For this research pick I’ll go over a recent trial which investigated the effect of terbutaline on body composition in healthy men.
Beta2-agonists like clenbuterol and terbutaline have long been thought to increase muscle mass. A wealth of animal research clearly shows this, however most trials in humans fail to demonstrate this. The obvious reason could be that they simply don’t increase muscle mass in humans. There might be some other reasons though. Animal studies usually employed fairly high dosages, well above the therapeutic dosages used in clinical trials. Also, most clinical trials had small sample sizes. As such, they might’ve lacked statistical power to detect an effect, especially if the effect size is modest. Moreover, beta2-agonists have some annoying side-effects like tremor and high heart rate (tachycardia), which might lead to the subjects not taking their doses as indicated (besides that people suck at taking something regularly anyways). So poor compliance might be a factor too.
That said, a recent well-designed study did find an increase in lean body mass (LBM) after 4 weeks of terbutaline usage. Notably, they used a terbutaline inhaler, instead of the pills, leading to less systemic exposure of the drug [1].
In this double-blind randomized controlled trial, 67 healthy young men (!) were assigned to one of the following groups:
- Habitual & placebo
- Habitual & terbutaline
- Resistance training & placebo
- Resistance training & terbutaline
- Endurance training & placebo
- Endurance training & terbutaline
The dosage of terbutaline was 0.5 mg 8 times per day. Although a high dosage, it’s still in the therapeutic range (at the upper limit).
Both the habitual & terbutaline and resistance training & terbutaline groups gained 1 kg more LBM than their placebo counterparts (both statistically significant). Interestingly, the endurance training & terbutaline group did not gain more LBM than its placebo counterpart. Apparently endurance training negates the effect terbutaline has on LBM. The reason behind this is uncertain (I’m not gonna guess either).
Now I can hear some of you thinking: but LBM can be something else than muscle too. Yes, that’s true. Luckily, the same research group has done a similar investigation in the past, in which they took muscle biopsies to assess what happened to the muscle fibers [2]. In this study, the subjects also took terbutaline, albeit orally. They found that terbutaline increased the muscle fibers size, which strongly suggests the measured increase in LBM is muscle hypertrophy. I would not expect inhaled terbutaline to suddenly increase LBM of a different tissue than muscle mass. The difference I’d expect with a different route of administration is simply a different magnitude in the increase. So gaining a bit less muscle when inhaling it compared to taking it orally.
Concluding, terbutaline does seem to help in gaining muscle mass when combined with resistance training (and not endurance training). Whether it will stay effective when taking it longer than 4 weeks? We don’t know.
References
- Jessen, Søren, et al. “Hypertrophic effect of inhaled beta2‐agonist with and without concurrent exercise training: a randomized controlled trial.” Scandinavian journal of medicine & science in sports (2018).
- Hostrup, Morten, et al. “Mechanisms underlying enhancements in muscle force and power output during maximal cycle ergometer exercise induced by chronic β2-adrenergic stimulation in men.” Journal of Applied Physiology 119.5 (2015): 475-486.