Rats! Clenbuterol may reduce, not enhance, performance, and stiffen heart muscles to boot

Clenbuterol is a popular - but banned - drug used by athletes in body-building, power-related, and even endurance sports. The chemical is attractive to athletes because it appears to have an anabolic effect on human muscles, and it may also increase fat metabolism. However, the actual effects of long-term clenbuterol intake on performance, muscle power, and overall health have been unclear.

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To determine some of clenbuterol's actions, scientists at the University of Melbourne in Australia recently gave clenbuterol to laboratory rats at dosages of 2 milligrams per kilogram of body weight per day. Some of the rats followed a completely sedentary lifestyle, while others sprinted on treadmills or engaged in endurance swim training. Clenbuterol did have a couple of potentially positive effects: Sedentary rats which ingested clenbuterol had larger muscles than clenbuterol-free, sedentary rodents, and clenbuterol users also transformed leg-muscle cells from slow-twitch to fast-twitch fibres, a surprising change which would tend to increase anaerobic energy production and magnify muscle power during short, intense exertions.

However, clenbuterol also yielded three very negative changes. First, after just four weeks, clenbuterol-treated rats were unable to maintain their normal swimming or running training intensities, while clean rodents were quite capable of continuing. Secondly, the hearts of the clenbuterol-taking, trained rats increased dramatically in size compared to the hearts of sedentary rats, but the heart expansion was probably due to the infiltration of collagen fibres into the heart walls, not an increase in heart-muscle cells. Collagen is a tough connective tissue which doesn't augment heart-muscle power but in fact stiffens the heart, potentially leading to a decrease in cardiac output. Increases in collagen may also produce cardiac arrhythmias. Thirdly, clenbuterol rats suffered from noticeable cardiac-cell degeneration.

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In addition, although sedentary, clenbuterol-treated rats were more muscular than clenbuterol-free, sedentary rats, clenbuterol was unable to boost muscle mass among either the swim- or treadmill-trained rodents. It appears that, in spite of its popularity, clenbuterol is a potentially dangerous drug which offers very few positive effects for either the power or endurance athlete.

('The Effects of Clenbuterol on Skeletal and Cardiac Muscle of Rats when Combined with Aerobic and Anaerobic Exercise, ' Biochemistry of Exercise Ninth International Conference Abstracts, #117, p 75, 1994)

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