A few minutes into your high-intensity intervals, your legs feel sluggish. You shrug it off as just a bad day at the gym. During the same workout the following week, you're flying high and feel like the treadmill belt can't keep up with you.
Why the difference? You ate the same Wheaties breakfast both times, you slept the same amount the night before, you even wore the same lucky underwear. The answer may be your hormones.
Estrogen is a woman's primary sex hormone and it distinguishes you from that grunting guy doing bench presses on the other side of the gym. It influences everything from metabolism to muscle glycogen storage to bone health. It's so important to bones that its deficiency—often caused by irregular or absent menstruation from a high level of training and not consuming enough calories—is the biggest risk factor for osteoporosis in active women.
Your hormonal environment constantly changes and the levels of estrogen and its sister hormone progesterone fluctuate throughout your menstrual cycle. During the first two weeks—the follicular phase—of your cycle, your body is dominated by estrogen; the second two weeks—the luteal phase—begins with ovulation and is dominated by progesterone, although estrogen is also elevated in the middle of the luteal phase. At the end of that phase, you start your period and re-start the cycle.
So what does all this mean for your workouts? In general, you can expect to feel better and have better workouts when estrogen is high, and feel the worst during your period and when progesterone dominates. But there's a lot of variability between women.
"While some women may have few noticeable effects during their cycle, others may notice fatigue, difficulty working out, cramping, bloating, and increased perception of effort, particularly in the days leading up to and the first few days of their period," says Carolyn Smith, executive director of the Marquette University Medical Clinic and medical director for the Department of Intercollegiate Athletics and the Athletic Training Educational program and co-author of Running for Women.
This is because of the changes of your hormones. Smith advises keeping a menstrual log to determine how your menstrual cycle affects you and your training.
Perhaps the most significant effect of estrogen is a shift in your metabolism when you run.
"Estrogen enhances fat use, which spares glycogen," Smith says. "Because your ability to run for long periods is influenced by the amount of glycogen in your muscles, by sparing the amount used and relying more on fat for energy, fatigue is delayed and your endurance is improved."
And because your muscles use less carbohydrate during running, they also use less protein, since protein only provides significant energy for muscle contraction when glycogen is low. To capitalize on this shift in your metabolism, run more when estrogen is high.