Clenbuterol, also known as “clen” is a beta-2-adrenergic agonist, that has steroid-like effects. Used as a bronchodilator for asthma, clen is used off-label for weight loss, since it has some structural and pharmacological similarities to epinephrine and salbutamol as a thermogenic. We’re going to talk more about clenbuterol, how it works, and what the benefits are.

What Is Clenbuterol [Clen]

Clen or Clenbuterol, is a beta-2 agonist prescribed as a bronchodilator for asthma. Clen is used off-label by athletes and bodybuilders, for the purported anabolic effects and reduction of subcutaneous fat.

How Does Clenbuterol [Clen] Work

To fully understand the etiologies of and how clenbuterol works, we must first discuss what beta-2 agonists are and what they do physiologically.

Beta-adrenergic receptors are transmembrane glycoprotein structures that elicit a response inside the cell when interacting with catecholamines. Beta receptors can be divided into three subtypes, beta-1 (B1), beta-2 (B2), and beta-3 (B3). 

Beta-2 receptors are present in smooth muscles. Smooth muscle differs from skeletal muscle in function. Unlike skeletal muscle, smooth muscle is capable of maintaining tone for extended periods and often contracts involuntarily. At a cellular level, smooth muscle can be described as an involuntary, non-striated muscle.

Smooth muscle fibers are located in the walls of hollow visceral organs (intestines, liver, uterus). Beta-2 receptors also exist in skeletal muscles, and cardiac muscles.

Endogenous hormones stimulate B2 receptors and synthetic compounds; epinephrine (adrenaline) is the most effective natural catecholamine agonist of B2, while norepinephrine (noradrenaline) is less effective on it.

Epinephrine is the hormone responsible for B2 receptor stimulation in the physiological state.

B2 receptor agonists, are pharmacologically designed to modulate the natural effect of endogenous epinephrine and norepinephrine hormones. This is called sympathomimetics.

B2 agonists are indicated to treat, respiratory disorders such as chronic obstructive pulmonary disease (COPD) and asthma. Bodybuilders and athletes, use B2 agonists off label for their anabolic and lipolytic properties.

What Does Clenbuterol, Clen Do? 

So what does clenbuterol or clen do? It burns more body fat, by mimicking epinephrine, your body’s hormone which controls your fight or flight response, and norepinephrine, increasing your heart rate, energy expenditure, and acting as a thermogenic, by increasing metabolic rate.

Clenbuterol [Clen] Benefits

Fat Loss

A small study conducted at the University of Copenhagen, School of Integrative Physiology, Department of Nutrition in Exercise and Sports, investigated the effect of clenbuterol ingestion on metabolic rate as well as skeletal muscle mammalian target of rapamycin (mTOR) phosphorylation and protein kinase A (PKA)-signaling in 6 healthy young, male subjects.

Before and 140 min after ingestion of 80 μg clenbuterol, resting metabolic rate and contractile function of the quadriceps muscle were measured, and blood samples as well as vastus lateralis muscle biopsies were collected. 

Study results indicated that clenbuterol increased resting energy expenditure by 21% (P < 0.001), and fat oxidation by 39% (P = 0.006), whereas carbohydrate oxidation was unchanged. 

mTOR phosphorylation and PKA substrates increased by 121%. Collectively, these findings indicate that clenbuterol is an efficient thermogenic that possibly also exerts muscle hypertrophic actions in humans [R].

Clenbuterol [Clen] Side Effects

A systematic review investigated the adverse events of clenbuterol amongst athletes, when used recreationally. Of the 23 identified studies, 24 individuals experienced adverse events, or side effects [R].

Cardiac-related complications and side effects were the most commonly occurring adverse events.

The daily administered dose of clenbuterol ranged between 20 µg to 30 mg. Major adverse events experienced by athletes were

• supraventricular tachycardia
• atrial fibrillation
• hypotension
• chest pain
• myocardial injury
• myocarditis
• myocardial ischemia
• myocardial infarction
• cardiomyopathy
• hepatomegaly
• hyperglycemia
• death

Clenbuterol is notorious to produce life-threatening adverse events including death. Lack of evidence regarding the performance-enhancing effects of clenbuterol combined with its serious toxicities questions the usefulness of this drug in athletes.

Is Clenbuterol [Clen] Legal

Clen is not approved for use in humans by the federal drug administration (FDA). A liquid form of the drug is approved by the FDA for treatment of airway obstruction in horses. Clenbuterol is also available and prescribed outside of the U.S. for asthmatics.

Clenbuterol: Takeaway 

As a performance enhancing substance, preliminary evidence shows that clenbuterol, can in fact increase metabolic rate and energy expenditure. This is very appealing to bodybuilders and athletes alike, to obtain a better more lean physique. Although, the fact remains, that the side effects of clen, largely overshadow the benefits.

Inhaled beta-2 agonists are banned from sports and on the WADA prohibited substance list, due to their improvement of airflow and ergogenic benefit. However, a review published in the British Journal Of Sports Medicine, found that there really  is no ergogenic potential of inhaled B2 agonists in non‐asthmatic athletes. Of the 20 identified studies, only three resulted in ergogenic benefits [R]. Those were observed in untrained or recreational athletes, not competitive athletes. Thus, ergogenic effects in trained athletes, would be minimal.

Although this review was conducted in 2006, and the study at Copenhagen was published in 2019, it seems this is the only evidence, that shows slight ergogenic benefit.

As a fat burner, and thermogenic, it does seem that clenbuterol does elicit weight loss benefits, and may support cutting phases for bodybuilders and optimized body recomposition goals for recreational athletes as an off-label use.

Disclaimer: Swolverine does not condone or sell beta-2 agonists or anabolics. The contents of this article are for informational purposes only. Clenbuterol is not for sale in the United States. Possessing, using, or distributing these substances may lead to serious legal consequences. Clenbuterol is on the WADA prohibited banned substance list and will cause a failed drug test.

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