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Timing of Exercise and Your Insulin Levels

Dec 30, 2008

Timing of Exercise and Your Insulin Levels By Sheri Colberg, Ph.D., FACSM, is a must read for everyone. Not only does this feature apply to insulin dependent patients, but the information is for all patients whether they have diabetes or not.

Timing of Exercise and Your Insulin Levels


By Sheri Colberg, Ph.D., FACSM

SheriThe timing of exercise may also play a big role in your body’s responses. For instance, you’re less likely to experience low blood sugars if you exercise before breakfast, especially before taking any insulin. At that time of day, you have only your basal insulin (the insulin that covers your body’s need for insulin at rest separate from food intake) on board, so your circulating levels will generally be low, but you usually have higher levels of cortisol, a hormone that increases your insulin resistance, to compensate.

If you exercise after breakfast and a quick-acting insulin injection, your insulin dose may affect whether you get low because the dose will affect your levels of circulating insulin. In one study, exercisers with type 1 diabetes did 60 minutes of moderate cycling starting 90 minutes after taking their regular dose with an insulin pump and eating breakfast. To prevent lows, they reduced their rapid-acting insulin boluses by 50 percent and took no basal insulin. Their morning insulin reductions, however, turned out to be less than afternoon ones made for a similar workout. Thus, if you often develop hypoglycemia during exercise, you might be better off exercising before taking any insulin to cover breakfast instead of afterward or later in the day.

Anyone with type 2 diabetes who still makes insulin is also more likely to have glucose levels drop if exercising after breakfast or another meal (as opposed to before) because of the insulin that is released in response to eating that elevates the levels. Keep in mind, though, that if you exercise long enough without eating, whether you have diabetes or not, you can develop hypoglycemia because of running low on fuels and liver glycogen after not eating overnight, so running a marathon without eating anything beforehand isn’t a good idea. You are likely to develop hypoglycemia when you exercise moderately early in the morning before breakfast or whenever your insulin levels are lower.

Regulating Insulin Levels during Exercise

Physical activity is one of the main causes of hypoglycemia in people with tightly controlled diabetes. Exercising with low levels of insulin is indeed a much more normal physiological response. To lower yours, you may need to lower (if possible) your premeal insulin doses. The following table gives some general recommendations for insulin changes but refers primarily to rapid- or short-acting insulins, not basal ones. (Basal insulins can also be reduced, but for guidelines on doing so, refer to the recommendations for individual sports.)

How much insulin you have in your system between your exercise sessions can also affect how well you do during the next workout. You may end up restoring less muscle glycogen after exercise (or any time) if you don’t have enough insulin or your insulin action is diminished. Although your muscles can take up glucose and restore glycogen mostly without insulin for the first hour following an intense or long bout of exercise, after that time, you need to have enough insulin available to continue stimulating glucose uptake and glycogen storage. If you end up not storing as much glycogen, the next time you exercise your body may depend on greater use of fat, which will lower your ability to exercise and likely cause you to fatigue much more quickly, especially if having low glycogen levels causes you to take up more blood glucose. Keeping your blood sugars closer to normal after exercise also helps you restore glycogen more effectively than if your sugars run high during that time. So, you’ll likely need some insulin after exercise for any carbohydrate that you eat, albeit it a reduced amount.

Table: General Insulin Reductions for Endurance Sportsa


Low intensity

Moderate intensity

High intensity


Insulin reductionsb

15 min



0–15% c 

30 min




45 min




60 min




90 min




120 min




180 min




Notes: aThese insulin recommendations assume that no additional food is eaten before or during the activity to compensate. For insulin pump users, basal rate reductions during an activity may be greater or lesser than these recommendations, and they may be done alone or along with reduced bolus amounts. bThese insulin reductions apply to the specific insulin peaking during exercise (usually rapid-acting ones). A lesser insulin reduction may be needed if exercise occurs more than three hours following the last injection of rapid-acting insulin. Postexercise insulin reductions may also be necessary. cFor intense, near-maximal exercise, an actual increase in rapid-acting insulin (rather than a decrease) may be necessary to counter the glucose-raising effects of hormones released during exercise.

This column is excerpted from Diabetic Athlete’s Handbook (available November 6, 2008 from Human Kinetics), which contains essential exercise-related information and examples for type 1 and type 2 diabetic exercisers. Look for it in stores or find links to places to buy it online on www.shericolberg.com, along with additional information.