L-Carnitine: Mechanisms, Performance Benefits, and Optimal Use
By Jim Stoppani, Ph.D. | August 27, 2021 | 6 min read
When used appropriately, L-carnitine can enhance both athletic performance and body composition. During exercise, it may increase endurance and promote more pronounced muscle pumps. Aesthetically, it supports greater muscle retention and reduced fat mass. Importantly, contrary to some misinformation, L-carnitine is safe when used as directed.
Avoid misconceptions about L-carnitine. Understanding its mechanisms and proper usage can make it a valuable addition to your supplementation regimen.
What Is L-Carnitine?
Though frequently classified as an amino acid, L-carnitine is not a true amino acid. It is a vitamin-like, amino-acid-like compound associated with the B-vitamin family. Initially investigated in the 1950s, L-carnitine was originally termed "vitamin BT."
L-carnitine is synthesized in the liver and kidneys from the amino acids lysine and methionine. It is stored predominantly in muscle tissue (including cardiac muscle), the brain, and sperm. Dietary sources are primarily animal-based, with meat being the richest source—red meat in particular. Plant sources such as avocados and soybeans provide minimal amounts.
Carnitine occurs in two stereoisomeric forms:
L-carnitine: The only naturally occurring, biologically active form. Supplement labels list it as L-carnitine, L-carnitine L-tartrate, or propionyl-L-carnitine—all structurally similar and equally effective.
D-carnitine: Biologically inert and not marketed as a supplement.
Acetyl L-Carnitine (ALCAR)
Acetyl L-carnitine (ALCAR) is another widely used supplemental form. It is present throughout the central nervous system, where it contributes to energy production and the synthesis of the neurotransmitter acetylcholine. The acetyl group on ALCAR enhances its ability to cross the blood-brain barrier, allowing it to act as a potent antioxidant in the brain. Emerging research suggests ALCAR may exert protective effects against age-related processes and neurodegeneration.
What Does L-Carnitine Do?
L-carnitine’s primary role is fatty acid transport: It shuttles long-chain fatty acids into the mitochondrial matrix—the cell’s "power plant." Once inside, fatty acids undergo β-oxidation to produce adenosine triphosphate (ATP), the cell’s primary energy currency. This process occurs during both rest and exercise, but research highlights enhanced efficacy during high-intensity physical activity.
Conditional Essentiality
Insufficient carnitine impairs the entry of most dietary fats into the mitochondria, preventing their utilization as fuel. Carnitine deficiency is a severe medical condition characterized by muscle weakness, stunted growth, hepatomegaly, and other systemic issues. This underpins its classification as a conditionally essential nutrient—while the body synthesizes it, inadequate production can have profound health consequences.
Insulin Sensitivity and Glycogen Synthesis
A Scottish study found that beyond its role in fat transport, L-carnitine potentiates insulin signaling in muscle cells.[1] This translates to:
Improved postprandial blood glucose regulation (even after high-carbohydrate meals).
Enhanced glycogen synthesis (critical for recovery).
This is why I recommend pairing L-carnitine with a post-workout meal.
Performance and Physique Applications of L-Carnitine
L-carnitine is a well-documented aid for both fat metabolism and athletic performance. Below are its most evidence-backed benefits:
1. Enhanced Fat Oxidation During Exercise
A 24-week University of Nottingham study assigned athletes to either:
2 grams of L-carnitine + 80 grams of high-glycemic carbohydrate (twice daily), or
Carbohydrate alone.[2]
Key findings:
Low-intensity cycling: The L-carnitine group used 55% less muscle glycogen and increased fat oxidation by 55%.
High-intensity cycling: Lower blood lactate levels (reduced fatigue) and higher creatine phosphate (ATP precursor) concentrations.
2. Improved Fatigue Resistance
The same Nottingham study measured time to exhaustion during cycling: The L-carnitine group extended their performance by 25%+. This is attributed to:
Preserved muscle glycogen (from increased fat burning).
Reduced lactate accumulation.
Elevated creatine phosphate levels.
3. Reduced Muscle Soreness and Accelerated Recovery
Multiple human studies demonstrate that 1–2 grams/day of L-carnitine reduces exercise-induced muscle damage and enhances recovery.[3-6] It also attenuates delayed-onset muscle soreness (DOMS). Critically, these findings are from trials involving healthy, active men and women—not animal models.
4. Better Blood Flow and Muscle Pumps
Performance benefits also derive from L-carnitine’s ability to enhance muscle perfusion.[7] Increased blood flow delivers more nutrients (e.g., glucose, amino acids) and hormones (e.g., testosterone) to working muscles.
Mechanistically:
L-carnitine reduces oxidative damage to nitric oxide (NO), a molecule that dilates blood vessels.
It upregulates enzymes involved in NO synthesis—boosting circulating NO levels.
This supports both exercise performance and post-workout recovery.
Safety and Side Effects
L-carnitine is well-tolerated at reasonable doses. High doses (typically >5 grams/day) may cause gastrointestinal distress (nausea, cramps, vomiting, diarrhea).
Rare adverse effects include:
Muscle weakness in uremic patients.
Seizures in individuals with preexisting seizure disorders.
How to Stack L-Carnitine
The key to maximizing L-carnitine’s benefits is to pair it with carbohydrates. Early studies showing no benefit failed to raise muscle carnitine levels—likely because supplements were not taken when insulin levels were elevated (to drive muscle uptake).
Modern research confirms that high insulin levels are required for sufficient L-carnitine entry into muscle cells (where it exerts most effects).[8] Conversely, L-carnitine enhances insulin sensitivity, promoting greater glucose and L-carnitine uptake.
General Stacking Guidelines
Dose: 2–3 grams per serving (1 gram can be effective, but higher doses maximize benefits).
Carbohydrates: Pair with 30–40 grams of high-glycemic carbs (e.g., dextrose, white rice).
Protein: Add 20–40 grams of protein (to support muscle protein synthesis).
Forms: This applies to L-carnitine, L-carnitine L-tartrate, and propionyl-L-carnitine.
Acetyl L-Carnitine (ALCAR)
ALCAR is unique: It is readily absorbed by the intestines, muscle, and brain without food. For between-meal stacking with fat-burning ingredients (e.g., caffeine, green tea), use ALCAR.
Should You Cycle L-Carnitine?
Current evidence does not support the need to cycle L-carnitine. Chronic use maintains efficacy over time.
Timing Recommendations
Post-workout is an ideal time to take L-carnitine, but it can be paired with any high-carbohydrate, high-protein meal. For between-meal stacking with fat-burning ingredients, use acetyl L-carnitine.
A strategy I’ve found effective for fat loss and performance:
Take 2–3 doses of ALCAR (with fat-burning agents) between meals.
Take 1 dose of L-carnitine/L-carnitine L-tartrate with a post-workout meal.
References
Galloway, S. D., Craig, T. P., & Cleland, S. J. (2011). Effects of oral L-carnitine supplementation on insulin sensitivity indices in response to glucose feeding in lean and overweight/obese males. Amino Acids, 41(2), 507-515.
Wall, B. T., Stephens, F. B., Constantin‐Teodosiu, D., Marimuthu, K., Macdonald, I. A., & Greenhaff, P. L. (2011). Chronic oral ingestion of l‐carnitine and carbohydrate increases muscle carnitine content and alters muscle fuel metabolism during exercise in humans. The Journal of Physiology, 589(4), 963-973.
Ho, J. Y., Kraemer, W. J., Volek, J. S., Fragala, M. S., Thomas, G. A., Dunn-Lewis, C., ... & Maresh, C. M. (2010). l-Carnitine l-tartrate supplementation favorably affects biochemical markers of recovery from physical exertion in middle-aged men and women. Metabolism, 59(8), 1190-1199.
Volek, J. S., Kraemer, W. J., Rubin, M. R., Gómez, A. L., Ratamess, N. A., & Gaynor, P. (2002). L-Carnitine L-tartrate supplementation favorably affects markers of recovery from exercise stress. American Journal of Physiology-Endocrinology and Metabolism, 282(2), E474-E482.
Kraemer, W. J., Volek, J. S., French, D. N., Rubin, M. R., Sharman, M. J., Gómez, A. L., ... & Hakkinen, K. (2003). The effects of L-carnitine L-tartrate supplementation on hormonal responses to resistance exercise and recovery. The Journal of Strength & Conditioning Research, 17(3), 455-462.
Spiering, B. A., Kraemer, W. J., Vingren, J. L., & Hatfield, D. L. (2007). Responses of criterion variables to different supplemental doses of L-carnitine L-tartrate. Journal of Strength and Conditioning Research, 21(1), 259.
Atalay, G. N., Erikoglu, O. G., Sezen, B. F., & Coskun, C. S. (2015). Effects of acute L-carnitine supplementation on nitric oxide production and oxidative stress after exhaustive exercise in young soccer players. The Journal of Sports Medicine and Physical Fitness, 55(1-2), 9-15.
Stephens, F. B., Constantin-Teodosiu, D., Laithwaite, D., Simpson, E. J., & Greenhaff, P. L. (2006). Insulin stimulates L-carnitine accumulation in human skeletal muscle. The FASEB Journal, 20(2), 377-379.
