A recent study investigated whether circulating testosterone levels were associated with muscle mass, function, or the muscle anabolic response to resistance training in premenopausal females.
Overview
- What did they test? The authors conducted a 12-week study that examined the relationship between bioavailable testosterone, androgen receptor activation, and muscle mass and strength in females.
- What did they find? Total circulating testosterone was not related to muscle mass or strength before or after resistance training in pre-menopausal females. However, bioavailable testosterone and androgen receptor activation was positively related to exercise-induced muscle hypertrophy in pre-menopausal females.
- What does it mean for you? For females, this means that simply having higher total testosterone doesn’t necessarily help with strength or muscle gain. What really matters is having enough bioavailable testosterone and making sure the muscles can respond to it effectively via androgen receptors.
What’s the problem?
Purpose
Testosterone is a hormone often associated with building muscle and strength, and although it’s 10-fold lower in females compared to males 1, it still plays an important role. There are two types of testosterone in the body:
- Total testosterone. This is all the testosterone in your body, but a big part of it is “bound”, meaning it’s not available for your muscles to use.
- Bioavailable testosterone/ This is the free or active testosterone that your body can actually use for muscle building purposes.
Most of the testosterone in the body is attached to proteins that help transport it through the bloodstream. About 45% is bound to a protein called SHBG (sex hormone-binding globulin), and around 50% is bound to albumin 2. Only a small amount (about 3-5%) is "free", meaning it is not attached to any protein 3.
Testosterone that is bound to SHBG is inactive, meaning it can’t be used by the body. However, testosterone that is bound to albumin is weakly attached and can easily separate, making it available for use by the body. Both free testosterone and albumin-bound testosterone are considered “bioavailable,” meaning they can enter cells and bind to a receptor called the androgen receptor (AR) 1.
One of testosterone’s important roles is in skeletal muscle, where it binds to the AR. Since this receptor is found in many different tissues, testosterone influences various functions throughout the body, including muscle growth and repair.
The AR can be found in two locations within muscle cells: in the cytosol 4 or in the sarcolemma 5. Androgen receptors in the sarcolemma activate pathways like mTOR that promote protein synthesis, although this has only been confirmed in rodent cells 6. Androgen receptors in the cytosol move to the nucleus, triggering gene expression of over 1000 genes that supports muscle growth 7.
Both circulating endogenous testosterone and muscle AR protein content are positively associated with muscle mass and strength in males 8, but there is no such evidence in females. Previous research has shown that total testosterone levels do not correlate with muscle mass or strength in pre- or post-menopausal females 9. However, bioavailable testosterone, measured by the free androgen index (FAI), shows a weak association with muscle mass in premenopausal women 10. These findings, however, come solely from cross-sectional observations with a lack of control for confounding factors, which means they can show associations but not prove cause and effect. In addition, no studies have yet explored how strength training affects natural androgen levels and in turn, muscle mass and function in females over time.
Therefore, in this article, we will be looking at a cross-sectional study that investigated whether circulating testosterone levels were associated with muscle mass, function, or the muscle anabolic response to resistance training in premenopausal females.
Hypothesis
The authors hypothesized that bioavailable, but not total testosterone, would be associated with muscle mass and strength before and after a 12-week resistance training intervention in pre-menopausal females.
What Did They Test and How?
Participants
A total of 27 healthy females aged 18−40 years were recruited from the general population. Study participants were not resistance-trained (defined as having performed structured resistance training at least twice per week in the previous six months), pregnant or breastfeeding, didn't smoke and displayed no contraindications to exercise according to the Exercise and Sports Science Australia Adult pre-exercise screening system. Participants were excluded if they had a history of anabolic hormone use, used medications or supplements that could affect the anabolic response to training, or if their daily protein intake was outside the Australian dietary guidelines of 15−25% total macronutrient intake, measured through a mobile phone application for four days including one weekend day.
Baseline characteristics of the participants are shown in Table 1. The average age was 23.5 years and average BMI was 23.5kg/m2. There were no differences in baseline anthropometric or strength data across participants. There was also no between-group difference in the trajectory of working weight progression for any exercise.
