Low carb + intensive training = fall in testosterone levels

Trained athletes who a) cycle hard for an hour a day and b) eat little carbohydrate produce less testosterone and more cortisol [structural formulas shown below], write physiologists from the University of North Carolina in the European Journal of Applied Physiology.

Intensive training requires energy. If there’s no more glucose available for your muscles and they are forced to turn to fat for burning, your body starts to produce more cortisol to provide more energy. Cortisol stimulates the conversion of proteins into glucose. This is not desirable, but even worse; cortisol also depresses the production of steroid hormones like testosterone.


In a nutshell, this is what’s behind the advice to athletes to keep their training sessions down, both in time and intensity, and to eat before, during and after a session. In the eighties, experts claimed that it was carbohydrates above all that protected against muscle breakdown.

Nowadays sports scientists and nutritionists are less enthusiastic about carbohydrates. The low-carb diet is wildly popular, and comes in all shapes and sizes. This research suggests that there may be a shadow side to the low-carb diet.

The physiologists did a study of 20 well-trained endurance athletes, all of whom trained more than five times a week. Half of them were given a diet for five days in which 60 percent of the energy came from carbohydrates [Control CHO]; the other half got a diet in which 30 percent of the energy came from carbohydrates [Low CHO]. On days 2, 3 and 4 the subjects had to cycle for an hour at 75 percent of their maximal oxygen uptake.

During the cycling sessions the athletes in the Low CHO group were given a glucose sports drink. The athletes in the Control CHO group were given a protein shake containing 45 g protein.

When the researchers measured the concentrations of free testosterone and cortisol in the subjects’ blood, they noticed that in the Low CHO group these declined during the trial.


Sports scientists are interested in the ratio between free testosterone and cortisol. This remained more or less constant in the Control CHO group. In the Low CHO group, however, it declined rapidly.


The experiment only lasted 5 days. From other studies – and also from the experiences of people who go over to a low-carb diet – we know that the body takes a few weeks to get used to a lower carbohydrate intake. The athletes in this experiment may well have reacted differently if the experiment had lasted a few months instead of a few days.

One of the take-home messages of this study is that, especially if you’re on a low-carb diet, it seems like a good idea to consume the carbs you are allowed near to your training session.

Influence of dietary carbohydrate intake on the free testosterone: cortisol ratio responses to short-term intensive exercise training.


This study examined the effect of dietary carbohydrate (CHO) consumption on the free testosterone to cortisol (fTC) ratio during a short-term intense micro-cycle of exercise training. The fTC ratio is a proposed biomarker for overreaching-overtraining (i.e., training stress or imbalance) in athletes. The ratio was studied in two groups, control-CHO (approximately 60% of daily intake, n = 12) and low-CHO (approximately 30% of daily intake, n = 8), of male subjects who performed three consecutive days of intensive training (approximately 70-75% maximal oxygen consumption, 60 min per day) with a dietary intervention (on the day before and during training). Resting, pre-exercise blood samples were collected under standardized-controlled conditions before each day of training (Pre 1, 2, 3) and on a fourth day after the micro-cycle (Rest). Bloods were analyzed for free testosterone and cortisol via radioimmunoassay procedures. Subjects performed no additional physical activity other than prescribed training. Statistical analysis (ANCOVA) revealed the fTC ratio decreased significantly (p < 0.01) from pre-study resting measurement (Pre 1) to the final post-study resting measurement (Rest) in the low-CHO group (-43%), but no change occurred (p > 0.05) in the control-CHO group (-3%). Findings suggest if the fTC ratio is utilized as a marker of training stress or imbalance it is necessary for a moderately high diet of CHO to be consumed to maintain validity of any observed changes in the ratio value.

PMID: 20091182 [PubMed – indexed for MEDLINE]

Source: http://www.ncbi.nlm.nih.gov/pubmed/20091182

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