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There has been much talk about the popular and well hyped fat-loss (thermogenic) supplement weight loss product that contains coleus, or also known as
forskolin. The question tends to be, what exactly is it and perhaps more importantly how does it work if at all?
Coleus is the root of a plant from the mint family called Coleus forskohlii. It was first used in ancient Ayurvedic and Hindu medicine and, at one time or other, has been used to treat asthma, some types of heart disease, painful urination, insomnia, convulsions, chest pain and skin disorders. One of the most active ingredients in coleus is a chemical called forskolin. Like ephedrine and caffeine, forskolin increases the amount of cyclic adenosine monophosphate (cAMP) within cells. In fat cells, a rise in cAMP causes thermogenesis (fat burning). Forskolin increases cAMP in cells directly, while ephedrine and caffeine do it indirectly. Forskolin binds to adenylate cyclase, the enzyme that makes cAMP, and revs it into high gear. Ephedrine also increases cAMP production, but in a more roundabout way (through norepinephrine release and several other steps). As with ephedrine, forskolin acts in synergy with caffeine.
Does forskolin cause fat loss? In a test tube, definitely. It breaks down human fat in a dish. It also "fries" brown fat in living rats (P.J. Scarpace et al., "Effects of age on beta adrenergic subtype activation of adenylyl cyclase in brown adipose tissue," Proceedings of the Society for Experimental Biology and Medicine, 213:262-67, 1996). Even so, it doesn't seem to be a good idea to take it orally, unless the goal is to be able to reduce the dosage of ephedrine - and possibly the temporary blood pressure rise that ephedrine sometimes elicits.
Forskolin acts everywhere in the body, even in the brain, so it may cause side effects at fat-burning doses. For example, according to the rat study mentioned previously, a dose that increased brown fat thermogenesis in young rats "caused cardiac distress in [old] rats." Obviously, forskolin isn't very selective. When it is taken orally, it has a scattershot effect, like a shotgun. As a result, it may be tough to get enough forskolin by mouth to see fat loss without also risking cardiac distress and other side effects. Luckily, toxicity is unlikely in the amounts people tend to take - about 9 mg of forskolin two or three times per day (for example, as 50 mg of coleus root extract standardized to 18% forskolin). Fat loss is unlikely at that dose, though.
That's not to say that forskolin isn't promising. In fact, it may prove useful as a topical cream, according to research done at UCLA School of Medicine some years ago. World-renowned obesity experts Frank Greenway and George Bray published a report demonstrating a local fat-loss effect for forskolin when applied to the thighs of 28 obese women ("Regional fat loss from the thigh in obese women after adrenergic modulation," Clinical Therapeutics, 9:663-69,1987). After a similar study eight years later, they reported the same loss of subcutaneous fat (F. Greenway et al., "Topical fat reduction," Obesity Research, 3 Supplement :561S-568S, 1995).
Indeed, some commercially available thigh creams do contain coleus as one of the active ingredients. These thigh creams have names such as Forever Young - Erase Thigh Gel. Clearly, they're not targeted toward bodybuilders. Nonetheless, I think there's enough evidence to give thigh creams a try on small localized fat deposits. As for taking it by mouth, avoid it if you have a heart condition or other medical problems; in any event, don't exceed 30 mg per day until more is known about possible risks.