Need help navigating life with diabetes? You can always Ask D’Mine… Welcome back to our weekly Q&A column, hosted by longtime type 1 and diabetes author Wil Dubois.

Today, Wil’s focusing on the challenges that come along with fitness and activity when you happen to live with diabetes — especially rollercoaster blood sugars.

{ Got your own questions? Email us at }

*   *   *

Jill, type 1 from Montana, asks: I thought exercise was supposed to lower blood sugar, so why does it make my BG go way up sometimes?

Wil@Ask D’Mine answers: Great question, and very timely, as we just recently published a comprehensive guide to exercise with type 1 diabetes.

To answer you specifically: Exercise always lowers blood glucose (BG) on a 24-hour basis, but sometimes exercise also briefly raises BG following a workout. Here’s how all of that works: Globally, even moderate exercise—such as a nice walk with a sweetheart, grandchild, or dog—raises your heart rate, increases your breathing, and boosts muscle activity. All of those increased utilizations of your body’s systems must be fueled.

Meaning, they require sugar.

Now, some of that sugar comes from your bloodstream, but some of it comes from glucose stored in your muscles and liver, and that’s the key to the day-long BG-lowering effect of exercise. Over the 24 hours following exercise your body re-stocks these reserves—slowly but steadily sucking sugar out of your bloodstream to accomplish the task. I like to picture it like a biological vacuum cleaner. Can’t you just see a fleet of miniature Roombas cruising through your bloodstream, cleaning up the sugar all day long, and dumping the refuse in the trash can of the liver? Exercise is like plugging the little robots into the wall to charge them up.

OK, so much for how exercise lowers sugar. But why does it sometimes raise BG levels for a short time? There’s actually more than one way that exercise can spike glucose, but the most common way is from stress hormones. Any workout that’s greater than moderate is likely to release adrenaline, which is basically pouring sugar into your system. Metaphorically, it’s like sticking a Tootsie Pop in your mouth. That’s not to say that strenuous exercise is a bad thing, it has many benefits, but you just need to accept the fact that you may see a bump in your BG if you break a sweat. The good news is that adrenaline is a short-acting sugar that your body will mop up quickly, and you’ll still get that day-long lowering effect from the exercise, despite the brief excursion.

Another way that exercise can increase blood sugar for a time is more complex, and it has to do with the availability of insulin in your system. If you’re short on insulin, your body can’t move sugar from your blood into the cells where it’s needed. This leaves the cells blind to the sugar on their doorsteps; they assume there’s none to be had, and they send out signals for reserves to be brought up from the liver and kidneys, further increasing blood sugar levels that may be too darn high to start with.

Oh, and speaking of blood sugar levels that are too darn high, the Mayo Clinic advises against even starting exercise when your blood sugar is above 250 mg/dL, especially for type 1s like us. Why is that? It has to do with insulin again, and the reaction of the body to exercise in low-insulin environments. If you have little-to-no insulin in your system, and start working out, two things are going to happen. First, the cells are gonna cry out as we talked about, and the liver and kidneys will dump more sugar into your blood stream. But it won’t help. There’s not enough insulin to move this new sugar into the cells. The blood sugar level soars, and then the second thing happens. The hard-working hungry cells turn cannibal and start consuming the body’s fat. Uh-oh. You all know what the byproduct of the body burning fat for fuel is, right? Yep, ketones. And too many ketones leads to diabetic ketoacidosis (DKA).

They’re wrong when they say a little exercise won’t kill you. It can, if your blood sugar is way-high and your insulin is low.

Speaking of low, there are opposite risks of exercising when your blood sugar is too low, as well. Again, the folks at Mayo advise against starting exercise at a blood sugar level below 100 mg/dL, because while we’ve been focused on blood sugar spikes with exercise, the opposite can happen as well: some PWDs (people with diabetes) see profound drops in blood sugar—especially if insulin levels are on the high side—when they start exercising, especially if it’s very strenuous. 

Exercise-induced lows can also strike well beyond the time period of the workout. Remember that the body will take up to 24 hours to re-stock its reserves, and the most profound action comes in the 4-8 hour range, which is considered the riskiest time period for post-workout hypos. This is why, especially if you’re starting a new workout regime, you need to test your blood sugar frequently until you get a good handle on how your body will react. Here, check out results of the Big Blue Test to see how various PWDs responded to even a short workout.

In fact, even if you are just on the lower side of that 100-250 mg/dL safe exercise range, many experts recommend that PWDs eat a pre-exercise snack of between 15 and 30 carbs to avoid being sent into a hypo state by the exercise. 

Oh, and speaking of eating, should you exercise before or after you eat? Although not all exercise experts agree, a good case can be made for timing exercise right after a meal. Why? Glucose levels typically hit their peak levels about an hour and a half after eating, so setting the body up to use extra glucose can help blunt the post meal spike, as well as improving blood sugar for the next 24 hours. 

So there you have it: For the most part, exercise does lower blood sugar, but like all things diabetes, there will always be a few bumps in the road.


Will Dubois lives with type 1 diabetes and is the author of five books on the illness, including “Taming The Tiger” and “Beyond Fingersticks.” He spent many years helping treat patients at a rural medical center in New Mexico. An aviation enthusiast, Wil lives in Las Vegas, NM, with his wife and son, and one too many cats.

This is not a medical advice column. We are PWDs freely and openly sharing the wisdom of our collected experiences — our been-there-done-that knowledge from the trenches. Bottom Line: You still need the guidance and care of a licensed medical professional.