Type 1 Diabetic Athletes: Physiology Part II

During brief high-intensity exercise, your muscles rely on their glycogen stores and to a lesser extent blood glucose, but relatively little fat. Shorter, intense activities such as sprinting or power lifting are mostly anaerobic activities that cause your muscles to rapidly use high-energy phosphate compounds (ATP and more), and, if they last longer than ten seconds, glycogen stores in muscles. High-intensity exercise causes a strong response of your nervous system (the one that controls “flight or fight” responses, the sympathetic nervous system), which can be further worsened by the mental stress of competition, resulting in the release of adrenaline and other hormones that raise your blood glucose levels. Such activities generally cause a transient rise in your blood sugars that may require additional insulin to correct. Uncorrected, blood glucose levels may remain elevated for two to three hours afterwards, even if normal prior to exercise. Athletes, however, must guard against hypoglycemia resulting from ongoing muscle glycogen replacement in their muscles post-exercise, which is largely insulin independent until glycogen levels increase, particularly during the “window of opportunity” from 30 minutes up to two hours after physical activity.

Athletes with diabetes who train regularly, on the whole, exhibit a heightened sensitivity to insulin, which allows blood glucose to enter their muscle cells more efficiently both acutely and chronically with exercise. Acute changes likely result from heightened muscle glycogen repletion following workouts. However, one study reported that following a competitive marathon, athletes with type 1 diabetes who kept their blood sugars normal had unchanged insulin sensitivity on the morning after the event despite significant glycogen depletion. Under such conditions, it is likely that enhanced fat use following exhaustive exercise, which occurs normally even in non-diabetic individuals, combined with some degree of muscular damage to create a transient state of insulin resistance that usually resolves in a day or two.

Chronic changes in insulin sensitivity, on the other hand, are attributed to adaptive changes in your muscle tissue resulting in enhanced insulin-mediated glucose transport by insulin-sensitive glucose transporter (GLUT4) proteins and lower glucose production by the liver. Aerobic training also results in an increase in the proportion of fats used during low- or moderate-intensity activity. Using fats more effectively spares some muscle glycogen and blood glucose and allows for better control of your blood sugars during activities. These changes in fuel utilization in response to training, though, will result in a need for smaller adjustments to your carbohydrate or insulin intake to maintain control over your blood sugars compared to the period of time prior to training. Therefore, training adaptations lower your overall insulin needs, regardless of the insulin regimen that you use. Your exercise must be consistent, though, as this heightened state of insulin action begins to decline after only one to two days of inactivity.

Similar to aerobic activities, resistance training also enhances insulin sensitivity and blood glucose utilization. Skeletal muscle is a metabolically active tissue that takes up more glucose (through glycogen synthesis) than many other body tissues; therefore, athletes with diabetes who gain muscle mass from doing resistance work also have lower overall insulin requirements and must lower their insulin doses both acutely and chronically.

In the next blog, I’ll talk more about the importance of nutrition and carbohydrate intake in controlling your blood sugars during exercise.

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2 thoughts on “Type 1 Diabetic Athletes: Physiology Part II

  1. Anonymous

    My experience (I am type one) on days where I anticipate a log bike ride (80-100 miles) is to cut back my normal initial dose of humalog by 75 to 90% (relatively big breakfast). The ride leaves around 8:30 am. I ride and eat all day taking no additional humalog but eating lunch and snacks or gatorade. I am not a fast rider and we finish around 4, sometimes as late as 6 depending on wind, hills, distance, etc. As soon as I finish with no intent to get back on the bike my sugar will rise if I eat. I fear taking insulin during the course of the exercise. Do you (or anyone with an opinion) suppose that during my eight hour ride that the non insulin dependent pathways for BG to my muscle cells take over completely? What do you think might happen if I stretched the duration of my aeorbic excercise out another 8-9 hours? Would I have to take insulin in the latter hours, 9-16, to get the nutrition to the cells? I am a little worried about what is out there in the hours after 8 to 10 hours of aerobic exercise.
    Larry

    Reply
  2. Sheri Colberg PhD

    Larry,
    You probably will need at least some basal insulin (albeit less than normal) when exercising for longer amounts of time. The non-insulin pathways to take up BG help, but your body also releases hormones that are glucose-raising. Certain ones of these like cortisol increase during long rides and make you more insulin resistant. Whenever you slow your pace or stop exercising for a while, you won’t be taking up as much glucose through the contraction-induced mechanisms, and your body may even switch to using a lot more fats when glycogen levels get low. As a result, you will need some insulin to get glucose into cells at some point during a longer ride, particularly if you don’t make any of your own. (Some type 1s still do).
    Hope this helps answer your questions. Sheri

    Reply

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