What is Glutamine?

Glutamine is the most abundant free amino acid in the human body. It constitutes 50-60% of the total free amino acid pool in skeletal muscle and about 20% of the plasma amino acid pool1.

It is essential for many important homeostatic functions (which include body fluid and pH balance, and regulation of body temperature and heart rate) and the optimum functioning of a number of body tissues, particularly the immune system and the gut.

Glutamine is involved in nitrogen transport from organ to organ and directly influences the balance of protein synthesis (an anabolic process) and protein degradation (a catabolic process).

Glutamine is found naturally occurring in almost all animal and plant protein sources. As a dietary supplement, Glutamine is available as free form L-Glutamine: a white, free flowing fine powder which has no odour and a slightly characteristic taste.

Glutamine: A Conditionally Essential Amino Acid

Glutamine is a non-essential amino acid under normal conditions, meaning the body is able to produce enough to meet its own requirements. Glutamine becomes essential during certain physiological states, which is why it is termed a 'conditionally essential' amino acid2.

Physical conditions such as critical illness, stress conditions including heavy resistance exercise, endurance exercise, post-operative burns and trauma and overtraining syndrome result in an increased requirement for glutamine.

Under these conditions, skeletal muscle and the liver are no longer able to maintain normal plasma glutamine concentrations because of strongly increased requirements for glutamine by the gastrointestinal (GI) tract, immune system and inflammatory cells. The extra glutamine essential for healthy cellular function must therefore be provided in the diet.

Glutamine occurs naturally in a wide variety of foods.  Supplementation with glutamine has been shown to result in improved nitrogen balance and preservation of skeletal muscle3.

Glutamine Metabolism

Glutamine is synthesised in the body from glutamate and glutamic acid by the action of glutamine synthetase3. The primary site for glutamine synthesis is in skeletal muscle. The lungs, liver and brain are the secondary sites 4.

Glutamine's key structural feature is that it contains two nitrogen atoms. This enables it to function as a nitrogen shuttle, carrying nitrogen between different organs and tissues.

Glutamine is utilised primarily as a fuel source by tissues such as the small intestine, immune system and hair follicles (Figure 1). The GI tract alone accounts for 40% of the total glutamine utilised by the body3. It is also utilised by a smaller extent in the kidneys and in the liver where it plays a vital role, assisting in numerous detoxification processes.

Glutamine is able to cross the blood-brain barrier, where it is used as an energy source and a precursor for neurotransmitter substances.

Why is Glutamine Important in Sports Nutrition?

Because athletes subject their bodies to high levels of physical (and mental) stress, many nutrients, including glutamine, are required at levels above those of a non-training person. If a sufficient quantity and quality of a nutrient is not obtained in the diet, or is unable to be manufactured by the body at the required rate, depletion occurs. This can contribute to sub-optimal performance and slow recovery times.Supplementation to insure dietary intake is adequate becomes an important consideration.

Glutamine supplementation can be beneficial for athletes for the following reasons:

Support of Immune System

Glutamine increases the numbers of circulating lymphocytes and macrophages5 , which can be useful in decreasing the incidence of infections or illness resultant from over-training6. Glutamine helps to protect the body from cellular stress and the build up of waste substances that accompany bouts of heavy resistance and endurance training.

Restoration of Protein in Skeletal Muscle

Glutamine restores protein in skeletal muscle by both anabolic and anti-catabolic mechanisms. The anabolic effect is an increase in protein synthesis7 and the promotion of muscle cell volumisation8. Additional glutamine affects nitrogen balance, ensuring it remains positive and thus preventing a decline in muscle protein synthesis10.

Cortisol is an adrenal stress hormone that triggers catabolic (tissue breakdown) activity in muscle, whereas the hormone testosterone stimulates anabolic (tissue building) activity. Increased levels of cortisol and decreased levels of testosterone are suggested to indicate a disturbance in the anabolic-catabolic balance, which can lead to decreased performance9. Glutamine exerts an anti-catabolic effect in muscle tissue by counteracting the proteolytic effect of cortisol.

Fuel for Cells in Different Organs of the Body

The GI tract is the primary site of glutamine utilisation. Other organs that use glutamine include the liver, kidneys, brain and immune cells. The provision of supplemental glutamine could assist in providing the requirements of these organs, thus sparing muscle protein3.

Glucose Regulation

Glutamine has an integral role in glucose regulation. After an acute or high intensity training/activity session, muscle and liver glycogen stores become depleted. Glutamine acts as both a substrate for glucose formation and a regulator of this process, increasing glucose production and muscle glycogen storage12.

Safety of Orally Administered L-Glutamine

Glutamine is absorbed efficiently by the human gastrointestinal tract. Acute oral ingestion of glutamine at doses up to 0.3 grams per kg of body weight showed no evidence of clinical toxicity (i.e. approximately 21 grams for 70kg person)14.

Oral supplementation is an effective13 and safe14 means of providing additional free glutamine to the body.

Glutamine: When and How Much

It is known that muscle glutamine levels decline in a dose-dependent manner to the degree of stress (i.e. greater the stress, the greater the loss), thus the need for glutamine is variable for different athletes.

The daily requirement of glutamine for an athlete is between 8 to 20 grams, depending upon the dietary intake of glutamine containing foods, health and frequency and intensity of exercise. Glutamine supplementation ranging from 2 to 6 grams, two to four times throughout the day is suggested15.

There is no preferred time for glutamine administration. It is suggested one dose early in the morning and one either prior to or after a workout or training session will assist in maintaining plasma levels of glutamine and may confer anti-catabolic protection.

References:

  1. Glutamine Peptide: A New Perspective in Sports Nutrition. DMV International Business Unit Nutritionals Communication Pg. 3, June 1996.
  2. Lacey, J.M., and Wilmore, D.W., 1990. Is Glutamine a conditionally essential amino acid?. Nutr. Rev. 48: 297-309
  3. Antonio, J. and Street, C.,1999. Glutamine : A Potentially Useful Supplement for Athletes. Can. J. Appl. Physiol. 24: 1-14
  4. Rowbottom, D.G. et al., 1996. The Emerging Role of Glutamine as an Indicator of Exercise Stress and Overtraining. Sports Med. 21 (2), 80-97.
  5. Newsholme, E.A. et al, 1999. The Proposed Role of Glutamine in Some Cells of the Immune System and Speculative Consequences for the Whole Animal. Nutrition 13 (7-8), 728-730.
  6. Castell, L.M. et al., 1996. Does Glutamine have a Role in Reducing Infections in Athletes? Eur. J. Appl. Physiol. 73(5), 488-490.
  7. Hankard, R.G. et al., 1996. Effect of Glutamine on Leucine Metabolism in Humans. Am. J. Physiol. 271: E748-E754.
  8. Haussinger, et al., 1994. Regulation of Cell Function by the Cellular Hydration State : Am. J. Physiol. 267: E343-E355.
  9. Hoogeveen, A.R. and Zonderland, M.L., 1996. Relationships between Testosterone, Cortisol, and Performance in Professional Cyclists. Int. J. Sports Med., 17: 423-428.
  10. Hammarqvist, F. et al., 1990. Alanyl-glutamine Counteracts the Depletion of Free Glutamine and the Postoperative Decline in Protein Synthesis in Skeletal Muscle : Ann. Surg. 212: 637-644.
  11. Perriella, G. et al., 1997. Regulation of Gluconeogenesis by Glutamine in Normal Post-absorptive Humans . Am. J. Physiol. 272: E437-E445.
  12. Varnier, M. et al., 1995. Stimulatory Effect of Glutamine on Glycogen Accumulation in Human Skeletal Muscle. Am. J. Physiol. 269: E309-E315.
  13. Dechelotte, P. et. al., 1991. Absorption and Metabolic Effects of Enterally Administered Glutamine in Humans . Am. J. Physiol. 260: G677-682.
  14. Ziegler, T.R. et al., 1990. Safety and Metabolic Effects of L-Glutamine Administration in Humans .J. Parenter. Enteral Nutr. 14: 137S-146S.
  15. Burke, E.R., 1999. Optimal Muscle Recovery. Avery Publishing Group, New York, pp 80-84.