Immunity and the menstrual cycle: what all female athletes should know!

Andrew Hamilton looks at research suggesting the phase of the menstrual cycle can significantly impact post-exercise immunity in female athletes. Why is this the case and what are the implications?

Numerous studies have demonstrated that exercise is beneficial for fitness and health and (In the longer term at least), it also boosts immunity. The caveat however is that immediately after training (especially long or hard sessions), your immune system becomes temporarily depressed, increasing the risk of coughs, colds, sore throats and other upper respiratory tract infections (URTIs). One theory to explain this phenomenon is that the increase in the body’s core temperature produced by exercise temporarily interferes with the normal functioning of the body’s immune cells, increasing the risk of infection and illness. This has implications for women: due to hormonal changes, the body’s core temperature fluctuates during the menstrual cycle, which begs the question: are exercising women more at risk of URTIs at certain times of the month?

Immunity, cycling and time of the month

To try and answer just this question, Japanese scientists looked at the immune responses in women who undertook cycling trials at different times in their menstrual cycle (no pun intended!). These were the follicular phase (typically around 5-14 days after the start of menstruation) and the luteal phase (typically around 14-28 days after menstruation). They also investigated whether giving carbohydrate during exercise affected the immune responses in any way. Six healthy but untrained women completed four separate cycling trials. Each consisted of 90 minutes of cycling at 50% peak aerobic power (easy intensity) followed by a high intensity time-trial performance test, which lasted 10 minutes. The four trials were however conducted at two different points in the menstrual cycle and were as follows:

  • Follicular phase consuming a carbohydrate drink
  • Luteal phase consuming a carbohydrate drink
  • Follicular phase consuming a carbohydrate-free (placebo) drink
  • Luteal phase consuming a carbohydrate-free (placebo) drink

In all four trials, the ambient temperature was maintained at a very warm 30C – the reason being that the added heat stress would make any core temperature changes more pronounced and therefore make it easier to detect changes in immune function.

Effects on core temperature

The first main finding was that when the women were in the luteal phase of their menstrual cycle, their core temperatures were higher both before and during exercise than when they were in the follicular phase. A second finding was that compared to the follicular phase, when the women were in the luteal phase of their cycle and drank the placebo beverage, the exercise trial resulted in a larger rise in immune cells called leucocytes – basically, a measure of how much stress the immune system is under (see figure 1). High post-exercise levels of blood leucocytes indicate compromised immunity and increased risk of an URTI. However, when the women had consumed the carbohydrate drink while performing the luteal phase trial, the rise in leucocytes was not nearly as great – ie consuming carbohydrate during exercise helped to reduce the amount of immune stress that resulted.


Figure 1: Immune stress after training

Circles = luteal phase; triangles = follicular phase

Solid circles/triangles = consuming carbohydrate; open circles/triangle = no carbohydrate


Implications for female athletes

Previous studies have already demonstrated the link between immune stress and exercise – for example there’s a substantial increase in leukocyte concentration during endurance exercise, and this increase depends on the intensity and duration of exercise(2). We also know from studies on male subjects that consuming carbohydrate during exercise suppresses the rise in leucocytes, thereby helping to prevent a post-exercise dip in immunity(3). Although this study was small and more data is needed, what it indicates is that women exercising during the luteal phase may be especially at risk of a post-exercise immune dip. Based on their findings, the researchers also recommended that women should consume carbohydrate drinks when doing hard/long training sessions during the luteal phase of their cycle.

References

  1. J Int Soc Sports Nutr. 2014 Aug 12;11:39
  2. Nutrients. 2012;4:1187–1212
  3. J Sport Nutr Exerc Metab. 2002;12:145–156

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