This week’s EXSS Impact post was developed by Dr. Anthony Hackney and discusses the influence of sex hormones on physical performance in female athletes. Many thanks to Dr. Hackney for his contributions.
To investigate the influence of estrogen on energy metabolism and performance during prolonged endurance exercise in women.
Background. Intensive exercise training programs, emotional stressed, and poor nutritional practices can result in the development of amenorrhea (loss of the menstrual cycle) in women. There a number of health consequences from this development, perhaps the most severe being bone-mineral loss and an increased risk of musculo-skeletal injuries. This is definitely a research question that focuses upon “how exercise influences the menstrual cycle”, which is a question of critical importance. However, several years ago my students and I started to ask a different question – “how does the menstrual cycle influence exercise”?
Across a menstrual cycle the reproductive hormones of a woman change dramatically to bring about the necessary changes in the reproductive organs to allow fertility and gestation (if need be). One of the most important of these hormones is estrogen (actually it should be estrogens as there are several types; but for simplicity I will use the singular). Estrogen is not only a critical reproductive sex-hormone but also a vital metabolic hormone. Animal-based research has shown estrogen can directly and indirectly promote the increased usage of fats as an energy source as well as spare the use of carbohydrates as an energy source, the latter being important in endurance performance. That is, in most individuals there are ample fat reserves for energy that can last days while the carbohydrate reserves typically are only enough for a few hours of activity.
SHAPE Project. To this end, my research group has done a number of human-based studies in what we call the “SHAPE Project” over the last few years to examine the use of fat as an energy source in women during prolonged endurance activity and to question whether the level of performance in these activities is affected by estrogen changes over the menstrual cycle. SHAPE stands for Sex-Hormones and Performance Enhancement. These studies have shown that when estrogen is elevated (this occurs from ovulation and through the last two weeks of the menstrual cycle [luteal phase]) there is a substantially greater reliance on fat as an energy source in continuous exercise lasting an hour or more. Furthermore, our evidence supports that carbohydrate reserves in the body are spared because of this increased reliance on fat for energy.
We have also examined in these studies relationship between the estrogen and performance. Perhaps the most dramatic study involved an experiment where we simulated a 2+ hour marathon-like competition in our laboratory. Instead of running a distance for time, we had women run until they reached exhaustion. In this study, the time until the women stopped running was nearly 10+ minutes longer when estrogen was elevated; that is, they would have covered a fixed distance that much faster. These findings certainly imply that the changes in estrogen levels during the menstrual cycle affect fat-carbohydrate utilization and potential stored fat losses occurring from exercise, as well as having a powerful positive ergogenic enhancement on endurance performance.
In these studies on estrogen we have typically measured circulating levels of fat and carbohydrate biomarkers of metabolism in the blood, hormones, respiratory gases that indicate metabolism rates, and taken muscle biopsies that indicate the degree to which carbohydrate is being utilized at the skeletal muscle. Additionally, when doing the performance-based studies we have typically performed running to exhaustion on a treadmill or biking to exhaustion on a cycle ergometer as our outcome measures (it’s challenging to mimic a fixed distance race event in a laboratory setting).
How do these findings impact the public?
These findings suggest that women that are endurance athletes may discover their performance times in events lasting over an hour may vary in relation to their changing estrogen levels throughout the menstrual cycle. This also means that the rate at which they lose/use body fat as an energy source when exercising may change across their menstrual cycle. This might mean that woman athletes with a normal menstrual cycle may want to select performance competitions based upon the menstrual phase. Additionally, they may need to be mindful that the issue of weight loss and body composition may not be consistently the same as they move through the different phases of their cycle.
One question we hope to pursue in future research is the role that oral contraception (OC) use might have to mitigate some of the effects of estrogen fluctuation across the menstrual cycle, and whether OC use itself is a potential ergogenic aid to women endurance athletes.