Menstruation and Athletic Performance

Menstruation is one of the most natural aspects of life. Yet, it often gets unnecessarily criticized and unfairly portrayed as a performance-blocking monster. While it can certainly affect performance in many cases, it can also be used to your advantage. The question isn't whether the cycle matters, but how much it matters and how you can apply that knowledge to your training. Individual variations are significant, but with a better understanding of your body's signals, you can make smarter decisions, optimize your training, and minimize the risk of unnecessary performance dips. 

This article doesn't focus on individual experiences but provides the available knowledge so you can make informed decisions when planning your training. As a bonus, we have doctor and triathlete Sara Svensk featured in the podcast episode for more practical tips and experiences. 

Estrogen levels rise significantly in the last week before ovulation, peak at ovulation, then decrease. Estrogen enhances insulin sensitivity and improves glycogen storage, which is interesting during carbohydrate loading. Estrogen also increases glucose uptake in type I muscle fibers during shorter, more intense efforts, which is beneficial in high-intensity aerobic sessions like threshold runs and intervals.

Progesterone has the opposite effect: it can decrease insulin sensitivity and reduce glucose uptake and its oxidation in muscle fibers during intervals or thresholds. This is unfavorable when intensity is high and glucose is the main fuel.

Estrogen also increases lipolysis and the use of free fatty acids during activity. However, the area is poorly researched. There are indications that the luteal phase—when both estrogen and progesterone are elevated—may be favorable for endurance sports, but the research is not yet conclusive, and more studies are needed.

The findings of menstrual phase studies may be confounded by the high variability in the concentrations of ovarian hormones between subjects and from day to day within subjects during any particular menstrual phase. For this reason, investigating the relationships between metabolic and exercise performance parameters and the change in the ovarian hormone concentrations between the menstrual phases and/or the E/P ratio should increase the sensitivity of studies for identifying metabolic or performance changes caused by the naturally cycling ovarian hormones.

Overall, estrogen primarily acts to prepare the body for a possibly fertilized egg, by storing more glycogen and ensuring energy reserves. A practical tip from the above: schedule more long-distance sessions during the luteal phase and more high-intensity training just before ovulation. Research also suggests a so-called "The Sweet Spot" for endurance performance, approximately 1–2 days before ovulation.

Moreover, there may be a "sweet spot" a day or two before ovulation where a spike in estrogen leads to optimal endurance performance. Oosthuyse and Bosch found some hints of this effect in a cycling time trial study.

The quality of iron is crucial, and absorption varies significantly. The absorption of non-heme iron from plants ranges from 2–5 percent, while heme iron from animal sources is absorbed at 10–35 percent. Eating animal products alongside plant sources improves the absorption of non-heme iron due to the effect of heme iron. Vitamin C enhances iron absorption, whereas calcium, polyphenols (found in tea, coffee, wine), and phytic acid (in grain husks) inhibit it. That's why you had orange juice instead of milk with black pudding at school — the vitamin C from a glass of orange juice can triple the absorption of non-heme iron. (Ref: Exercise Physiology 7ed, McArdle & Katch).

In a study included in the review, researchers measured body weight and fluid levels daily over 28 days in women of childbearing age. They found that participants were heaviest and most fluid-filled on the last days of the luteal phase and the first day of menstruation. This was followed by a drop in body weight with the onset of menstruation. A brief peak in body weight due to fluid was also noted around ovulation.

However, it's important to note that most studies have not been able to show differences in body weight over the cycle when controlling for energy intake, salt intake, and fluid intake — something the above study did not do. It is, therefore, possible that the feeling of fluid retention in many cases is due to a changed dietary pattern. One hypothesis is that hormone levels alter the distribution of fluid in the body rather than the total amount of water, which can make one feel puffy in certain areas during certain parts of the cycle and thus experience an impact on performance.

In terms of strength, there is nothing indicating that the ability to develop maximal power is affected by menstruation. When it comes to the body's handling of lactate, the research is more varied: studies use different protocols and dietary controls, which influence the results. Two studies with strict dietary controls found no impact on participants' lactate levels, regardless of the cycle phase. The conclusion is that the impact is minimal at best or difficult to measure.

The same applies to maximal oxygen uptake (VO2max) — there seems to be no clear impact from menstruation.

Here we find one of the most noticeable effects. On average, body temperature increases by 0.3–0.5 degrees during the luteal phase (after ovulation) due to higher levels of progesterone compared to the follicular phase. The increased body temperature also affects the heart rate at a given intensity: it's estimated that the heart rate increases by about 7 beats/min for each degree higher core temperature. An increase of 0.3–0.5 degrees means approximately 2–3 extra beats per minute at the same intensity. In most studies, this is too small to show statistical significance, but for you as an athlete, it can be clearly felt, especially when it comes to perceived exertion.

This can be particularly noticeable during interval training where you push hard for 20 minutes, or if you're competing in longer distances in a hot climate—this is something to keep in mind.

An alternative explanation for this mid-luteal phase increase in heart rate may be based on the increased body temperature at that time. Increased heart rate has been shown to occur with increased body temperature, at a rate of 7 beats/min for each 1°C rise in core temperature.

Your heart rate can vary during different phases of your cycle at a given intensity — partly due to changes in substrate conditions, and partly due to an increased body temperature of about 0.5 degrees during the luteal phase. Know your cycle: if you know where you are in your cycle, you can adjust your intensity and eventually your race pace accordingly.

A menstrual cycle isn't always 28 days, that's an average; cycle length usually varies between 23–36 days. Many regulate their cycle more precisely with the help of contraceptives. We chose to omit the contraceptive aspect in this article, partly because it would make the text much longer, and partly because it would add another complex factor on top of everything else.

To summarize the main points from the article in bullet form:

• If there is an optimal performance window, it is 1-2 days before ovulation
• Schedule longer, more intense sessions during the Follicular phase when body temperature isn't elevated
• At a sufficiently long distance where you can benefit from better fat oxidation, it's better to train/compete the week after menstruation and just before ovulation. Your ability to use ingested carbohydrates is, however, impaired, so it's essential to have well-filled glycogen stores before this phase and know that your carbohydrate oxidation may be slightly lower. But in return, your fat oxidation is elevated, so over longer distances (Marathon and beyond), you can likely benefit from the glycogen-sparing effect.

For those interested in reading more about contraceptives and their impact on sports performance, we found a bachelor thesis that covers the topic. We haven't reviewed it ourselves, and it is not a peer-reviewed article, but it's a good thesis in Swedish that provides insights into how contraceptives might affect performance. References are available for further reading if you're extra nerdy. ;)