This is another excerpt from The Women’s Book, looking a bit at the cause of menstrual cycle dysfunction and, specifically, secondary amenorrhea. Even more specifically it looks at the cause of that dysfunction as it occurs in exercising women. Early theories revolved around body fat percentage although it would turn out that energy availability (EA) played a much larger role.
Since this is a short excerpt from a much longer chapter, I want to define a few terms so it will make sense.
When a woman is menstruating “normally” this is called eumenorrhea (“eu” = good). In contrast, the lack of a menstrual cycle is termed amenorrhea.
Primary amenorrhea is when the onset of menstruation (called menarche) is delayed. As frequently as not this occurs in females involved in sports such as gymnastics, ballet, ice skating and other sports that “emphasize thinness” (as the literature puts it). Their intense training and dieting simply delay the normal onset of menstruation.
Secondary amenorrhea is the loss of the menstrual cycle that occurs in women after menarche has already occurred. This can occur for many reasons but here I’m focusing on the amenorrhea seen in exercising women, an issue that started to occur more frequently as women started to really enter sport.
A related concept oligomenorrhea refers to an infrequency menstrual cycle, occurring every 35 to 90 days.
And yes this chapter addresses other related concepts, not just this one.Oligomenorrhea is an infrequent cycle, defined as a cycle occurring between 35+ and 90 days (the “normal” menstrual cycle runs from 24-32 days or so). Hyperandrogenism refers to women with relatively higher testosterone than normal.
Body fat, Energy Balance or Hormones
As I mentioned above, it was originally thought that some critical level of body fat was required for a woman to start menstruating. Nobody knew why this was the case, it was just an observation. The idea was simply that until she had enough body fat to safely become pregnant and carry a baby to term, her body would not become reproductively active and there is certainly some logic to that.
This was before the discovery of leptin so nobody really knew why this case the case even if we know now that until leptin hits a certain level, reproduction will not start (1). It’s interesting to note that in recent years the average age of the start of menstruation has gone down (girls are starting earlier) and at least part of that is thought to be related to increasing levels of body fat in children. They are hitting whatever critical point earlier in life.
Because of this observation, it was basically assumed that the system worked in the opposite direction, that when women dropped below a certain level of body fat, that was the trigger for them to lose their menstrual cycle. Given that amenorrhea was often seen in sports which require body fat to be lowered and which place a primacy on thinness (for either performance or aesthetic reasons) this seemed completely logical. It just happened to be wrong.
Female endurance athletes were frequently found to have lost their cycle and usually had a low body fat. Female physique athletes, due to the nature of their sport, often have the same issues. All of this seemed to implicate body fat per se as the problem and many still think this is true. And while body fat level does have an indirect effect but it’s not the level of fat per se that is the issue.
As I mentioned above, it was also long assumed for the longest time that oligomenorrhea and amenorrhea were just different degrees of the same situation but this no longer looks to be the case. The hormonal profile is different, the cause appears to be different (the hyperandrogenism that goes along with certain types of sports and the women that pursue them) and this means that the treatments will be different (2). Once again, the hyperandrogenic oligomenorrheic may still be a in sport which requires them to lose weight/fat (i.e. weight class or some endurance sports) and the eventual loss of the cycle can still occur.
In any case, it was also found that female endurance athletes with roughly the same levels of body fat either did or did not maintain their menstrual cycle and body fat levels weren’t an absolute determinant of whether or not a female lost her cycle or not. The normally cycling women did show what looked like subclinical effects on their menstrual cycle but it was still present. For whatever reason, they had escaped the loss of their cycle despite being of similarly low body fat levels.
Other studies found the same with the menstrual cycle shutting down at different body fat levels. Certainly the problem was more frequently seen at low levels of body fat but there is a big confound here: to reduce body fat levels to the extremes requires a great deal of calorie restriction and exercise. And it would eventually be shown conclusively that it was not body fat per se but energy availability that was causing the problems (3).
Energy availability (EA) is defined as the differences between energy intake and exercise energy expenditure. Basically it’s the calories that are left over for other biological processes. So consider a female eating 1600 calories/day who is performing 400 calories/day of exercise. She has an energy availability of 1200 calories. That is, her body has 1200 calories per day to allot to other biological processes.
And the basic idea is that, when those “left over” calories are limited, her body will have to prioritize some processes over others. Since some are required for life and others are not, the relatively less important ones (such as reproduction) are reduced or shut down completely. Since lean body mass (LBM) tends to be the most metabolically relevant aspect of physiology, EA is defined relative to LBM. So if the female above has 120 lbs of LBM, her EA is 10 cal/lb LBM.
And through a variety of studies, research found that when a woman’s energy availability dropped below 9.6-13.6 cal/lb LBM (20-30 cal/kg) her system would be negatively affected. Levels of active thyroid (T3) drop quickly (although the threshold for this is a bit lower, about 11.3 cal/lb LBM) and LH pulsatility is lost along with a host of other adaptations. And, as would be expected, this is related to the rapid drop in leptin levels that occurs when women are in a caloric deficit (4).
In support of this cutoff value, studies of amenorrheic compared to normally cycling athletes show that the first group is consistently below 13.6 cal/lb LBM (30 cal/kg LBM) while the normally cycling women are above that. As well, even bringing energy availability even slightly above the cutoff value (i.e. to 31 cal/lb LBM or 14 cal/lb) by either decreasing exercise or increasing energy intake eventually restores the menstrual cycle.