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L-carnitine is a trimethylated amino acid that plays essential roles in many areas of the body, including fatty acid translocation and muscle function. Carnitine is also acetylated into the ester Acetyl-L-carnitine (ALCAR) in the brain, liver, and kidney. ALCAR also plays a variety of roles in the body, including increasing acetylcholine production and stimulation of protein and membrane phospholipid synthesis. Orally administered L-carnitine and ALCAR have profound nutrient repartitioning properties, and the effects of supplementing with them have been extensively researched in many areas.
2. What form is best?
Pharmacologically speaking, there is little difference between supplementing with L-carnitine and supplementing with ALCAR. This is because ALCAR is deacetylated during or immediately after intestinal cell uptake, and then a certain amount of free carnitine is later reacetylated (1, 2). Similarly, it has been shown that supplementation with both L-carnitine and ALCAR increase tissue levels of both substances, and that the intestine creates significant amounts of ALCAR from carnitine (2, 3).
3. What application does carnitine have?
The carnitines are excellent supplements for athletes, as well as anyone who wants to look and feel their best. Studies with both ALCAR and L-carnitine have shown that they have potent nutrient repartitioning effects, and there are some studies indicating that L-carnitine can increase exercise performance.
In one study, both young and old rats were supplemented with ALCAR and compared with age-matched controls. Nutrient partitioning and muscle mass and function were among the factors measured. Body weight did not differ significantly between the two groups, but there were strong differences in the fat to protein ratios. The young rats fed ALCAR had slightly higher body protein levels (not statistically significant) and significantly less body fat, while the old rats fed ALCAR had much high body protein levels and the same amount of body fat (1). ALCAR may be especially useful for older people, as one rat study found that long-term administration prevents the progressive increase in the size of fat cells that normally occurs with age (4).
Another study looked at the effects of carnitine (.68 g) and choline (.94 g) supplementation on fat metabolism in healthy women. In the treatment groups, body fat decreased .7-1.3% over 35 days (as opposed to no difference in the placebo group). They also found that when the supplemented groups exercised, the ability to utilize fat as energy substrate increased (5). Animal studies have also shown L-carnitine to reduce obesity (6) and improve nitrogen utilization (7). However, there has been one human study in moderately obese premenopausal women in which no weight loss or repartitioning effect was found with L-carnitine (13).
When looking at the effects of the carnitines on exercise performance, we no longer have to rely so much on animal studies. Various studies have shown that L-carnitine improves exercise performance in people with heart problems or impaired exercise tolerance, as well as people recovering from hemodialysis (8-12). However, the influence on exercise performance on healthy individuals is less clear. L-carnitine definitely changes the response and effects of exercise, as evidenced by decreased muscle soreness and decreased pyruvate and lactate concentrations and higher ALCAR concentrations post-exercise (14, 15), as well as studies showing it to increase utilization of fat as a fuel source during exercise (5, 16). A study with L-carnitine L-tartrate also showed it to favorably effect many markers of recovery from squat exercises (19). The real world effect of this is not conclusively known, and two recent reviews stated that further studies are needed before the effect can be determined on healthy individuals (17, 18). Regardless of the present lack of human studies showing a definite ergogenic effect, the nutrient partitioning properties and improved recovery alone make carnitine a worthwhile supplement, and there are also many added benefits.
4. How does carnitine work?
Carnitine's primary job is in the regulation of cellular metabolism, and it closely interacts with coenzyme A in a variety of reactions. It is required for fatty acid oxidation, and this is the primary theoretical reason for improved exercise performance, as improved fatty acid oxidation will preserve muscle glycogen and improve ATP production (17, 18). It was recently confirmed that supplemental carnitine increases long-chain fatty acid oxidation in healthy individuals without carnitine deficiency (20), providing more evidence for an ergogenic benefit. Enhanced fatty acid oxidation and cellular metabolism is also the proposed mechanism of action for the nutrient partitioning benefits (1).
Additionally, carnitine is a potent antioxidant (especially in combination with ALA) (21) and there is evidence that it increases exercise performance for this reason. A Russian study found that both L-carnitine and ALCAR increased running speed and endurance in trained animals, and that the increase was proportional to their antioxidative activity (22). Another strong antioxidant is melatonin, and ALCAR has also been shown to increase melatonin levels (23).
5. What other benefits does carnitine have?
- Supports cognitive function - ALCAR plays a strong role in the
brain in many ways, and has beneficial effects. Studies in aged rodents show markedly
improved memory and learning capacities (32, 35-37), while studies in younger
rodents show a variety of promising effects as well (40, 41). Other rodent
studies have shown that ALCAR significantly protects the brain against a
variety of stresses, such as ischemia and repurfusion (26) and mitochondrial
uncoupling (27). It also protects against peripheral nerve trauma, "almost
eliminating neuron loss" (28), and in vitro neuronal apoptosis (29, 30).
ALCAR also increases levels of dopamine, amino acids, and acetylcholine in
the brain, as well as facilitating cholinergic activity (31, 32).
ALCAR supplementation is also accompanied by many positive structural changes in the brain in both the young and the old. It stimulates nerve growth factor (NGF) binding (28, 42), and rodent studies indicate significantly more regenerative elements and reduced degenerative elements (40, 43). A study that measured the regenerative capacity of myelinated fibers in young and old rats found that ALCAR significantly increased the density of regenerating myelinated fibers (RMF) and increased the density of axon diameters in both, as well as reducing degenerative elements (40), and another rat study with ALCAR found increased synaptic numeric densities and improvements in energy provision at nerve terminals in both young and old rats, as well as 10-20% increases in synapses smaller than .08 microns (41).
6. Are there any side effects?
Carnitine has no toxicity, teratogenicity, contraindications, or drug interactions
(65). There are very few side effects associated with carnitine use, and
no serious side effects have been reported (58, 64, 65). Some users report
nausea or stimulation, but these tend to be transient and disappear with
time (65). There are also many anecdotal reports of more vivid dreams.
7. How should carnitine be taken?
The typical dosage for carnitine is 1-4 g/day (65). A study on the pharmacokinetics
of oral L-carnitine in human subjects found no differences in plasma carnitine
after 2 g vs. 6 g, indicating that 2 g is already more than the saturable
dose (66). The half-life of the 2 gram dose was 6.5 hours, and this would
imply that the ideal dosing schedule would be 1-2 grams 2-3 times daily,
although many studies indicate beneficial effects with only .5-1.5 grams
daily.
8. What are some good supplements to stack with carnitine?
- Alpha lipoic acid
- Studies show that carnitine and ALA or R-ALA have additive benefits in
protecting against oxidative damage, improving substrate utilization, improving
neurological function, and improving the function of mitochondria (21, 33,
35, 55). They also may have an additive benefit in improving hearing function
(61).
- L-ornithine - A widely quoted study found 500 mg of ALCAR and 25-100 mg of L-ornithine taken before bed after a three hour fast to improve GH release, but this regimen must be followed exactly (69), and the effects of fasting for three hours before bed every night should be weighed against improved GH release.
- Choline - Two studies, one in human and one in rats (which also
included caffeine), indicate that carnitine and choline have synergistic
effects in decreasing body fat (5, 67). The dosage of choline is generally
1-2 times that of carnitine.
- Caffeine - The rat study with carnitine, choline and caffeine found that it produced body fat reductions and physiological responses similar to those induced by exercise, and that the rats subjected to both regimens (supplementation and exercise) had improved substrate utilization during exercise (67). Another study in endurance athletes found a carnitine and caffeine combination to improve exercise times more than either substance alone (68).
If you have any questions or comments regarding this article, please email dvdtlsn@bulknutrition.com.
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