Exercise and Blood Glucose Levels

Exercise is good for you. It’s good for the heart, good for losing weight, makes you feel better (really — it releases endorphins that elevate mood), and it’s good for blood glucose — well, sort of. It is good for blood glucose, but it can be tricky at the same time. So today I’m going to talk about how to deal with blood sugar when you’re exercising so that you can minimize the negative effects and enjoy the positive.

OK, let’s start with some basics. Aerobic exercise, or cardio, is what we call activity that requires “the pumping of oxygenated blood by the heart,” to be delivered to working muscles. A general rule of thumb is that aerobic exercise is achieved when our heart rate and breathing rate rise in a sustainable way (in order to maintain this pumping of oxygenated blood — the heart rate to circulate the blood, the breathing rate to increase the oxygen intake). Anaerobic exercise occurs when the activity is simply too much for the heart rate and breathing to keep up with, causing you to become out of breath, and it includes activities such as sprinting and weightlifting. Here, we’ll be talking about aerobic activities, such as swimming, running, or dancing.

Advertisement

So, what happens with aerobic activity? First, it lowers blood glucose. Why? Because the muscles are working harder and they need energy. The glucose in our blood is energy for our cells. Insulin is the hormone that transfers the glucose from our blood to our cells. So when we Diabetians exercise, we often “go low.” This is because the glucose in our blood is quickly moved into our cells, but the insulin in our blood is still active. Unlike non-Diabetians, the insulin we’ve injected doesn’t go away once the glucose has been moved. It keeps moving glucose out of the blood (and out of the glycogen stores in our muscles and liver), even if we’re going low. So we find ourselves having to take in more sugar to “mop up” that excess insulin. The trouble, of course, is that by doing this, we are ingesting MORE calories, while performing an activity meant to BURN calories. You see, for non-Diabetians, the body will begin burning stored fat for energy once the glucose supply is going low, resulting in increased weight loss. For us Diabetians, it’s harder to achieve that burning of fat since we often have to give our body more glucose to avoid the low, and then our insulin simply delivers THAT glucose to our cells instead of forcing the body to burn the fat.

One solution to this is using a pump. Since the basal rate of a pump can be temporarily lowered, it’s possible to tell the pump to deliver LESS insulin during aerobic activity, and this means we’re less likely to go low, may not have to take in more calories, and will more likely burn fat. However, not all of us use the pump. I, for instance, tried making the switch and found that the pump simply didn’t work for me. And so I’m back to multiple daily injections (or MDI). For the most part, I find my diabetes easier to manage on MDI, but exercise is a little trickier.

For those of us on MDI, preventing lows is the name of the game. We can’t lower the basal rate, so generally we have to accept that we WILL need to ingest some extra calories for exercising. This doesn’t mean exercising doesn’t do any good. We still benefit from the increased metabolic rate that comes with aerobic exercise, we still burn some fat, and it’s still very good for us. But we need to be proactive. I have found that “front-loading” is best for me. I test before exercising, and if my blood glucose is low, or even on the low side of normal, I will take some sugar right at the start of the activity. In fact, I will often aim to raise blood sugar UP to about 130–140, knowing full well that over the course of exercising that sugar WILL come down as the glucose is used. In the past, I would be ingesting glucose in small amounts continuously while exercising, with the goal of keeping blood glucose around 100. But there are a few issues with this. First, I found myself constantly dipping low and correcting over and over throughout the activity. It left me with low energy during the exercise and made the exercise quite a chore. But more importantly, it increased the POST-exercise high. Yes, that’s right. Exercise, that thing that is supposed to LOWER blood glucose, can also raise it immediately afterward.

Why does glucose sometimes shoot UP after exercise? Because during exercise, the activity prompts the liver to deliver stored glucose to the blood to keep energy up. While we’re exercising, this bit of extra sugar is being hauled to our cells by our insulin. But AFTER exercising, that extra glucose can cause a spike for us. The glucose in our system is still elevated, but the cells aren’t asking for anymore “extra” deliveries (or we may not have enough insulin in our system to make those deliveries), so the glucose builds up in the blood. Think of it this way: During exercise, each unit of insulin has a glucose-lowering effect that is GREATER than average, because of the extra demand from the body for the glucose in the blood. But afterward, the glucose-lowering effect goes back to normal, AND…the level of glucose is GREATER than average. Again, for non-Diabetians, that extra sugar is no problem. The body puts out a bit of extra insulin and regulates itself. But our insulin level is set.

So here’s what we can do about it. If we’re using a pump or MDI, the solution is pretty much the same. We can give ourselves a small bolus of extra insulin right when we’re finishing our aerobic activity. This can lessen the spike or head it off completely. The amount of insulin differs from person to person and should be determined with your diabetes care team. And it may take some trial and error. For those of us on MDI, this is where that “front-loading” idea can help. I’ve noticed that if I’m ingesting glucose continuously while exercising, then when I stop, not only is my liver potentially giving me extra glucose, but I ALSO have that extra ingested glucose still floating in my system. However, if I front-loaded it, there is less LEFTOVER glucose to add to the post-exercise high that can be so frustrating.

When all is said and done, exercising is incredibly important. The positive effects far outweigh any negatives. And if we’re smart, we can manage those potential negative effects so they don’t derail us.

Want to learn more about exercise and diabetes? Read “Exercise Myths and Facts” and “Picking the Right Activity to Meet Your Fitness Goals.”