Oops, I lied.
In my last article on Diabetes and Exercise, I promised that this article, Part III, would contain some tips on insulin dosing in light of special considerations for diabetics. However, when I sat down to write about those special considerations, I realized that I had left out an important aspect of exercise physiology: what happens after you exercise?
Understanding this question is vital to understanding how the diabetic's body reacts to exercise. The more intense the exercise, the more disruptive this reaction can be. So today, I'd like to provide some (very) basic information about the post-exercise phase of physiology. Just as in Part II, I'm going to keep this as simplified as possible, so that we can focus on the important information and not get caught up in a description of cellular respiration and biochemistry.
After The Workout: Anaerobic Exercise
Recall that in Part II, we learned that anaerobic energy is "ready-to-use" energy stored within the cells, waiting for the moment you move a muscle. It requires no insulin for immediate use, it simply fuels cell work and expires.
Think carefully about what this might imply about the body's next course of action. The stored energy that was once contained in the cells is now gone. The body will naturally act to replenish this energy. This act of replenishment does, in fact require insulin (and other hormones).
When you digest a meal, insulin helps carry glucose into the cells so that it can be converted into this stored energy. When you exercise, this stored energy is depleted and your body becomes "hungry" again. Whenever your body is hungry, it "feeds its cells" with whatever glucose is on-hand.
Therefore, immediately following a big burst of anaerobic exercise, the body starts drawing on its various sources of glucose (either the blood stream, fat deposits, as-yet undigested food, a post-workout Gatorade, whatever) and acts quickly to put that glucose back inside the body's cells.
It happens rapidly. Your body needs that energy immediately, not merely for the sake of exercise, but for the sake of getting around. What I mean is, let's assume you sprint as fast as you can for 200 meters. If you have given it a good effort, the muscles in your legs have expended so much of their stored energy that if that energy isn't replaced, you won't be able to walk. Indeed, track and field spectators have seen many a 200-meter athlete collapse at the finish line, unable to continue to walk, much less run.
This is due to the fact that the body doesn't distinguish between "anaerobic energy used for sprinting" and "anaerobic energy used for walking." It's all the same stuff, and once it's gone, it's gone.
Now, here's what you need to know as a diabetic: When you engage in anaerobic exercise, your body will immediately act to replenish the energy you've used up. If the body does this too quickly or if you have used up too much of your stored energy, this will induce hypoglycemia.
After the Workout: Aerobic Exercise
When we engage in aerobic exercise, things follow a different path.
Recall that we learned in Part II that during cardiovascular exercise, we use up a little anaerobic energy to get started, and then the body phases-in aerobically generated energy to keep you going throughout your workout. This is what causes the heavy breathing, the elevated heartrate, and so forth. Your body is rapidly taking in oxygen to mix with sugar and insulin, and delivering it to your cells.
Once the workout is over, our bodies continue to have an elevated heartrate, we continue to breathe heavily, and our respiratory system continues to deliver aerobic energy into our cells. The process doesn't immediately end, it takes a little time to wind down, gradually.
Not only that, but aerobic respiration itself isn't a perfectly efficient process. You may have heard that your body "doesn't start to burn fat until twenty minutes into your workout." Twenty minutes is a ballpark figure, and in reality, the process is gradual rather than binary. But the bottom line is more or less true: It takes time for the body to be able to use the energy produced during aerobic respiration; it also takes time for the process to taper off.
This, too, functions as much logically as it does physiologically. What you might expect to occur within the body really does occur: Namely, our blood sugar remains elevated for some time after the workout, thanks to the gradual and inefficient nature of aerobic respiration. Eventually, though, it does taper off, and the energy must be replaced. The body once again gets "hungry" for more energy, and starts to transport any and all sugar from available sources into the cells so that it can be stored for future use.
Exactly how long this aerobic cycle lasts depends not only on your own unique body, but also on how fit you are, how hard you pushed yourself, and for how long you were exercising aerobically. It will not occur exactly the same way twice. In some cases, it will persist for twelve hours. Some endocrinologists have suggested to me that periods of longer than twenty-four hours are not uncommon.
Regardless, what's important for the diabetic to understand is this: Aerobic exercise will initially raise your blood sugar, and that rise in blood glucose will persist for quite some time after exercise. Eventually, however, the elevated blood sugar will subside as your body acts to replenish its stored energy - and this may induce hypoglycemia.
Some Closing Remarks/Discussion
I suppose the most vitally important thing to remember in all of this is that both anaerobic and aerobic exercise can induce hypoglycemia in a diabetic, for reasons I will (hopefully) elucidate in my next article. Therefore, always remain vigilant. Monitor yourself very carefully during and after exercise to ensure you are safe. It takes some experimentation to find out what your limits are and when you hit them.
I have to reiterate that because at every point in time, it is a diabetic's first priority to avoid life-threatening hypoglycemia. It is not the keystone of exercising as a diabetic, but it never goes away. We diabetics must simply and at all times think about hypoglycemia and how to avoid it. It is a fact of life for us.
So, the best recommendation I can give here is to always carry a little sugar with you when exercising, and always have a snack after the workout. No more than 15g of carbohydrates, and if I were you, I would not take it in the form of sugar.
Having looked after that top priority, we can then dive into the details and nuances of the remaining information. We can then start analyzing how our bodies will react to various workouts to which we might subject it.
For example, an interval workout at the track will result in a particular "blood sugar reaction" afterward; a fifteen-mile long run will result in a different, no less unique reaction. And not only is the workout important, the sequence is, too. Lifting weights and then doing some cardio will do something different to your body than doing cardio and then lifting some weights.
In subsequent articles in this series, I will explore some of the considerations involved in the types of workouts you might choose, and the types of reactions we can expect from them.
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