilization (i.e. increased rates of glycolysis and flux through the TCA
      cycle), compared to the administration of CHO alone.

      Eligible young men who have consented to participate will visit the laboratory on 4 occasions
      each separated by a washout period of approximately 1 week. Following a series of baseline
      assessments to evaluate body composition, fitness etc.(Visit 1), participants will complete 3
      trials (Visits 2, 3 and 4) in a randomized order. For each of the 3 trials, participants will
      arrive by car or via public transportation at the laboratory after an 8-12 hour overnight
      fast (no food or drink except for water after midnight the night before). A catheter will be
      inserted into an antecubital vein and a fasting blood sample will be drawn (~8 mL).
      Participants will then consume either 150 mg/kg body mass MSG or a placebo within 5 min.
      Blood (~8 mL) will be drawn at the following time points: 10, 20, 30, 40, 50, 60, 75, 90,
      105, and 120 min. Immediately following the 30 min blood draw, participants will consume a 75
      g carbohydrate (dextrose) beverage or a second placebo. After the last blood sample is
      obtained the catheter will be removed. The total amount of blood drawn during each trial will
      be ~88 mL.
    

Intervention name(s): MSG; CHO; Placebo A; Placebo B
Intervention type(s): Dietary Supplement; Dietary Supplement; Dietary Supplement; Dietary Supplement
Intervention description(s): 150 mg/kg body mass monosodium glutamate (MSG).; 75g CHO (i.e. dextrose); A non-caloric placebo composed of table salt (NaCl) and sucralose (Splenda).; A non-caloric placebo composed of flavoured water.
Allocation: Randomized
Intervention model: Crossover Assignment
Masking: Double (Participant, Investigator)
Primary purpose: Basic Science
Keywords: Monosodium glutamate; Amino acids; Glucose tolerance