Clenbuterol: Fat Loss and Anabolic
Muscular Development|March 2021
Clenbuterol is the undisputed champion in the arena of fat-burning drugs used by bodybuilders, models and trophy wives.
By Daniel Gwartney, M.D.

There are certain drugs that are more potent, but carry hazardous or even deadly risks (e.g., DNP, T3). The worst scenario is the skeletal-like catabolic form that results from abusing addictive drugs (e.g., crystal meth) that ravish spirit and body.

Contrary to its lipolytic peers that catabolize muscle mass as readily as fat, clenbuterol generally has a protective effect. In fact, it gained considerable attention due to its use as an anabolic agent in animal feed. Unfortunately, the dose of clenbuterol needed to induce skeletal muscle hypertrophy (mass gain) is toxic to humans without protecting against dangerous changes in heart rate and rhythm.2 Clenbuterol works by selectively activating a specific class of receptors called beta-2 adrenergic receptors (B2AR). There are two major classes of adrenergic receptors, alpha and beta. The beta receptors activate the “fight or flight” effects of adrenalin, including accelerating heart rate, dilating airways, increasing force production and metabolism in skeletal muscle, and releasing stored fat. These effects are relatively specific to subtypes of the beta-adrenergic receptor. Beta-1 receptors increase heart rate; beta-2 dilate airways and promote changes in skeletal muscle; beta-3 stimulates the breakdown and release of stored fat. However, it is not as cut-and-dried as that, as there is crossover between the subtypes.

Clenbuterol has been shown in human adipocytes (white fat cells) to stimulate the breakdown and release of stored fat, and also inhibits insulin’s fat-storing signal at the fat cell.3-5 Further, it stimulates brown fat to increase thermogenesis. Brown fat is different from white fat in that instead of storing fat, it “wastes” the calories to generate heat instead of ATP energy. It is critical to increase metabolic demand when stored fat is being released, otherwise it just redeposits back in white fat or worse, in “wrong” places like the liver or plugging arteries. Clenbuterol accomplishes this (via B3AR rather than B2AR, surprisingly) by increasing the production of uncoupling proteins in brown fat, a crucial factor in “wasting” fat calories.6 If research from rat studies translates to the human experience, it appears that white fat cells become resistant to clenbuterol’s signal to break down stored fat by nearly 50 percent when consuming a ketogenic diet.7 Certainly, there are many bodybuilders who have used clenbuterol during different diet schemes. It would be interesting to hear feedback regarding whether they subjectively experienced less than-expected results from clenbuterol when following a very low-carbohydrate diet.

Clenbuterol increases the metabolic demand of the muscle. Oddly, the relative rate of fat burning in muscle is reduced in both skeletal muscle and the heart (cardiac muscle); carbohydrate metabolites account for a greater percentage of mitochondrial fuel.8,9 Further, more fat is stored in skeletal muscle, which would tend to increase insulin resistance in that tissue (all this from rat studies). Combined with the reduced effectiveness seen in very low-carbohydrate diets, this suggests that clenbuterol and related drugs might work best in people consuming balanced macronutrient diets (e.g., 40/30/30 Zone Diet or something similar).

20-Inch Guns and an Enlarged Heart

As the more serious adverse effects associated with clenbuterol are heart-related, some research suggests protective measures that can be taken – though by no means making it a safe or recommended practice outside the direction of a licensed health care provider. Clenbuterol can induce muscle growth in the heart as well as the biceps and other muscles. While a 20-inch set of guns might be appealing, an 800-gram heart (over twice the normal size) spells disaster – meaning death. Again, in rats, the use of a non-steroidal anti-inflammatory drug (NSAID) similar to ibuprofen prevented the increase in heart mass seen in clenbuterol-treated rats when the drugs were used concurrently, without affecting muscle mass increase.10 Many drugs taken by bodybuilders are associated with cardiomegaly (enlarged heart), as is resistance exercise.

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