Of all the difficulties I encounter when helping patients, no challenge is tougher than helping type 1 individuals to manage their blood glucose levels around exercise.
When referring to type 1 diabetes and exercise I am specifically referring to those who are still following a basal bolus regimen (long duration insulin once or twice daily with a rapid acting insulin administered at meals) and do not have the added benefit of pump therapy where they can adjust their insulin levels minute by minute.
Why so difficult
The major difficulty in type 1 diabetes and exercise arises because once insulin has been administered it is in the system until it has run it’s natural course. Therefore, if you have got it wrong, you will either suffer a hypoglycaemic or hyperglycaemic episode.
Adding to this difficulty, during exercise the body does not need insulin to transport glucose into the cells of the body. Transporters known as GLUT – 4 – receptors are able to transport glucose from the blood into the cells where it is needed independent of insulin. Therefore, if you have given an insulin dose which usually would translate into good control, the added action of the GLUT – 4 – receptors may cause your blood glucose level to drop more than usual.
The different types of exercise
On top of this already difficult balance is the different types of exercise having different effects on your blood glucose levels.
We can broadly split exercise into two classes from a physiological perspective.
The first is known as aerobic exercise commonly known as cardio. Aerobic literally means with oxygen. Aerobic activity is usually performed at intensities up to 80% maximum capacity although this number varies depending on your fitness levels. When you are in your aerobic zone you are able to sustain this for some time. Examples include jogging, swimming, walking, gardening, etc.
When you exercise your body produces by-products, one of which is lactate or as you may know it lactic acid. Without getting too scientific, when you are in a comfortable aerobic zone, the oxygen you breath combines with the lactate to produce more energy for you muscles to continue to exercise.
As intensity increases more lactate is produced meaning more oxygen is required to produce more energy. Eventually, you will hit a point where you are unable to breath in enough oxygen to match the lactate. At this point, the excess lactate is broken down into acid resulting in a large increase in your breathing and the discomfort you feel. This is known as your lactate threshold.
Athletes like Mo Farrah have a very high lactate threshold and can remain in their cardio zone up to very high intensities i.e. >90%. Hence why Mo can sustain 20km/hr for an entire marathon. If you’ve ever taken a long time out of exercise and jumped back in at very high intensities and felt sick, it this mechanism at work.
When you pass this lactate threshold you enter our other form of exercise. This is known as anaerobic exercise. This means without oxygen. Anaerobic exercise cannot be sustained for very long. Typically under 2 minutes because the demand on your oxygen supply is too great for your breathing to meet it. Examples include sprinting, sports with quick bursts of activity, HIIT or weight lifting.
Aerobic and anaerobic activities are relevant because they have different effects on your blood glucose levels. Aerobic, which is typically slow and steady, will gradually deplete your glucose levels from your muscles, liver and circulating blood glucose. It will do this through your GLUT – 4 – receptors. Those individuals without diabetes will have their insulin supply switched off during periods of exercise to allow the GLUT – 4 – receptors to do their thing. Then once activity is stopped and you begin to recover the body will sense how much insulin it needs to release from the pancreas. Those with type 1 diabetes do not have this benefit. You will have your background insulin on board anyway (and it’s important you take this and do not withhold it) and possibility some rapid acting insulin from earlier meals. As a result, you are prone to hypo’s.
Furthermore, because aerobic activity drains your body of glucose your supplies later in the day may be impacted leaving you prone to hypo’s much later in the day or even into the next day.
Anaerobic exercise will also deplete your glucose stores but has one major difference. Due to it’s high intensity nature, anaerobic exercise causes your body to release stress hormones such as adrenaline and cortisol. This is a good response for burning fat and building muscle but stress hormones push up blood glucose levels. Therefore, with anaerobic activity you may find you see an initial spike in your blood glucose levels followed by a later decline.
Hopefully you can see how this can be tricky and there is no definitive answer as to how to manage this. However, there are general principles which you can follow.
Lets start with aerobic. For planned activity you may find reducing your rapid acting insulin at the meal prior to and subsequent meals later in the day by 10-50+%. How many meals you need to reduce your dose for will depend on the type, duration and intensity of the activity. For unplanned activity it is wise to reduce your rapid acting insulin only at later meals.
If going for an hour walk, you may find reducing only at the next meal is necessary whereas if going for a 50 mile bike ride, you may find reductions at all meals and even into the following day are required.
If having very high training volumes you may find a reduction in your basal insulin is required. This will only become apparent if you are reducing or even withholding your rapid acting at all meals but still experiencing low blood glucose levels. Additionally if you are dropping low overnight or between meals when no rapid acting insulin is on board, this may be another indication your basal insulin is too much.
Anaerobic activity is trickier. Patients I have worked with have found potential split doses useful where they take half of their dose with pre exercise food and half directly following the activity to reduce the high blood glucose level. If experiencing high levels of activity, some people consider splitting their basal insulin taking different doses morning and evening. However, this needs to be done alongside your diabetes specialist as some insulins should not be split.
Others may take a small correction dose at half time if playing sports or during the activity if possible and noticing their blood glucose levels are increasing.
Another trick is to combine the anaerobic activity with aerobic activity afterwards. The aerobic activity will help to bring down the initial spike.
With anaerobic activity however, do remember the long term effect will be that it reduces your blood glucose levels because it is depleting your body’s glucose levels. It is only the hormones being released which cause the spike and therefore you may still be prone to hypo’s later.
You may not have to administer or may significantly reduce your correction doses if exercising and therefore be mindful if reacting to high glucose levels around exercise.
Some healthcare professionals may tell you to take an additional snack prior to exercise. This can be good advice to keep the patient safe but it does not account for the fact many individuals exercise to help manage their weight. Therefore, eating more often counteracts the effect of the exercise from a weight management perspective. Therefore, in my opinion insulin should always be adjusted to match your activities and diet and not the other way around. That said, if you are happy to take a snack prior to exercise this too is a good strategy.
It is generally recommended not to exercise if your blood glucose are on the low side i.e. <5.5mmol/l and you may wish to take a snack bring them up prior to exercise.
You could also try to mitigate against this earlier in the day by eating a slow releasing carbohydrate such as wholegrains, new or sweet potatoes, oats, vegetables or fruits.
Likewise it is not a good idea to exercise if your blood glucose levels are over 14mmol/l prior to exercise. The reason being levels of 14mmol/l or above are when theoretically your body can start producing ketones. Ketones in small amounts are harmless but in abundance are lethal. Ketones are produced from fat breakdown which is common in someone without diabetes who may follow a low carbohydrate diet or exercise. In type 1 diabetes, ketones are also produced when they have too little insulin on board. If your blood glucose levels are high prior to exercise it is likely your body will begin to break down fat to produce energy for the cells of the body resulting in some ketone production. Combine this with exercise where you will breakdown more fat and produce more ketones could produce an environment where the ketones begin to accumulate uncontrollably pushing you into DKA.
If exercising with high blood glucose levels it is clear for whatever the reason, you do not have enough insulin on board. Therefore, producing more ketones through exercise may be dangerous. The temptation may be to give yourself a correction dose prior to exercising but this too could cause a swing in your blood glucose levels as the exercise effect is combined with the extra insulin causing dangerous episodes of hypoglycaemia.
It is wise then, to analyse as to why your blood glucose levels were high in the first place and use this as a learning experience. Over time, you will develop the skills to manage this more appropriately.
A final word on Pumps
Insulin pumps allow patients to adjust their insulin minute by minute. Therefore, they are able to reduce their basal insulin for a time period and then increase the dose later in the day. Those without a pump do not have this benefit and once the basal insulin is administered it is in the system. Changing your basal dose when not on pump is dangerous because you may see your exercise glucose levels stabilise but the rest of the day may be thrown out.
Therefore, if you are a keen exerciser and have to test regularly, experience many hypo’s, have lost hypo awareness or have other issues with insulin management, it may be worth having a conversation with your GP whether pump therapy is for you.