Dr Ziv Haskal, professor of radiology and surgery at the University of Maryland Medical Center in Baltimore, said the energy-boosting results of studies conducted on those with vascular and heart disease, could not necessarily be extrapolated to healthier people.
Professor Haskal began to form his view of the amino acid-derived molecule in 2005 when involved in an L-carnitine review for the American College of Cardiology Foundation and the American Heart Association.
That review concluded that while L-carnitine could benefit those with cardiovascular problems, little evidence demonstrated benefits beyond that.
The research has led to some doctors like Dr Mary Hardy, medical director of the Simms/Mann UCLA Center for Integrative Oncology in Los Angeles, prescribing it to cancer patients. But she told the LA Times she made no further extrapolations.
"It's kind of a leap, to conclude that it would have a similar effect in healthy people," Hardy said.
The article notes mixed results for L-carnitine in studies conducted on healthy populations, with energy-giving links growing in the 1990s after studies indicated L-carnitine supplementation could improve oxygen uptake and power.
Other studies have not found statistical difference between athletes that take dietary supplements and those that do not.
L-carnitine ferries fatty acids into the cells, but its exact mechanism is not completely understood, hence the ambiguity among scientists about its role and the benefits of supplementation for a molecule the body makes naturally at certain levels.
It can also be found in vegetables such as asparagus.
Lonza, which claims to be the world's largest manufacturer of L-Carnitine, has said that extensive scientific research shows the supplement promotes cardiovascular health and that other studies suggest the nutrient may be useful in weight management.
According to Lonza, previous studies have indicated that restoring plasma levels of carnitine via supplementation during pregnancy may also help prevent the development of gestational diabetes, especially in overweight women, by decreasing elevated plasma free fatty acids.