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Why is it that some people have no period, where others, in seemingly the same situation with exercise and energy consumption continue to menstruate regularly?

(I’ve been told this post is very “science-y” – if you’d like some help with interpreting and applying to your situation, schedule a call with me! Also, there is a lot of specific calorie discussion in this post. If that is likely to be triggering for you, the take home message is that the best way to recover a missing period is to provide your body with the energy it needs throughout the day.)

A possible explanation for this difference was offered by researchers in Sweden, Farenholtz et al., "WITHIN-DAY ENERGY DEFICIENCY AND REPRODUCTIVE FUNCTION IN FEMALE ENDURANCE ATHLETES." The really fascinating and novel contribution of this paper is that 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, its results, and my analysis, here are 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 - the things your body needs to do to keep you alive: 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 surprising; the average time with a negative energy balance of < -300 kcal for athletes with amenorrhea in this study was four more hours per day than athletes with regular periods. This negative energy balance means that their bodies are in an energy deficit and have to obtain fuel from body stores.

What I found really interesting was a diagram the researchers included, illustrating the hourly energy balance. 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 people can be “all in” for 6+ months without period restoration, where others resume cycles within just a few months.


Reading this paper and the results led me to explore this a possible explanation for why some people restore periods after only a month or two of being "all in" (as described in No Period. Now What?), where others might take six months or longer (and of course, there is the usual caveat that there is no one-size-fits-all recovery, so I’m sure this is not true for everyone!).

I examined some hypothetical within-day-energy-balance graphs. I started by trying to recreate the graph from the paper based on numbers provided for the participants. The information I used was:

  • predicted resting metabolism (63 kcal/hour, 16 waking hours)

  • predicted sleeping metabolism (57 kcal/hour, 8 hours sleep as described in the paper)

  • daily non-exercise activity thermogenesis (NEAT – basically daily living activities, 446 kcal/day = 28 kcal/hr during waking hours)

  • diet-induced thermogenesis (DIT – the energy it takes to digest consumed food, ~10% of calories consumed, in the hour after consumption)

  • exercise as needed to fit the graph (25 kcal / hr between 7 and 9 am, 500 kcal exercise at 5:30 pm)

  • exercise post-oxygen consumption (EPOC, 8.6% of cal burned through exercise, 2/3 in the first hour post-exercise, 1/3 in the second hour)

And here’s my result:

This agrees with the graph from the paper to a large degree, with around 7 hours in the day with energy balance (EB) < 300 kcal, around 5 hours with a positive energy balance, and 12 hours in negative energy balance (between 0 and – 300 kcal) that would in theory be compensated for by glycogen stores.

To accomplish this, I included the following activities during this hypothetical day:

  • Sleep: 10 pm – 6 am

  • Wake at 6 am

  • A bit of extra walking beyond what was encompassed in NEAT, 25 kcal / hr, from 7 am – 9 am

  • Breakfast between 9-10 am of 850 kcal (note the large increase in EB at that time)

  • Snack of 80 kcal at ~10:30

  • A long lunch of 850 kcal between 12:30 – 1:30 pm

  • Coffee, 200 kcal, just before 3 pm

  • Going to the gym, 500 kcal between 5 – 6 pm (sharp decrease in EB)

  • Dinner of 500 kcal between 6 – 7 pm

  • Snack of 100 kcal just before bed at 10 pm

  • The only way I could explain the increase in EB at the end of the paper’s graph was that the subject got up from bed to have a “midnight snack” of 225 kcal.

  • Total caloric intake = 2805 kcal, total expenditure including all factors listed above = 2805 kcal, daily EB = 0.


Now we get to the interesting part. I wanted to see what the same graph would look like for the amenorrheic athletes. The paper specified that the average maximum deficit was -3181 kcal: almost 2500 kcal lower than in the example above! And these women were spending on average only one hour a day with a positive energy balance – so their within-day energy balance would look a bit more like this:

Note only one hour with a positive energy balance, and ~12 hours with an energy deficit < 300 kcal.

The major difference here versus the graph above was exercise in the morning prior to eating anything, a slightly larger lunch, and a larger dinner. This caloric increase mostly but not quite offset the higher amount of exercise.

Looking at this graph, it’s easy to understand why these athletes lost menstrual cycles. Hormones take fuel to create; these bodies are conserving every bit of fuel they can!


Looking at energy balance in this way led me to thinking about within-day energy balance as a potential difference in those who are “all-in” (in these examples, eating 2500 kcal / day with planned walking for exercise, and a daily positive energy balance of ~200 kcal – this would go toward things like needed repair, bone density increase, added fat storage). Here’s the next example graph:

In this example, the person eats (please do not get hung up on these numbers, I’m including them simply so you can understand how the examples are the same and where they differ) a breakfast of 500 kcal within an hour of waking. Two snacks of 100 cal between 9-10 and 10-11. A 700-kcal lunch at around 12:30, a 200-kcal coffee just before 3, then a walk at around 4:30 that expends 250 kcal. Next is dinner at 7pm (700 kcal), a 200-kcal snack just before bed, and sleeps through the night :). As a reminder, I do NOT recommend ongoing calorie counting!!

Energy balance is nicely in positive territory throughout the day, and never gets into the < -300 kcal territory.

Here’s a very different example – but note the overall energy intake and energy balance are the same as in the above graph:

This shows the hourly energy balance for someone eating and exercising the same amount as in the previous example; 2500 kcal / day of food and drink, and walking for ~45 minutes. The big differences a walk in the morning, before eating breakfast, and no breakfast until getting to work at around 9:15. Nothing else is consumed until a 500-kcal lunch three hours later. Coffee in the early afternoon, 250 kcal, and then a 250-kcal snack at 4:30. Dinner is between 6 – 7 pm, and this person is used to "saving calories" to eat a larger meal so has 1000 kcal. This last meal bumps nicely into positive energy balance territory… but notice how, contrary to the previous example, there are almost 5 hours in the day with an energy balance below -300 kcal, and another 9 hours between 0 and -300 kcal. It’s hard to overcome the big deficit from the morning, from both overnight metabolism, and exercise without eating beforehand. Even though the total number of calories in and out, and daily positive energy balance is the same as the previous example, you can imagine how this body is not feeling nearly as “safe.”


Based on all this, I recommend that when you are working on recovery, you should try to adjust caloric intake to lessen time with an energy deficit throughout the day. I would strongly suggest eating a good breakfast within an hour of getting up in the morning, and certainly before any planned exercise. Consume meals and snacks throughout the day rather than having fewer, larger, meals. Perhaps eat just before going to bed to provide energy for the work your body is doing while sleeping.

What do you think of this theory? If you have recovered your cycles, what does your within-day energy balance looks like? If you have not recovered cycles, does this ring true for you? Are there changes you could make to keep your energy balance in positive territory for more of the day?

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