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  • Sheri Colberg, PhD

What Causes Blood Glucose to Go Down or Up During Exercise

For all the time that I spend praising the “miracle” of being physically active to help better manage diabetes and health, there are times when exercising does lead to better manage blood glucose and times when it does not. It is not always possible to predict the glycemic outcomes in all cases either, although individual patterns and responses can be determined over time. It is helpful to know the main factors that are predictive of outcomes, however, as detailed below:

Exercise Generally Lowers Blood Glucose When:

  • Circulating levels of insulin are higher (such as after eating in those who make their own insulin and within 2-3 hours of the last bolus of mealtime or correction insulin in those who take insulin)

  • Prolonged and aerobic in nature (30 minutes or more when moderate in intensity, an hour or longer when easier)

  • Blood glucose levels are normal (or near normal) at start of activity

  • Muscle glycogen stores are insufficient (either to start or later during activity)

  • On a low-carbohydrate diet and not fully adapted to eating that way

  • Still recovering from recent prior physical activity

  • Done after a recent hypoglycemic episode (particularly if a more severe low)

  • Doing a new or unaccustomed physical activity (greater reliance on blood glucose)

Exercise Tends to Raise Blood Glucose When:

  • Active first thing in the morning when circulating insulin levels are low and cortisol levels are higher (before taking or releasing any insulin)

  • Short and intense (such as heavy weightlifting, sprinting, or high-intensity interval training)

  • Hyperglycemic, especially when ketones levels are also elevated (i.e., relative insulin deficiency)

  • Eating too much during physical activity (or a large amount right before starting)

  • Dehydrated to start or if get dehydrated while active

  • Exercising in environmental extremes (too hot or cold, high humidity, high altitude)

  • A cold, virus, or other type of infection lowers insulin action and raises physical stress

  • An exaggerated release of glucoregulatory hormones (epinephrine, norepinephrine, glucagon, cortisol, and/or growth hormone) occurs for any reason

Despite all the potential influences, the biggest overall impacts on glycemic responses arise from the timing of being active and the activity itself. Those two factors likely explain most of the variance, while the rest comes from people not being able to anticipate what insulin levels are likely to be during an activity and other unexpected environmental or bodily concerns.

Despite any aggravations associated with balancing blood glucose during physical activity, it is still worthwhile to be regularly active to gain all the physical and mental health benefits associated with it. To help establish patterns and trends, check blood glucose levels before, during, and after various activities and circumstances until it is as predictable as possible.

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