Mastering Exercise With Type 1 Diabetes

Exercise is the double-edged sword of type 1 diabetes management. Though there is zero doubt about exercise’s comprehensive health benefits, the actual practice of exercise can be exceptionally challenging for blood sugar management. Diabetes, to put it simply, can ruin your workout, and exercise can change a steady blood sugar day into a roller coaster.

The Type 1 Diabetes and Exercise Initiative (T1DEXI) was founded to help people with type 1 diabetes get exercise and blood sugar management right. It is the largest real-world study of exercise in type 1 diabetes yet performed, and has been designed to drill deep into the data for insights that regular people can put into practice.

Mike Riddell, PhD, a professor of kinesiology and health science at Toronto’s York University, helms the initiative. Dr. Riddell and his team recently published their first analysis of the new data, and it’s chock full of high-quality observations that regular people with diabetes can put into practice. He was kind enough to walk Diabetes Daily through the work.

Here are some of the major takeaways:

Exercise affects us all differently — really, really differently

It’s difficult to overestimate just how variable the effect of exercise is. Your diabetes experience will be radically different from someone else’s.

Riddell’s work has generally confirmed what we already knew about the impact of exercise on blood sugar — namely, that exercise tends to cause blood sugar drops, and that cardio causes larger drops than high-intensity or strength-building exercises. But it’s one of the first efforts to assess the scale of variability.

The new analysis showed that 30 minutes of aerobic exercise caused an average blood sugar drop of 18 mg/dL, and resistance training caused a drop of 9 mg/dL. Riddell does not want you to focus on those numbers:

“I don’t like it when researchers show averages, and make us believe that we people with diabetes should behave like the average. We really don’t.”

The more important finding was that margin of error was vast. Aerobic exercise caused a drop of 18 mg/dL plus or minus 39 mg/dL, meaning that most participants fell somewhere within a very wide range of -57 mg/dL and +21 mg/dL. A sizeable minority had even more extreme responses than that.

The upshot is that the average experience is practically meaningless to any one person. It kind of mocks the idea of standard management recommendations. All we can really say is that your response to exercise may be unique, and that you’ll need to test your blood sugar.

Remember, these are real-world numbers, and they include all of the messy variables that go into diabetes management — differences in insulin use, carb consumption, baseline glucose level, and so on. Your blood sugar behaves very differently during a jog if you chugged a Gatorade before starting. Should we assume that some people experience extreme blood sugar effects because of their other management habits?

Riddell told me that his team is working on a new analysis, as yet unpublished, that will likely show that surprising blood glucose effects are often consistent within any one person. This accords with my anecdotal understanding: I have an acquaintance with type 1 diabetes who needs to use insulin immediately before a workout because her blood sugar levels always go up during cardio. (Mine always go down; if I were to use rapid insulin before a jog or hike, it could put me in the hospital.)

“Clearly some people have a rise, even with aerobic exercise.”

The wealth of new data allowed Riddell to isolate new factors explaining these inconsistencies.

“What’s novel is that because the database is so big, we can start to understand why there’s variability. And what we found was pretty startling.”

Surprising blood sugar variables

Most of us know the basics. Carbs make your blood sugar go up, insulin makes it go down, and exercise can supercharge your insulin sensitivity, causing any insulin “on board” in your body to have an especially powerful effect. Adrenaline, meanwhile, can spike your blood sugar, which causes some athletes to experience blood sugar rises during important competitions.

T1DEXI has revealed some new variables of which we were previously unaware:

People in good shape experience larger blood sugar drops during exercise.

This was a counterintuitive finding, not at all what Riddell expected. You might have thought that frequent exercises were more inured to extreme blood sugar effects, but the truth was precisely the opposite.

Riddell speculated that “people who are fitter — they have a larger muscle mass, they use glucose more efficiently — they must use up the glucose in the bloodstream more quickly.”

The more you work out, the more glucose-lowering benefits exercise might have.

People with lower A1Cs experience larger blood sugar drops during exercise.

Another counterintuitive result! “If you have a higher baseline A1C,” even though you have more room for improvement, “you actually drop less.” Riddell suspects that “might be because people with a higher A1C have less insulin sensitivity, but we don’t know.”

Nevertheless, this was another finding suggesting that the glucose-lowering power of exercise just gets better the more you engage in it.

Any type of exercise improves time-in-range over the next 24 hours

Riddell told me that the strength of this effect was “really quite startling.”

“On days in which volunteers did 30 minutes of exercise, their time-in-range was much better compared to sedentary days. Much, much better. We’re talking about 5 to 7 percent better.”

If the study produced a single piece of evidence that everyone with type 1 diabetes should be exercising regularly, this was it. Work out today — any kind of exercise — and enjoy steadier, healthier blood sugars tomorrow.

“I think it might have to do with a short-term improvement in insulin sensitivity, but we don’t have the mechanisms sorted out yet.”

“A little bit of work pays off in the long run. Do as little as 30 minutes, you can expect to see an improvement in time-in-range.”

Morning exercise results in the steadiest blood sugars, by far

Exercising in the morning, before you eat breakfast, may be the best way to lock in the glucose-lowering benefits of exercise while reducing your risk of hypoglycemia.

“I think that is critical. That is really a safe time of day to be exercising. Hypoglycemia is a big threat when you exercise later in the day.”

There are probably several factors at play. If you work out before you eat breakfast, you won’t have any mealtime insulin in your system. Morning exercise can also counteract the blood sugar’s natural tendency to rise in the morning, known as dawn phenomenon. Even absent those factors, this time of day seems to just be more conducive to steady blood sugars.

“We have some of the best biostatisticians working on this data, and they’ve convinced me that time of day is independently a factor, independent of insulin on board.

“It’s a safe time of day to get a bit of exercise in, as compared to later in the day, when you’re more insulin sensitive and you might have more mealtime insulin circulating.”


New research from the Type 1 Diabetes and Exercise Initiative shows that exercise improves blood sugar levels over the subsequent 24 hours, and that the more you exercise, the more powerful its glucose-lowering effect.

Exercise can have a huge immediate effect on blood sugar levels — an effect that is highly personal and varies widely among individuals. The morning, before breakfast, appears to be the safest time to exercise, leading to the steadiest glucose concentrations.

It’s important for everyone with type 1 diabetes to know how their bodies respond to different types of exercise so that they can participate in this vitally healthy habit without undue risk of hypoglycemia.

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