In the Shape Up Challenge™, you recommend L-carnitine. What is the difference between acetyl-L-carnitine and carnitine? I was told that carnitine is usually used to treat heart ailments and was not recommended for weight loss. Please clear things up for me. Thanks!

L-carnitine acts as a kind of escort service for fat, moving it into the little power centers of the cells (called mitochondria) so that the fat can be burned for energy. Quite simply, without optimal amounts of carnitine, there is not optimal fat burning. Humans have gotten carnitine in their diets for more than two and a half million years. However, here's the question: Are we getting the optimal amount for vibrant good health?

The main source of carnitine is red meat. Babies get theirs through breast milk, or through fortified formulas. Vegetarians or other people who eat very small amounts of red meat are most probably not getting enough. If you're breastfeeding and a vegetarian, supplements would be a very good idea.

Carnitine is "the most important nutrient for naturally supporting the weight loss process," according to Robert Crayhon, author of The Carnitine Miracle. Crayhon is one of the best and most respected nutritional educators in the country, and his book is a wealth of information, not just on carnitine, but on diet in general. According to Crayhon, whose book is meticulously documented with more than 60 published references, carnitine is useful for obesity, fatigue, athletic performance, general energy and well-being.

To get the weight-loss benefits of carnitine, you must take a meaningful dosage. Some manufacturers stick it in their "fat burning" formulas so that they can say their product contains carnitine, but many of these commercial "fat burners" contain only about 50 mg of carnitine -- a meaningless amount for promoting weight loss and energy. Carnitine advocates often recommend dosages anywhere from 1,000 mg to 4,000mg (1 to 4 grams) or even higher, in some rare cases.

For best absorption, carnitine should be taken on an empty stomach -- half an hour before a meal, right before a workout or a couple of hours after eating. According to Crayhon and other experts in the field, it works best with a diet moderately low in carbohydrates (50 percent or less of calories consumed). Why? Because high levels of carbs promote high levels of insulin, which in turn inhibits carnitine activity. Carnitine also works synergistically with a diet rich in omega-3 fats, such as those in fish and flax.

That carnitine is only used for people with heart problems is simply not true. But carnitine is a very heart-friendly nutrient. Since the heart depends on energy from fats, and because carnitine makes fats available for energy, carnitine is very helpful to the heart. One study demonstrated that carnitine can protect the heart from damage when a heart attack or spasm cuts off the oxygen supply. That may be the source of the rumor that it is only used for heart patients.

Acetyl-L-carnitine, on the other hand, is best described as a kind of "supercarnitine." According to Dr. Richard Firshein, author of the excellent book The Nutraceutical Revolution, acetyl-L-carnitine (abbreviated as ACL) "promotes efficient energy use in the brain." It does that by increasing the brain's supply of acetylcholine, a neurotransmitter crucial for memory function. Indeed, ACL is often used as a kind of "brain food" -- it appears to have a positive effect on memory and mood and seems to slow down the aging of brain cells and the associated deterioration of a lot of mental functions. Acetyl-L-carnitine, however, is very expensive.

Both nutrients are valuable and can certainly be used together, but if general good health, energy and weight loss are your primary concern, L-carnitine will do just fine.

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