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What happens to your BGL when you exercise?

Well, isn’t that a loaded question! In short, it can go up, down or stay the same. The long answer I will attempt to explain includes why.

By Nicole Walker

Well, isn’t that a loaded question! In short, it can go up, down or stay the same. The long answer I will attempt to explain includes why.

Target BGLs for exercise

  • Ideally our BGL will be between 7-10mmol/L before and during exercise.
  • BUT it is safe to exercise up to a BGL of 15mmol/L
  • If you are over 15mmol/L make sure your ketones are <0.6mmol/L otherwise it is NOT safe to exercise

The reason for these targets? They are far enough from a hypo that we can catch it, our body has enough fuel for exercise and we have enough insulin to use this fuel properly. On the flipside if you aim for 7-10mmol/L, your BGLs are not too high to dehydrate you, cause thirst or the need to wee all the time. This is also the best target range for sporting performance and achieving muscle gains.

BGL testing/checking around exercise

  • Do a BGL Check before you go. You might want to do it 30 mins before so you can sort out a BGL that is unfavourable for exercise and still be able to start on time eg: <7mmol/L or >15mmol/L
  • But it is most important to check right before you go to ensure you are above 7mmol/L!
  • For new exercise check every 30mins during to make sure your BGL stays above 7mmol/L
  • If you know what your BGL does already, still take your testing kit/phone/reader with you in case you feel hypo but otherwise you might not need to check as often.
  • Check after exercise so you know how your body responds.
  • That might include a doing a few extra checks over the next few hours and an extra one overnight if you are prone to post-exercise hypoglycaemia or to see if your strategies for managing exercise worked

Why do BGLs go DOWN during exercise?

Glucose is often the primary source of energy for our muscles when we exercise. It is important firstly to tell you that our muscles are GREEDY! Keep that in mind throughout and you won’t ever forget why you hypo during or after exercise. When our muscles contract with exercise they stimulate little suckers called GLUT4 to be released to the outside of the muscle. What these GLUT4 suckers do is suck glucose from the bloodstream into the muscle to be used for energy.

Now, they will do this REGARDLESS of what your BGL is, how much you have eaten recently or how much insulin you have taken. See? Greedy huh? Think of it like wringing out the kitchen sponge (MUSCLE CONTRACTION) then putting that sponge back in the water (NOW GREEDY GLUT4 SUCKERS HAVE APPEARED) where it will soak up the water from the sink. Therefore, your BGLs might just steadily or suddenly drop over the time that you are exercising – it can feel pretty fast sometimes! Given you might only be exercising for 30 minutes, people can easily have BGL drops of 4-5mmol/L or more in that time – pretty normal, you are not alone!

Please note – it is not necessarily a level of fitness or bravado that determines how much or how little your BGLs change with exercise. If you do hypo – here’s a reminder on how to treat it.

Some scenarios where BGLs will drop MORE

  • If the exercise is new or unknown to your body eg: you are a fit rugby player and usually BGLs are fine but then go for a walk with your Mum one day and HYPO!
  • If you are using your upper body muscles (smaller ones that use more glucose!) more than your lower body muscles eg: hanging out the washing, cleaning the car or swimming  classics for HYPOs!
  • If it is hot you use more glucose as a fuel and insulin in used in the body faster
  • When you have eaten less carbohydrates
  • When you have given insulin within 2-3hrs of exercise
  • If you have already been exercising for more than 60 minutes
  • If you are exercising with alcohol in your system from Backyard Cricket to dancing the night away in town
  • When you go for LONGER
  • Sometimes a MORE INTENSE session (note below: more intense sessions can also send your BGLs up)
  • Afternoon exercise as generally in the afternoons we are more sensitive to insulin
  • If you have had a hypo recently
  • Men are more likely than Women
  • At different times in the menstrual cycle for women

Why do BGLs go down AFTER exercise?

During exercise the other source of glucose is from the glucose storage tank within the muscles themselves…. MUSCLE GLYCOGEN. Muscle glycogen starts to break down and convert to glucose when we exercise and the LONGER or more INTENSE we exercise for, the more these stores will be used up. There is only about 60-90mins worth stored FYI so if we get to that time point – we are going to be more at risk of a Hypo during exercise.

BUT Muscle Glycogen is important to know about because it’s the reason why the GLUT4 suckers stick around after exercise too! Remember the muscles are GREEDY and they will continue to suck extra glucose out of your bloodstream to fill the stores of glycogen back up for as long as this takes. This can take anywhere from 2 hours to even 12-24hrs!

Therefore, there is a risk of DELAYED HYPOGLYCAEMIA POST-EXERCISE. Ahhhhaaaaa you say?! I hope you are having an Ah Haaa moment right now. And if not – go back and read it again. ? . Then if you don’t get it a second time, fair enough – but ask me! Often these post-exercise delayed hypos will occur much later than the exercise itself. Eg: you might have a hypo during the night but because did your first Boot camp morning sesh many hours before with your Exercise Physiologist or Personal Trainer.

Some scenarios when your BGLs are more likely to go down after exercise?

  • If you have not eaten a meal or snack containing carbohydrates after the exercise
  • Doing incidental exercise such as Christmas shopping or moving furniture that you didn’t think was exercise
  • Exercise closer to bedtime, including sex
  • Often 12-24hrs (delayed) after High Intensity exercise (see why in future blogs!)

Why do BGLs go UP during exercise?

Sometimes when we exercise (or all the time for some people) our BGLs will rise. And they don’t just rise, they soar! So much of the traditional exercise advice you have probably not felt was very useful at all! Well the reason is, sometimes our body considers exercise to be a stress. And when you are stressed or anxious or nervous – you will have noticed your BGLs go UP!

This is because our body starts to produce stress hormones such as Adrenaline and Cortisol. The job of these hormones is to get your body ready for a fight or flight response. Part of that response involves making glucose available so your body can run away from something (your trainer perhaps?). It does this by giving your liver a bit of a kick, which then springs into action and dumps some of its contents of stored glucose LIVER GLYCOGEN into the blood stream. Key word here – dumps. Not trickle, not steady release, it dumps.

So, although you have started your exercise, the GLUT4 SUCKERS are out working, the BGL is getting sucked into the muscles. The liver then does this dump of LIVER GLYCOGEN, turning into a large rise in your BGL which is by far the more noticeable event. Once again – it is not necessarily a fitness thing when this happens! Some people just seem to have or need these hormones around for exercise. However, there is types of exercise that MOST people will find their BGLs rising rather than falling.

Types of exercise that tend to rise BGLs:

  • High Intensity Training (HITT, hard circuits, boxing class, spin class etc) during and sometimes initially after
  • Intervals eg: running or cycling sprints, stair training
  • Weights Training
  • When you are nervous or in a high pressure environment eg: before a competition, in a close basketball game or final few minutes of a football game

In brief – How do you keep your BGLs stable with exercise?

If your BGLs go down:

  • Eat carbohydrates before, during and/or after
  • Reduce insulin before, during and/or after
  • Do some high intensity intervals/weights at the end of your low intensity/cardio session
  • Exercise in the morning

If your BGLs go up:

  • Do a warm up/warm down of low intensity exercise eg: stretching, yoga, light jog or walk
  • Have carbs + insulin dose with it prior to exercise so there is insulin on board during exercise
  • Consider the need for Exercise Insulin before training (scary thought but yes some people need it and can take safely)

Is that it? No don’t worry I will go into more detail in future blogs! Stay tuned.

What if my BGLs stay the same?

Great! That’s easier, just aim to be between 7-10mmol/L before and during exercise and watch out for post-exercise hypoglycaemia. Be careful starting new exercise as your body might respond differently. And if you are going for more than 60-90mins you might consider needing some sports nutrition!

Perseverance pays off!

When you get stuck into a new exercise or after a period without it, your BGL swings at first can be more troublesome! As your body adapts to that type of exercise or you get fitter, your diabetes won’t be as affected by the exercise. This is because your body gets more efficient at using glucose for fuel so the muscles don’t need as much to do the same job. Keep a journal with each new exercise and how your BGLs react and what tools work from your toolbox.

In my last blog we prepared your Diabetes Toolkit for safe exercise. Don’t forget to check it out before you start! And the most important thing… Get out there and give it a go! Here are some cool groups to get you motivated: Typeonerun, Beat-It and JDRF walk. Get in touch with YOUR Dietitian, Sports Dietitian, Diabetes Educator, Endocrinologist or Exercise Physiologist if you need more help getting started.

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