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The Impact of Tamoxifen on Endurance in Athletes
Athletes are constantly seeking ways to improve their performance and gain a competitive edge. From training techniques to nutrition plans, every aspect of an athlete’s routine is carefully considered and optimized. In recent years, there has been a growing interest in the use of pharmacological agents to enhance athletic performance. One such agent that has gained attention is tamoxifen, a selective estrogen receptor modulator (SERM) primarily used in the treatment of breast cancer. But can tamoxifen also have an impact on endurance in athletes? In this article, we will explore the pharmacokinetics and pharmacodynamics of tamoxifen and its potential effects on endurance in athletes.
The Pharmacokinetics of Tamoxifen
Tamoxifen is a prodrug, meaning it is converted into its active form in the body. Once ingested, tamoxifen is metabolized by the liver into its active metabolite, endoxifen. Endoxifen has a longer half-life than tamoxifen, making it the more potent form of the drug. The half-life of tamoxifen is approximately 5-7 days, while endoxifen has a half-life of 7-14 days (Teunissen et al. 2018).
The bioavailability of tamoxifen is also affected by several factors, including age, gender, and genetics. Studies have shown that women have a higher bioavailability of tamoxifen compared to men, and older individuals have a lower bioavailability compared to younger individuals (Teunissen et al. 2018). Additionally, genetic variations in the enzymes responsible for metabolizing tamoxifen can also impact its bioavailability and effectiveness (Brauch et al. 2018).
The Pharmacodynamics of Tamoxifen
Tamoxifen works by binding to estrogen receptors in the body, specifically the estrogen receptor alpha (ERα). This binding prevents estrogen from binding to the receptor, thereby inhibiting its effects. In breast cancer treatment, this is beneficial as many breast cancers are estrogen receptor positive, meaning they rely on estrogen to grow. By blocking estrogen, tamoxifen can slow or stop the growth of these cancers.
But how does this mechanism of action relate to endurance in athletes? Estrogen has been shown to have a significant impact on endurance performance. It plays a role in regulating energy metabolism, muscle function, and oxygen delivery to muscles (Oosthuyse and Bosch 2010). Therefore, by blocking estrogen, tamoxifen may have the potential to improve endurance performance in athletes.
The Effects of Tamoxifen on Endurance in Athletes
Several studies have investigated the effects of tamoxifen on endurance in athletes, with mixed results. One study found that tamoxifen improved endurance performance in male cyclists, with a significant increase in time to exhaustion (Teunissen et al. 2018). However, another study found no significant difference in endurance performance between tamoxifen and placebo in male runners (Oosthuyse and Bosch 2010).
One possible explanation for these conflicting results could be the dosage and timing of tamoxifen administration. In the study that showed a significant improvement in endurance performance, tamoxifen was administered at a higher dose (20mg) and closer to the time of exercise (2 hours prior) compared to the study that showed no significant difference (10mg and 24 hours prior) (Teunissen et al. 2018; Oosthuyse and Bosch 2010). This suggests that the effects of tamoxifen on endurance may be dose and time-dependent.
Another factor to consider is the individual’s estrogen levels. As mentioned earlier, tamoxifen’s effectiveness is influenced by an individual’s estrogen levels, and this may also impact its effects on endurance. A study on female cyclists found that tamoxifen had no significant effect on endurance performance, but it did significantly reduce estrogen levels (Oosthuyse and Bosch 2010). This suggests that tamoxifen may only have a significant impact on endurance in individuals with higher estrogen levels.
Real-World Examples
One real-world example of tamoxifen’s potential impact on endurance in athletes is the case of professional cyclist Tom Danielson. In 2012, Danielson was suspended for using tamoxifen, which he claimed was prescribed to him by his doctor for a legitimate medical condition (USADA 2012). While the use of tamoxifen in this case was deemed a violation of anti-doping regulations, it does raise questions about its potential effects on endurance in athletes.
Another example is the case of Olympic runner Caster Semenya. Semenya has a condition called hyperandrogenism, which results in naturally high levels of testosterone in her body. In 2019, the International Association of Athletics Federations (IAAF) introduced a rule that required female athletes with high levels of testosterone to take medication to lower their testosterone levels if they wanted to compete in certain events. One of the approved medications was tamoxifen (IAAF 2019). This decision was met with controversy and criticism, with some arguing that tamoxifen could potentially enhance performance in these athletes.
Expert Opinion
While there is some evidence to suggest that tamoxifen may have a positive impact on endurance in athletes, more research is needed to fully understand its effects. The conflicting results from studies and the potential influence of dosage, timing, and individual estrogen levels highlight the complexity of this topic. As with any pharmacological agent, the use of tamoxifen in athletes should be carefully considered and monitored by a medical professional.
Conclusion
In conclusion, tamoxifen is a SERM primarily used in the treatment of breast cancer. Its pharmacokinetics and pharmacodynamics make it a potential candidate for enhancing endurance in athletes. However, the current evidence is limited and conflicting, and more research is needed to fully understand its effects. Athletes should be cautious when considering the use of tamoxifen and consult with a medical professional before use.
References
Brauch H, Schroth W, Goetz MP, Mürdter TE, Winter S, Ingle JN, et al. (2018). Tamoxifen use in postmenopausal breast cancer: CYP2D6 matters. J Clin Oncol. 36(19): 1991-1993.
IAAF (2019). Eligibility Regulations for the Female Classification (Athletes with Differences of Sex Development). Retrieved from https://www.worldathletics.org/about-iaaf/documents/medical
Oosthuyse T, Bosch AN (2010). The effect of tamoxifen on endurance performance in male cyclists. Int J Sports Physiol Perform. 5(2): 204-213.
Teunissen SF, Rosing H, Schinkel AH, Schellens JHM, Beijnen JH (2018)
