Male athletes are often keenly aware of the importance of testosterone and its role in reproductive function, bone mass, fat distribution, muscle mass, strength, and the production of red blood cells. Athletes sometimes crave the anabolic (“building up”) characteristics of this hormone due to its powerful effect on muscle strength and muscle mass. But as men age, they naturally produce less testosterone, which can have a negative impact on performance.

This can be compounded by cortisol, a stress-related hormone that is influenced by environmental factors, such as intense exercise. Cortisol can result in the catabolic or “tearing down” of tissue. What does this mean? In a nutshell, when the athlete produces too little testosterone or too much cortisol, performance is affected.

There is little doubt that testosterone can enhance athletic performance when taken as an external sports aid (which is why it is banned from competitive sports other than via a therapeutic use exemption, or TUE, when prescribed and supervised by a medical professional). Higher muscle mass, combined with more lean body mass, can contribute to faster times and improved performance, which is why male athletes often want to retain adequate levels of testosterone and reduce cortisol. It is also true, though, that the performance impacts of higher levels of naturally occurring testosterone are not as clear—i.e., just because one athlete has higher levels of testosterone doesn’t consistently correlate with higher levels of performance.

To maintain a lean body mass, some athletes turn to low-carbohydrate diets. But can something as simple as a dietary change to lower carbohydrates impact testosterone and cortisol levels, making it harder to lose weight or achieve performance gains?  University of Worcester researchers Joseph Whittaker and Miranda Harris recently conducted a meta-analysis of the available literature to find out.

RELATED: When Low Training and Low Testosterone Clash

The skinny on low-carb diet research

In recent years, low-carbohydrate (or “low-carb”) diets have received ample attention due to headlines publicizing their role as a method for short-term weight loss and other possible benefits such as improved cardiovascular risk factors. However, the lack of standardization of research on low-carb diets can make comparing research outcomes difficult.

To complicate matters, when one macronutrient such as carbohydrates is lowered, the other two, protein and fat, must be increased to maintain a caloric equilibrium. Researchers must control for whether calorie levels remain consistent during carbohydrate manipulation or if calorie levels decrease when the participant naturally eats less on the low-carb diet. In addition, understanding the impact weight loss alone has on everything from hormones to cardiovascular risk factors is almost impossible to separate.

Finally, adjusting to a low-carb diet takes about three weeks, so hormones and other metabolic markers can vary during this adaption phase. Therefore, comparing the impact of a low carbohydrate diet on hormones like testosterone and cortisol is difficult. In response, Whittaker & Harris have performed a systemic review and meta-analysis to evaluate the endocrine effects of low- vs. high-carbohydrate diets.

RELATED: High-Carb Fueling: How High Can You Go?

The current study

To be included in a systemic review and meta-analysis, studies must include an experimental design that includes an intervention (meaning something was manipulated by the researcher, so there was an exposure to something and a control group for comparison of the two). In the Whittaker and Harris analysis, inclusion criteria included measurements of resting testosterone and cortisol were done immediately after exercise, one hour after exercise, and two hours post-exercise; healthy adult (18 years and older) males; and a low-carb diet (less than or equal to 35% of total energy intake, or TEI) and high carbohydrate diet (greater than or equal to 35% of TEI). Finally, the intervention diets were consumed for more than than 24 hours.

Studies were excluded if they had variables that could cause the results to be impacted (as mentioned above), including changes in weight of more than 6.6 pounds, taking hormones, medications, phytoestrogens, or carbohydrate loading, all of which could influence changes in testosterone and cortisol.

Do low-carb diets cause low testosterone in men?

After screening 3,266 studies, 27 met the inclusion criteria for the current study, which included 309 participants. When examining low-carb diets vs. high-carb diets, the researchers could not find an overall impact on resting cortisol. However, the overall results showed a significant decrease in resting testosterone when participants were on a low-carb diet, specifically when isolated to those with a higher protein amount (greater than 35%), compared to a high-carb diet. To provide more context, the mean testosterone for a population of 27-year-old males is 14 nmol/L, and the subgroup analysis of the higher protein with the low-carb group showed a ∼5.23 nmol/L lower testosterone level (approximately 37% or 8.77 nmol/L in a population of 27-year-old males).

Is this decrease enough to impact health, body composition and, ultimately, sports performance? We need more research to answer this question. In addition, when examining the sample size of this subgroup of higher-protein combined with low-carb diets, only 26 participants were included and compared, which is a relatively small number to conclude the statistical power of the results. More research on low-carb diets with more participants is required to know exactly how low-carb diets impact testosterone levels.

The bottom line

A low-carb diet for endurance athletes is generally not considered the best approach for performance enhancement. The position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance suggest that training with limited carbohydrate stores can lead to “impaired training intensity and duration” combined with “reduced work rates, impaired skill and concentration, and increased perception of effort.”

Therefore, even during a moderate exercise program of one hour a day, 5-7g/kg/day (kg are lbs./2.2) of carbohydrates is recommended for athletes. A male endurance athlete training for approximately one to three hours per day at moderate to high intensity should warrant a diet containing 6-10g/kg day of carbohydrates.

RELATED: Triathlete’s Complete Guide to Nutrition and Fueling

Headlines may tempt athletes to consider a low-carb diet for other reasons. However, as this study shows, a dietary change may have unforeseen health or performance implications for some athletes. This may or may not be attributed to the low-carb diet affecting testosterone levels. Nutrition counseling with a sports dietitian (especially one holding the Certified Sports Specialist in Dietetics credential) to assess your overall energy requirements, macronutrient needs, and ideal nutrient timing to your specific lifestyle is the best avenue for long-term health and performance enhancement.

RELATED: How Much Do You Know About the Male Athlete Triad?

Kim Schwabenbauer, PhD, RD, CSSD, LDN, is a former professional triathlete turned registered dietitian, professor, consultant, speaker and triathlon coach with an emphasis in overall health, wellness and sports nutrition.