NEW YORK (Reuters Health) - Using oral contraceptives to manipulate the menstrual cycle will likely have no effect on athletic performance, a new study of female cyclists and triathletes shows.

Variations in levels of the hormones progesterone and estrogen that occur during the menstrual cycle may influence women's aerobic exercise response, Dr. Claire Rechichi of the Western Australia Institute of Sport in Perth and her colleagues note. However, such effects are difficult to study because hormone levels throughout the cycle vary widely from person to person, they add.

To provide a more standardized measure of the effects of hormones on athletic performance, Rechichi and her colleagues conducted a number of physiological tests in 13 trained female athletes taking an oral contraceptive that was monophasic, meaning it delivered the same amount of hormones every day. Women underwent the tests after they had been taking the pill for 13 to 17 days; 2 or 3 days after they stopped taking it; and again s6 to 7 days after stopping the pill.

The researchers hypothesized that the athletes would perform better while taking the pill, because the hormones would push them toward greater fat metabolism, reduced depletion of glycogen stores, and would also result in lower levels of blood lactate, a byproduct of strenuous exercise that contributes to muscle fatigue.

But the women's performance on a 1-hour cycling test that measured power output, oxygen consumption, blood glucose level, and heart rate was the same at each of the three test points. However, women's breathing rates were increased and they consumed oxygen less efficiently while they were taking the pill.

While this didn't affect performance in the current study, Rechichi and her team say, it is not clear whether this respiratory inefficiency might increase fatigue and worsen performance with longer or more intense exercise.

Contrary to the researchers' hypothesis, the athletes' blood lactate levels were actually slightly higher while they were taking oral contraceptives. Low blood lactate levels mean that the body's tissues aren't getting enough oxygen.

Women also reported higher levels of perceived exertion while taking the pill, but the difference was small and was not likely of physiological significance, the researchers say.

"Female athletes taking a monophasic oral contraceptive do not need to be concerned about the timing of their cycle with regards to optimized endurance performance and competition," the researchers write.

Nevertheless, they conclude, variations in respiratory efficiency and lactate levels "should be considered when interpreting physiological test results."

SOURCE: International Journal of Sports Medicine, April 2008.

• More News on Birth Control
• More News on Sports Fitness

Related MedlinePlus Pages:

• Birth Control
• Sports Fitness