L-arginine l-ornithine l-citrulline l-lysine l-glutamine l-proline l-taurine l-carnitine nac

Mechanism of action

Citrulline → increases plasma arginine (partly avoiding arginine’s hepatic first-pass metabolism) → greater substrate availability for nitric oxide synthase → potential increase in NO (vasodilation, perfusion, efficiency). Arginine → direct substrate for NO and for the urea cycle; however, it may be limited by intestinal/hepatic metabolism and by arginase. Ornithine and citrulline → intermediates in the urea cycle → potential support for ammonia detoxification during intense exercise. Taurine → organic osmolyte, modulation of intracellular calcium, membrane stabilization, possible effect on neuromuscular excitability and oxidative stress. Carnitine → transport of long-chain fatty acids into mitochondria (carnitine palmitoyltransferase) and buffering of acetyl-CoA (acetylcarnitine) → potential impact on energy metabolism and recovery; muscle retention is slow and may depend on insulin/carbohydrates. NAC → donor of thiol groups and precursor of glutathione (GSH) → modulation of redox status and potential reduction of exercise-induced oxidative stress; it may influence redox-dependent adaptive signals. Glutamine → fuel for enterocytes and immune cells, nitrogen transport, precursor for nucleotide synthesis; direct performance effects are often inconsistent. Lysine → essential amino acid involved in protein synthesis and collagen (cross-linking), endogenous carnitine production (together with methionine); Proline → key component of collagen and the extracellular matrix, relevant for connective tissues.

Supported benefits

• Improved performance in high-intensity/high-volume exercise (especially with L-citrulline or citrulline malate) and possible reduction in perceived fatigue in some protocols (moderate)
• Increased plasma arginine availability and NO markers with citrulline (more consistent than with oral arginine) (strong)
• Reduction in some markers of oxidative stress and possible support for exercise tolerance in selected contexts with NAC (moderate)
• Possible support for muscle contraction and reduction of DOMS/perceived fatigue with taurine in some studies (limited)
• Carnitine: possible benefits for recovery, muscle damage, and work capacity in some contexts; effects on “fat loss” in healthy, trained subjects are not robust and depend on diet/adherence/duration (limited)
• Glutamine: immune support/supplementation may be useful under conditions of high stress or deficiency; direct ergogenic effects in healthy subjects are often not evident (limited)
• Ornithine/arginine/lysine/proline: direct performance evidence is generally weak; usefulness is more tied to specific clinical contexts or tissue support (emerging)

Safety & side effects

• Arginine/citrulline/ornithine: gastrointestinal disturbances (nausea, cramps, diarrhea), headache; possible drop in blood pressure in sensitive individuals
• Taurine: generally well tolerated; occasional GI disturbances or drowsiness in some individuals
• Carnitine: nausea, cramps, diarrhea; possible “fishy” body odor (trimethylamine) in some cases
• NAC: nausea, reflux, diarrhea; rarely bronchospasm in predisposed individuals; possible headache
• Glutamine: generally well tolerated; at high doses, possible GI disturbances
• Lysine: at high doses, possible GI disturbances
• Proline: limited data on high isolated doses; possible GI disturbances

FAQ

Why is citrulline often preferred over arginine for the “pump”?

Citrulline tends to increase plasma arginine more reliably because it partly bypasses first-pass metabolism and arginase degradation. This can translate into more stable substrate availability for NO synthesis.

Is this stack really aimed at “fat loss”?

Direct evidence for fat loss in healthy, trained subjects is limited. Some components (e.g., carnitine) have a rationale related to lipid metabolism, but the impact on body composition depends above all on energy balance, training, and long-term adherence.

Is NAC always useful for recovery and performance?

NAC can reduce oxidative stress in some contexts and sometimes improve exercise tolerance, but the effects are not universal. In addition, excessive suppression of redox signals close to training could theoretically reduce some adaptations under specific conditions.

Does carnitine work acutely as a pre-workout?

Generally no: many studies suggest that any benefits require chronic use and that increasing muscle carnitine is slow. The acute effect is often minimal or undetectable.

Is glutamine essential for people who train?

In healthy individuals with adequate protein intake, it is often not essential and performance effects are inconsistent. It may be more relevant under conditions of high stress, marked energy deficit, or gastrointestinal issues, always with professional evaluation.