Lack of evidence for fat-burning supplement

The Irish Times
Health Supplement, p. 13
20 January 2009

DOES IT WORK? Carnitine and burning fat

CARNITINE IS a nutrient commonly found in dairy products and meat. Its name comes from the Latin “carne” meaning flesh. At one time it was thought to be a vitamin, but it is now classified as a “conditionally essential nutrient”.

This means that under most circumstances the body can make enough carnitine if there is insufficient available in the diet. Some people are born unable to produce enough carnitine or develop medical conditions where additional carnitine must be provided on prescription.

Recently, carnitine has developed a reputation as a sports supplement and dieting aid. Carnitine is required to transport long-chain fatty acids into the mitochondria of cells.

Mitochondria are the “powerhouses” of cells where the energy is released from fat. Carnitine accumulates in tissues that use fatty acids, especially muscles and heart tissue. As a result, carnitine is available as a dietary supplement that allegedly “burns fat better”, either to produce more energy during endurance sports or to get rid of excess fat to lose weight.

Evidence from studies 

The use of carnitine by athletes and dieters has a certain logic to it based on its biochemistry. An important assumption that people who use carnitine supplements make is that carnitine in supplements gets into their muscles.

However, less than 20 per cent of an oral dose of carnitine is absorbed into the body, and half of that is eliminated again in urine within 24 hours. Healthy people with sufficient carnitine in their diets show very little increase in muscle carnitine levels when they take supplements.

Controlled studies of healthy overweight (but not obese) people found no increase in muscle carnitine levels and no additional weight loss among those who took carnitine compared with those taking a placebo. In fact, carnitine was found to stimulate people’s appetites – the opposite of what was expected.

Endurance exercise, as expected, has been found to produce small decreases (less than 10 per cent) in the level of carnitine in muscles. Such reduced carnitine levels were not found in athletes who took supplemental carnitine for several months. People with medical conditions related to carnitine deficiencies typically have 25-50 per cent less than normal levels of carnitine in their muscles.

Numerous studies have tested athletes taking carnitine supplements using various laboratory tests. While individual tests have shown positive effects, most tests in any one study usually show no benefits.

The one exception is with high-intensity exercise which is no longer aerobic. This sort of exercise leads to lactic acid production which contributes to muscle soreness and stiffness.

Muscle carnitine levels decrease significantly during this type of exercise, and supplementation has been found to prevent much of this reduction.

Some, but not all, studies have found that high-intensity exercise improved after carnitine supplementation for several months.

Problematic aspects 

Carnitine is generally well tolerated and safe when taken at the usual dose of 2g per day. A few studies have used much larger daily doses and not found adverse effects.

Carnitine is available in a number of forms, with L-carnitine being the naturally active form. Acetyl-L-carnitine and L-propionyl carnitine are also commercially available and appear to be as effective as L-carnitine. D-carnitine does not have the same effects and can even increase the risk of L-carnitine deficiency. Products containing D-carnitine should not be used.


Carnitine has an important role in treating medical conditions resulting from carnitine deficiency. Its benefits as a dietary supplement in individuals who otherwise have sufficient carnitine is debated.

Most people in developed countries such as Ireland get enough carnitine from their diets. Red meats and dairy products are the main sources. However, even people who have no carnitine in their diet usually produce enough carnitine themselves.

Although carnitine plays an important role in how the body releases energy from fat, there is little or no evidence that carnitine supplements help people lose weight. For athletes training at the highest intensity, supplemental carnitine may help speed up recovery and maintain high-intensity exercise.

However, studies have not shown whether this leads to improved performances in competition.

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