L-Carnitine - Review of Scientific Evidence
By William R. Sukala, MSc., CSCS
Clinical Exercise Physiologist / Consumer Health Advocate
www.williamsukala.com
Discovered
in 1905, L-carnitine is a nitrogen-containing,
short-chain carboxylic acid—technically, it is not an amino acid. It is a
water-soluble, vitamin-like compound (Kanter &
Williams, 1995) that is readily synthesized in the body from lysine and methionine (Cerretelli &
Marconi, 1990). Although carnitine is not an
essential nutrient because it can be synthesized in the body, it is sometimes
considered conditionally essential in that a dietary deficiency may cause
adverse side effects in certain circumstances (Broquist,
1994). However, in an industrialized nation, such a deficiency is rare.
Dietary carnitine can be easily obtained in a number of foods.
Perhaps the best source is meat, particularly beef, sheep, and lamb. Other
animal foods such as milk, cheese, and poultry contain somewhat less carnitine, while fruits and vegetables have negligible
amounts (Kanter & Williams, 1995). In light of
this, one must recognize that a diet containing sufficient amounts of
essential amino acids will provide the necessary building blocks for our
bodies to synthesize sufficient quantities of carnitine.
L-carnitine functions in a three-part enzyme complex (carnitine acyltransferase I, carnitine translocase, and carnitine acyltransferase II)
that is responsible for transport of long-chain fatty acids across the inner
mitochondrial membrane to the cristae where ss-oxidative enzymes are active (Pande
et. al., 1980).
However, carnitine supplementation with supraphysiological doses above and beyond that which the body requires, does not result in increased fat oxidation at rest or during exercise in well-nourished individuals; thus, it appears that we can synthesize the necessary amounts from a diet adequate in its precursors (lysine and methionine). Those medically diagnosed as carnitine-deficient may benefit from a supplement, but this condition is uncommon.
STUDIES ON
L-CARNITINE
We have established thus far
that carnitine plays a vital role in the transport
of fatty acids across the inner mitochondrial membrane. Based on this
function, it has been postulated that carnitine
supplementation will enhance lipid oxidation and thereby improve endurance
performance by sparing endogenous carbohydrate. Similarly, in anaerobic
activity, it has been purported that oral carnitine
will improve performance by inhibiting lactic acid production. The following
literature review will address these claims for their validity.
In a randomized, double-blind crossover study by Decombaz
et. al. (1993), nine subjects were given 3 grams/day of L-carnitine
for 7 days. Then at the end of the seven days, they completed a 20 minute
bicycle exercise at 43% VO2 max. Respiratory quotient (RQ), heart rate (HR),
rating of perceived exertion (RPE), and various blood parameters indicated no
influence of carnitine supplementation on substrate
utilization.
Otto et al. (1987) completed a randomized, double-blind cross-over study
employing 10 conditioned subjects. Participants completed a 4-week carnitine (500 mg/day) loading period prior to beginning
a 60-minute endurance event. There were no demonstrable improvements in
expiratory ventilation (Ve), VO2, HR, RQ, or work.
In a separate study by Otto and colleagues (1987), 10 subjects participated
in a double-blind crossover study and were randomly assigned to two trials of
either 500 mg of carnitine/day or a placebo for 28
days. In this instance the authors were testing its effects on maximal VO2
and serum free fatty acid levels. There were no significant changes in VO2, Ve, anaerobic threshold (AT),
HR, or max lactate.
Fink et al. (1994) studied 8 subjects over 14 days of carnitine
supplementation to see what effect it would have on lactate accumulation
during high intensity exercise. Subjects performed supramaximal
cycling activities at 115% VO2. L-carnitine
supplementation had no effect on blood or muscle lactate accumulation.
Ransone et al. (1994) employed 26 highly trained
male distance runners for 14 days of carnitine
administration. They completed a 600 m bout of activity and were analyzed for
lactate accumulation. The researchers found no effect of carnitine
on lactate accumulation during maximal anaerobic effort.
Kasper et al. (1994) tested the effects of carnitine
on running performance. Seven competitive male distance runners consumed
4g/day
for two weeks prior to testing. They found no improvement in running
performance during a 5k run and no decrease in blood lactate and heart rate.
Gorostiaga and colleagues (1989) examined 10
subjects over 28 days of supplementation and its effects on respiratory quotient
during exercise. This double-blind crossover study found a non significant
increase in O2 uptake, blood glycerol, and free fatty acids, and a small down
shift in RQ with carnitine supplementation. The
authors noted that none of the data were conclusive and that further studies
were needed to make
any definitive statement on carnitine efficacy.
LIMITATIONS
AND APPLICATIONS
The available research on L-carnitine supplementation does not appear to support
claims of enhanced aerobic or anaerobic exercise performance. While it is
true that carnitine plays a vital role in energy
metabolism, additional carnitine from exogenous
sources does not appear to yield any benefit above and beyond the necessary
physiological dose.
Results from experimental data on dietary supplements must be judiciously
applied given the limitations of the research methods employed. Many authors
note that 'highly trained subjects' were used. However, there is no uniform
definition for training status. A 'highly trained' athlete in one study may
be a 'moderately-conditioned' athlete in another, and vice-versa.
It is possible that all the athletes used in these studies were already at
their physiological limit. If so, one would not expect to find any
significant changes, irrespective of the supplement. One the other hand,
employment of unconditioned athletes may yield invalid results because
subjects may not be able to adhere to a demanding exercise protocol. But then
it must also be noted that unconditioned individuals would probably not be
competing at the same level (if competing at all) as the trained athlete.
Thus, studies on unconditioned subjects would have no relevance. And studies
on trained athletes, despite no demonstrable effect of carnitine,
would still be the most
pertinent to competitors.
These data provide valuable insight into an area of sport nutrition that is
highly debatable. Despite these contradictory data, claims of carnitine's efficacy persist based on anecdotal
testimonials. Unfortunately, testimonials do not control for confounding
variables which can make it difficult to separate cause and effect, and
therefore are not considered valid in the scientific arena.
Athletes wishing to explore carnitine's purported
benefits must be aware that the dietary supplement industry is not regulated
and, therefore, product safety is not guaranteed; that is, just because it is
sold in stores, consumers cannot be certain that the contents of the bottle
(dose or purity) is consistent with its labeling. With this lack of
regulation creating such a conducive climate for
misleading and false claims, the public is well-advised to research all
products thoroughly before making a decision. After all, an educated decision
is a wise decision.
This article remains exclusive property of the author and may not be used without prior consent. Requests may be made to William@williamsukala.com
REFERENCES