Glutamine: The Conditionally Essential Amino Acid
Glutamine is one of the 20 amino acids our body uses to build proteins. These proteins are the building blocks for most components of our bodies, including muscles, bones, hair, hormones and more. Glutamine plays a vital role in the proper functioning of many body systems, including the immune and digestive systems. Due to its importance in the body, the use of glutamine supplementation is the focus of intense research efforts.
Of the 20 amino acids in our bodies, nine are considered "essential" and the other 11 are termed "non-essential." The essential amino acids need to be obtained from our diet, as opposed to the non-essential ones, which our body can manufacture on its own. When we eat, the proteins we ingest are broken down by our digestive system into their individual amino acids. By linking amino acids back together in various combinations, our body synthesizes the proteins it needs.
Since we are capable of making glutamine on our own, it was originally labeled a non-essential amino acid. However, most scientists now consider glutamine to be a "conditionally" essential amino acid, because under certain conditions we are unable to make adequate amounts and thus need to obtain it from outside sources. Studies have shown that our body's concentration of glutamine is markedly decreased during times of severe bodily stress, such as during major surgery, burns, starvation, serious infections and even prolonged exercise.1,2
Glutamine is the most abundant amino acid in our bodies, comprising approximately half of the free amino acids in our muscles and blood.3 The majority of our glutamine is manufactured and stored in skeletal muscle. While our bodies synthesize most of the glutamine that we need, we also obtain some from the foods we eat. Practically all proteins we consume contain some amount of glutamine, usually in the order of 4% to 8% of their total amino acid composition. Given the average adult's daily protein intake, we probably obtain less than 10 grams of glutamine from our diet each day.4
Glutamine plays many roles in our body. Research has shown glutamine to be integral in the proper function of our digestive system, our immune system (our ability to fight infection) and our muscular system, to name a few. It acts as a type of fuel for cells, especially for rapidly dividing cells such as enterocytes, colonocytes, lymphocytes and fibroblasts.5 During the manufacture of glutamine, a nitrogen molecule is taken from free ammonia in the body, thus it plays a role in protecting our bodies from high levels of ammonia and maintaining proper acid-base balance.4,6 When needed, our body can convert glutamine to sugar for energy. Glutamine is also involved in the manufacture of other amino acids, including glutathione, an important intracellular antioxidant.4
Can glutamine supplementation help us in our goal of living a longer, healthier life? To explore this, we will examine glutamine's role in its two most active body systems: the digestive and immune systems. We will also discuss the relationship between exercise and glutamine. While a comprehensive review of the research was performed in the preparation of this article, it is important to bear in mind that the references used do not necessarily represent an exhaustive list of published research on glutamine and its use as a supplement.
The digestive system
Numerous studies have shown glutamine to be a key component in the maintenance of healthy intestinal mucosa. In fact, glutamine was once termed "intestinal permeability factor."7 The small intestine is by far the greatest user of glutamine in the body. Enterocytes (epithelial cells lining the small intestine) use glutamine as their primary fuel for metabolic function.4 It is felt that a lack of glutamine leads to a loss of epithelial cell integrity in the lining of the intestines. This, in turn, may allow toxins and infectious agents to enter the body.8 Most research studies concerning glutamine and the gastrointestinal system involve the addition of glutamine to TPN (total parenteral nutrition), a nutritional supplement given to the critically ill. One study showed that the addition of glutamine to TPN solutions reversed intestinal mucosal atrophy.9 Although glutamine has been shown to be beneficial when added to TPN solution in critically ill patients, its role in other gastrointestinal disease is still controversial and more studies are needed to determine its potential benefits and drawbacks.
The immune system
Glutamine plays a major role in our infection-fighting "immune" system. Many immune cells, such as lymphocytes and macrophages, use glutamine as an energy source almost as much, if not more than, the amount of glucose they use for energy.10,11 Some studies have shown a benefit of glutamine in chemotherapy and bone marrow transplant patients, whom usually have a weakened immune system.12 Yet other research has shown that glutamine can help reduce infections in critically ill patients.13 In addition, glutamine may play a role in enhancing the weakened immune systems of athletes after prolonged strenuous exercise, which is discussed in further detail in the next section.
Glutamine regulation may be especially important in athletes, both to help ward off infections and to prevent the breakdown of muscle. Studies have shown that while glutamine levels rise after short-term exercise, they decrease after prolonged periods of exercise.14 It has been theorized that after exhaustive exercise, such as running a marathon, athletes are at an increased risk of acquiring an infection. This is possibly due to decreased glutamine levels.15 On the other hand, it should be noted that regular, moderate exercise has been shown to reduce the incidence of illness in sedentary individuals (yet another reason regular exercise can improve your health and longevity).14
In one study, marathon runners who consumed oral glutamine supplements immediately after and then two hours after a marathon reduced their incidence of developing an upper respiratory tract infection by nearly one-third.16 Although the findings of this and additional studies are encouraging, other studies have failed to show a direct link between glutamine and specific markers of immune function after strenuous exercise.17,18 So, if glutamine does truly have a benefit in preventing infections after physical activity, the exact mechanism is unclear and further research is warranted.
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