Women who have missing periods often question why their cycles aren’t returning when they get to a weight at which they formerly cycled, or at the same exercise level they were at when previously cycling.
The reason is that it takes more of an energy surplus / exercise decrease to get the hypothalamus to turn back on than it does to keep going once it has restarted. It reminds me a lot of the activation energy required to start a chemical reaction.
The amount of energy required to get the reaction to run is greater than the difference in the energy of the reactants (starting material) and products (ending material) because the chemicals involved need to meet in the correct orientation and with enough energy for the reaction to take place.
Similarly, it takes extra fuel, maybe a little more weight gain than where you cycled at previously, to restart your hypothalamus than it does to keep it running once started. It takes cutting high intensity exercise to a lower level than your exercise when previously cycling. It takes being more relaxed.
This is, in a sense, for the same reason as the activation energy for a chemical reaction. New neurons (nerve cells) need to be created, there are changes in the receptors that are expressed, changes in the hormones that are output by the hypothalamus. It is harder for the hypothalamus to start a new hormonal pattern than to maintain a pattern. So it often takes eating more than you think you need, getting to a weight higher than you might want to, cutting your exercise more, to break out of the old pattern and start a new one.
It is really important to note that once cycles have commenced, we recommend waiting until you’ve had three periods before making changes to exercise or eating to allow the new patterns to establish. We never suggest going back to old ways of restricting food intake (limiting yourself to a set amount each day and/or avoiding certain food groups) and overexercise. But once your hypothalamus is chugging along you can slowly add back some high intensity exercise (making sure to note any effects on your cycle – some people are more sensitive to exercise than others) and can follow hunger cues as long as doing so does not cause significant weight loss (many do settle at a weight slightly lower than what was needed to get cycles to return in the first place.)
Hopefully knowing this will help you to make the changes you need to and stay consistent, so that you get your cycles back for good. And our book can help!