When celebrity TV doctor Mehmet Oz touts a supplement as a fat-burning, metabolic-boosting, fatigue-fighting, depression-easing wonder pill, people listen. And why wouldn’t they ... who doesn’t want to burn fat while simultaneously kicking the blahs to the curb?

As we’ve seen with green coffee beans and raspberry ketone — other supplements lauded by the influential doc — the once-unfamiliar L-carnitine is now enjoying the spotlight ever since Oz gave a shout-out about its potent powers.

Oz says that L-carnitine “stimulates growth hormone and helps turn on the mechanism that burns off fat in specific areas.” The doctoroz.com site explains that it “acts as a shuttle, pulling fat into the cell so it can be burned as fuel. L-carnitine will give you energy and ease depression.”

So what’s the skinny? Is this really a nutritional supplement that can do it all?

L-carnitine is a naturally occurring amino acid found in red meat and other sources, and is naturally produced in the body. L-carnitine deficiency in people without metabolic disorders is very rare; most people can synthesize enough L-carnitine, and research shows that even strict vegetarians generally get enough. While bioavailability of L-carnitine from the diet is very high, absorption from oral L-carnitine supplements is considerably lower. According to one study, bioavailability of L-carnitine from oral supplements ranges from 14 to 18 percent of the total dose.

Although in the body, L-carnitine plays a role in the transport of fatty acids and in other important metabolic processes, there hasn’t been much authoritative science confirming that it can perform the wonders that some propose.

According to the Linus Pauling Institute (LPI) at Oregon State University, some studies in rats have suggested that L-carnitine supplementation may be beneficial in preventing age-related declines in energy metabolism and memory, yet it is not known whether the same may hold true for humans. There is often a wide discrepancy in the way nutritional supplements affect small furry critters versus people.

And although it’s a popular supplement at the gym, LPI notes that there is little evidence that L-carnitine supplementation improves athletic performance. As well, the National Institutes of Health notes that "20 years of research finds no consistent evidence that carnitine supplements can improve exercise or physical performance in healthy subjects."

Meanwhile, the University of Maryland Medical Center claims that although L-carnitine has been marketed as a weight loss supplement, there is no scientific evidence to show that it works. They note that although some early studies showed that L-carnitine might help “slow down the progression of Alzheimer's disease, relieve depression related to senility and other forms of dementia, and improve memory in the elderly, larger and better-designed studies found it didn't help at all.” Which leaves the jury out on that part as well.

That said, the UM Medical Center notes that there is evidence L-carnitine may help reduce symptoms of an overactive thyroid, ease male infertility, help improve male sexual function, and some clinical trials are looking at it for use in treating cardiovascular issues.

The supplement seems to have potential. But as far as fat-burning, depression-easing miracle cures go, science has yet to prove L-carnitine to be the magic bullet that many in marketing and the media have implied.

If you want to see for yourself, The Oz site recommends 500 milligrams or more daily as a “sneaky trick to burn fat” or 500 milligrams twice daily to fight fatigue. And as always, check with your doctor before adding any supplements to your regimen.

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What is L-carnitine?
L-carnitine is lauded as a supplement to burn fat, fight fatigue and ease depression — but the line between the hype and science is fuzzy.