Leaving aside bad carb counting and having ruled out the typical things that could potentially cause blood sugar chaos, such as illness or emotional stress, a woman's menstrual cycle and the hormonal changes involved in it can have a huge impact on managing type1 diabetes. I didn't really think it would have such an impact until I started seeing a pattern as to when levels were higher than normal "without obvious reason" and started to cross-reference this to certain phases during my monthly cycle.
What's happening during the menstrual cycle?
Without going into all the details of the complex menstrual cycle, I want to briefly explain its different phases because they can impact on insulin sensitivity and can give away clues on how to manage t1 diabetes.
The menstrual cycle last about 28 days and is split in two phases, the follicular and the luteal phase. Latter starts with ovulation which occurs around day 14 so about mid-way through your 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.
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
With the various hormones playing their part, insulin sensitivity can vary significantly during the menstrual cycle. I noticed being very sensitive to insulin and therefore more prone to hypos during my menstruation. My observation is in line with a number of studies I found which conclude that insulin sensitivity is greater during the follicular phase (day 1-14) of the menstrual cycle (see here for one study on this topic).
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.
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
When I did my first long endurance event in 2011, which was the 180km bike-leg of Challenge Copenhagen as part of a relay, it was sheer coincidence that the race happened to take place towards the end of my menstrual cycle. I raced really well without having to take on a significant amount of carbs, which really surprised me. Over time I completed other races which took place during earlier stages in my monthly cycle and I seemed to require a lot more carbs to keep blood sugar levels up. During training I noticed that towards the second half of my menstrual cycle, I was able to do long-endurance based training on few carbs without hypoing.
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.
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.
Varying insulin sensitivity leads to adjusted training volume during the menstrual cycle
Taking advantage of hormonal changes
Although I haven't found a miracle solution to avoid chaotic blood sugar levels during certain times of the month, I use the hormonal changes which affect the management of my t1 and my training to my advantage: By adjusting training duration and intensity and scheduling races on days when I am less insulin sensitive, I have seen some highly positives outcomes for maintaining stable sugar levels and getting some quality training and racing done without the struggle of keeping sugar levels at bay. Equally, during my highly insulin sensitive phase, (my menstruation) when I am very prone to hypos which makes exercising quite difficult and frustrating at times, I am reducing the overall training volume and intensity and making it my recovery week.
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