Why is it that some women have no period, where others, in seemingly the same situation with exercise and energy consumption continue to menstruate regularly?
A possible explanation for this difference was recently offered by researchers in Sweden. Instead of looking at energy balance or availability for a whole day, as is the standard in energetic research, they computed energy balance on an hourly basis.
Before we get into the meat of the paper, it’s results, and my analysis, a few terms that might be helpful to understand:
- energy balance: Total energy intake (kcal) minus total energy expenditure, i.e., resting metabolism + daily living + exercise.
- resting metabolism: calories burned to fuel involuntary processes: pumping blood, fueling brain, building muscle and bone
- glycogen: stores energy in the liver for short term use, ~300 kcal worth (Farenholtz et. al, 2017)
- kcal: short for kilocalorie, measure of energy. In the US we usually say “calorie” instead.
- negative energy balance: more calories expended than consumed; during small deficits, liver glycogen can make up the difference. During larger deficits (e.g., < -300 kcal), fuel is obtained from other body stores, for example, fat and muscle (called catabolism).
Alright – getting back to the paper. The findings were remarkable; the average time with a negative energy balance of < -300 kcal for athletes with regular periods was 17.6 hours (Interquartile range (IQR) 3.9 – 20.8 hours), versus 21.8 hours in athletes with no periods (IQR 17.8 – 22.4). Athletes with no periods were in a catabolic state (negative energy balance) for four more hours per day than athletes with regular periods.
What I found really interesting was a diagram the researchers included, illustrating how hourly energy balance was calculated. This example shows a period of significant energy deficit at night, with no positive energy balance until the middle of the day (presumably lunchtime). Thinking of energy balance on an hourly basis like this rather than simply the amount of energy consumed in a day leads me to wonder if this might be part of the reason some women can be “all in” for 6+ months without period restoration, where others resume cycles within just 6-12 weeks. Continue reading