What's happening during the menstrual cycle?
The follicular phase (days 1-14) starts with menstruation and lasts until ovulation occurs. Hormones levels start to increase with estrogen peaking just before ovulation and progesterone levels starting to rise (see above).
When ovulation occurs, LH (luteinising hormone) and FSH (follicle -stimulating hormone) rapidly peak and progesterone rises further during the luteal phase. For more on the roles of these hormones, the Society of Endocrinology has a useful website.
Insulin sensitivity during the menstrual cycle
Over time I also repeatedly noticed a number of days during the month which were a nightmare for managing my sugar levels and I needed significantly more insulin throughout the day (+40% more!). This is the time when ovulation occurs. For the latter part of my menstrual cycle, the luteal phase, I am relative insulin resistant and tend to run a temporary basal rate of 140% for the mornings. This seems to be in accordance with research done on this subject in both female t1 diabetics and non-diabetics.
Scheduling training and racing
Ben Greenfield wrote a great blog about the topic of planning exercising around the menstrual cycle and different metabolic rates during the two phases (read here). During the follicular phase the body is more primed to use glycogen in form of carbs whereas in the luteal phase, the body's primary source of fuel is fat. This also explains why aerobic exercise is preferable during this phase whereas high-intensity training is preferably done in the follicular phase. This is because carbs are the body's only source of fuel for high-intensity exercise.
Together with my coach I began to schedule important races in the luteal phase of my monthly cycle. This included my first Ironman (read my blog here) followed by a number of middle-distance triathlons. We also looked at the type of training, as well as intensity and duration I was doing and adjusted everything in line with my menstrual cycle:
On a week-to-week basis my training volume is lowest during the week of my menstruation when insulin sensitivity is very high. This is my recovery week in which I tend to do low-intensity and shorter training. For the remaining three weeks training starts with shorter, high-intensity sessions which gradually decrease and are replaced by lower intensity, higher volume training. During last week of my cycle it is particularly that I notice I am able to race and train very well on few carbs.