GlutamineAmino Acid

• Sickle cell disease, as a prescription drugLikely helps

  This is the only FDA-approved use. Prescription L-glutamine oral powder is approved to reduce acute complications of sickle cell disease [7][12].
  
  The approval covers that specific product at 5–15 g twice daily, not a generic tub of powder repurposed for something else [6]. ^[7]

• Post-infectious IBS with confirmed intestinal hyperpermeabilityPossibly helps

  A randomized placebo-controlled trial enrolled adults with post-infectious, diarrhoea-predominant IBS and measured hyperpermeability, dosing 5 g three times daily for 8 weeks [9].
  
  Symptom severity dropped significantly, stool form improved, and intestinal permeability normalized against placebo [9].
  
  That is one trial in a narrowly selected group. It is not a general-purpose gut remedy on this evidence. ^[9]

• Major burns and critical illnessPossibly helps

  Hospital medicine, not shelf supplementation — and the evidence moved recently.
  
  Several smaller trials pointed to lower mortality and shorter hospital stays with enteral glutamine in burns [3].
  
  The large multicentre RE-ENERGIZE trial then gave 0.5 g/kg/day enterally to adults with major burns. It found no difference from placebo in time to discharge alive, 6-month mortality, or infection rates [8].
  
  Newer and larger contradicting older and pooled: that is the shape of this disagreement, and nobody has resolved it. ^[8]

• Exercise recovery and muscle sorenessPossibly helps

  The literature is genuinely split, and both halves are recent enough to count.
  
  A 2026 narrative review cites trials reporting improvements in intestinal permeability markers, oxidative stress, muscle damage and inflammation — alongside others reporting minimal or no effect [2].
  
  A 2008 academic review found no scientific evidence that oral glutamine aids muscle repair after exercise-induced damage, or reduces soreness against placebo [5]. Influence on athletic performance itself came out limited [2]. ^[2]

• Building muscle massNot shown to help

  Cleveland Clinic states that research does not support glutamine building muscle mass in athletes [7].
  
  The 2008 review landed in the same place on muscle repair after exercise-induced damage [5]. Recovery markers are a separate and messier question, covered above. ^[7]

• Chemotherapy side effectsPossibly helps

  Glutamine has been studied against chemotherapy-induced mucositis, fluorouracil-related diarrhoea, anthracycline cardiotoxicity, and paclitaxel muscle and joint pain [16].
  
  The oncology review that catalogues this describes an area under investigation. No outcome in it is settled. ^[16]

• Body weight and belly fatPossibly helps

  Cleveland Clinic says there is no solid evidence that L-glutamine reduces belly fat [7].
  
  A small pilot randomized trial pointed the opposite way from what people usually worry about, reporting modest weight loss in non-dieting women with obesity [17]. One pilot trial is not something to plan around. ^[17]

• No named deficiency syndrome exists. Glutamine has no vitamin-style symptom checklist — the marker used instead is a drop in plasma glutamine [3].
• Low plasma glutamine at ICU admission, which is associated with increased mortality in critically ill patients [3].
• Depleted plasma glutamine after prolonged, intense exercise. On limited direct evidence, that state has been linked to infection susceptibility and overtraining syndrome in athletes [2].^[3]

• Free L-glutamine, powder or capsulesOrdinary oral use. No published evidence compares powder against capsules for absorption, and neither clinical nor cons…

  Elemental content

  Capsules and tablets at 250, 500 or 1,000 mg, plus loose powder [5]

  Absorption

  Peak plasma concentration arrives roughly 1 hour after an oral dose, per the FDA's pharmacokinetic data [6].
  
  Absorption is saturable, so a larger dose does not lift the peak proportionally [6].

  Best for

  Ordinary oral use. No published evidence compares powder against capsules for absorption, and neither clinical nor consumer dosing guidance distinguishes them [12][6].

• L-alanyl-L-glutamine dipeptide (Ala-Gln)Hospital nutrition, where stability and higher plasma levels are the point .

  Elemental content

  A more stable dipeptide form, used in clinical and parenteral nutrition rather than retail tubs [5]

  Absorption

  It travels on the intestinal PepT1 dipeptide transporter, which is not subject to the saturation limits of free-amino-acid transport [3].
  
  That can deliver a higher peak plasma concentration and area-under-curve than free glutamine [3].

  Best for

  Hospital nutrition, where stability and higher plasma levels are the point [3].

• Common and allergic reactions

  Constipation, headache, nausea or vomiting, bloating, dizziness, heartburn and stomach pain are the usual reported complaints [4][7].
  
  Serious allergic reactions — rash, hives, swelling, trouble breathing — are possible, and are a reason to stop immediately [4]. ^[4]

• Advanced cirrhosis — avoid

  Glutamine is metabolized to glutamate and ammonia, which is precisely the wrong direction in a failing liver [11].
  
  Oral doses of 10–20 g raised serum ammonia and worsened psychometric and EEG measures of hepatic encephalopathy in patients with decompensated cirrhosis [11].
  
  In people with normal liver function, plasma ammonia does not rise on supplementation [11]. ^[11]

• Seizure disorders

  WebMD advises people with seizure disorders to avoid glutamine supplements [4].
  
  The reasoning is mechanistic: the body converts glutamine to glutamate, an excitatory neurotransmitter [4]. A separate oncology review notes that this excitation may leave anti-seizure medication less effective [16]. ^[4]

• Bipolar disorder

  WebMD flags an increased risk of mania or hypomania with glutamine in people with bipolar disorder [4]. ^[4]

• MSG sensitivity

  The body turns glutamine into glutamate, so people who react to monosodium glutamate may react to glutamine as well [4]. ^[4]

• Children

  WebMD rates oral glutamine likely safe in children up to about 0.7 g/kg/day [4].
  
  Glutamine is also FDA-approved inside prescription regimens — with human growth hormone and a specialized diet for short bowel syndrome, and for sickle cell complications from age 5 [12]. Both are doctor-set doses, not over-the-counter gut care. ^[4]

• Pregnancy and breastfeeding

  Reliable safety data on supplemental doses — meaning above what food supplies — do not exist for pregnancy or breastfeeding [4][12].
  
  Both WebMD and Mayo Clinic advise avoiding it, or using it only under a clinician's direction [4][12]. ^[12]

• Kidney stones — the sources point opposite ways

  A controlled study of urinary stone-risk markers found L-glutamine significantly lowered calcium-oxalate relative supersaturation in white subjects, with no effect in Black subjects [15].
  
  Sports-nutrition reviews nonetheless advise caution with glutamine in anyone who has a kidney disorder [5].
  
  That study is small and population-specific, so neither 'safe' nor 'risky' is established here. ^[15]

• Lactulose

  An oncology-focused review identified no known drug-drug interactions for glutamine, with a single exception [16].
  
  Glutamine works physiologically against lactulose, which is given to lower blood ammonia in liver failure [16]. ^[16]

Glutamine rides in on ordinary protein, and the spread across foods is enormous: 0.01 to 9.49 g per 100 g, making up 1–33% of a food's protein [13].

The heavy contributors are animal proteins — beef, pork, poultry, milk, yoghurt, cheese, eggs — with smaller amounts from nuts, tofu, corn, red cabbage, rice and oats [7].

A large food-frequency cohort of women averaged 6.84 g/day (SD 2.19) [13]. For a 70 kg adult eating 0.8–1.6 g/kg of protein, the usual estimate is 3–6 g/day [5].

On cooking: boiling lowered free amino acid content in vegetables, but much of it moved into the cooking liquid rather than being destroyed outright [14]. Oven-roasting raised levels of some free amino acids instead [14].

No clinical guideline names a best time of day for it.

On the empty-stomach question, the FDA's pharmacokinetic data for prescription L-glutamine found no significant food effect on blood concentration — with or without food both work [6]. That contradicts the standard retail advice to take it fasted, and the FDA data is the better source.

Peak plasma levels land about 1 hour after a dose [6].

For exercise timing, the 2026 review concluded the evidence is insufficient to define an optimal strategy, and recommended individualizing instead [2]. Only two trials in it tested post-exercise dosing, with mixed results [2].