L-Glutamine

Most abundant free amino acid in plasma and skeletal muscle; conditionally essential during catabolic stress. ICU/burn/sepsis evidence is robust and life-saving; healthy-adult athletic recovery and cognitive evidence is weak. For Dylan: OPTIONAL-ADD at 5g/day as gut + immune insurance during high MMA training load — useful adjunct, not a cognitive primary, not a transformative supplement.

Glutamine is a non-essential alpha-amino acid that becomes conditionally essential under physiological stress (trauma, sepsis, burns, severe overtraining, prolonged catabolism). Endogenous synthesis from glutamate + ammonia via glutamine synthetase (GS) is normally adequate, but demand outstrips supply in catabolic states. It is the most abundant free amino acid in human plasma (~500-750 µmol/L) and skeletal muscle (~20 mmol/kg wet weight, accounting for 50-60% of intramuscular free amino acid pool).

Distinct mechanisms relevant to supplementation:

1. Primary fuel for rapidly dividing cells. Enterocytes (gut lining), lymphocytes, macrophages, and neutrophils preferentially oxidize glutamine over glucose. The gut alone consumes ~30% of dietary glutamine first-pass; almost nothing reaches the systemic circulation from oral supplementation in healthy adults — a significant pharmacokinetic limitation. Splanchnic extraction is the dominant fate.

2. Glutamate / GABA precursor. Glutamine crosses the blood-brain barrier via SLC1A5 (ASCT2) and SLC7A11, and is converted intracellularly to glutamate by glutaminase (GLS / GLS2). Glutamate is the primary excitatory neurotransmitter and the precursor to GABA via glutamic acid decarboxylase (GAD). The glutamine-glutamate-GABA cycle is the dominant CNS nitrogen flux. Despite the mechanistic story, oral supplementation produces minimal CNS glutamate elevation in healthy humans because of first-pass splanchnic extraction.

3. Ammonia scavenger. Glutamine synthetase combines glutamate + NH3 + ATP to form glutamine, providing a non-toxic nitrogen carrier between tissues. Skeletal muscle and brain export ammonia as glutamine; liver and kidney use it for urea production or renal acid-base balance. After exhaustive exercise, plasma ammonia rises 5-10x; glutamine supplementation modestly accelerates clearance. This is the main mechanistic basis for the "post-exercise cognitive fog" claim.

4. Heat shock protein (HSP70) induction. Glutamine increases HSP70 expression in stressed cells, contributing to its protective effect in ICU/sepsis populations. HSP70 stabilizes proteins, prevents apoptosis, and reduces inflammatory signaling. This mechanism likely underlies the mortality benefit in critical illness.

5. Glutathione (GSH) substrate. Glutamine donates carbon to glutamate, which combines with cysteine + glycine to form GSH. In glutamine-restricted cells, GSH synthesis falls and oxidative stress rises. Practically, NAC (cysteine donor) is the rate-limiting input; glutamine is rarely limiting for GSH in healthy adults.

6. Anaplerotic feed to the TCA cycle. Glutamine-derived alpha-ketoglutarate replenishes TCA intermediates. In catabolic states or rapidly proliferating cells (cancer, immune cells), this is metabolically critical. Cancer cells are notably glutamine-addicted — relevant for risk discussion below.

7. Nitrogen shuttle. Glutamine is the dominant inter-organ nitrogen carrier in humans. Muscle releases glutamine; gut, kidney, and liver consume it. This is why plasma glutamine drops in prolonged catabolism — muscle protein breakdown is the source.