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Regular physical activity and exercise are recommended for the general population for overall improved health. However, exercise of moderate intensity increases the risk of hypoglycemia during and following exertion in those with type 1 diabetes mellitus (T1DM). Accordingly, exercise guidelines for T1DM focus on prevention of exercise-induced hypoglycemia.
The risk of hypoglycemia may discourage some with T1DM from exercising. Children and youth may be embarrassed by the temporary reduction in coordination and physical performance associated with hypoglycemia and by the fear of letting teammates down while playing. Some parents may discourage their children from normal participation in physical activity and from playing competitive sports because of concern about severe hypoglycemia. However, this fear may reduce a child's physical activity, which may lead to reduced overall health, reduced enjoyment from exercise, and restricted confidence in meeting the demands imposed by living with a chronic health condition like diabetes. As a result, long-term exercise habits as well as personal growth and psychosocial development may be blunted.
Concern with hypoglycemia from exercise is probably even more heightened in those with hypoglycemia unawareness, or the absence of symptoms during hypoglycemia. However, some of these concerns might be allayed by the realization that vigorous exercise tends to raise BG rather than lower it.
The fact that vigorous exercise tends to raise BG appears to be relatively unknown to many with T1DM. However, the effect has been confirmed in a number of studies. These studies have identified an exercise threshold that elicits this response. The threshold exercise intensity occurs at or above 80% of a person's maximum exercise capacity, which is also known as the VO2 max (maximum or peak volume of oxygen consumed in exhaustive exercise). The relevant question then becomes, what type of exercise is performed at this 80% level? The following guidelines can be used to estimate whether or not you are exercising at or above this intensity:
School age youth on athletic teams such as track and swimming do much of their training at or above this threshold intensity when they perform a type of training known as interval training. This training technique alternates periods of fast pace with periods of moderate pace during which one partially recovers from the previous high speed interval. For example, high school milers might run a fast lap at race pace followed by a lap of jogging. They then alternate between fast and slow laps, which is very effective in conditioning for sport. The intensity during the fast laps would probably surpass the 80% intensity threshold and therefore raise BG.
The intensity of the short speed bursts while playing sports such as football, soccer, and basketball is also well above the 80% threshold level, which explains why BG tends to rise following such activities.
Why does vigorous exercise raise BG instead of lower it?
Exercise above the 80% threshold is very strenuous. Heart rate is elevated to about 90% of maximum or even higher; breathing is difficult, and we tend to huff and puff while straining to meet the energy demand of the activity. This level of exertion activates the sympathetic nervous system, which in turn elicits the "fight or flight" response. The sympathetic nervous system is excitatory, in that it helps us to meet emergency needs such as fighting for survival or completing a race or vigorous game. Strenuous exertion requires high energy expenditure, and the sympathetic nervous system helps achieve this by exciting endocrine organs such as the adrenal gland. The adrenal gland releases the stress hormones adrenaline and noradrenaline into the blood, which then stimulate the liver to release glucose at a faster rate than normal. When this rate exceeds the rate at which glucose is absorbed by active muscle tissue, BG rises.
In one study, subjects with T1DM cycled for alternating periods of maximal exertion for 4 seconds followed by 2 minutes of easier cycling, for a total of 30 min. This alternating pattern of high and low intensity exertion was used to mimic metabolism of many sports and games. After the exercise bout, BG was elevated as compared to the BG level just before exercise. In comparison, a 30-minute session consisting of moderate pace exercise work at only 40% of maximum intensity decreased BG, although less total work was done in this time.
Consequently, exercise intensity strongly influences the BG response of exercise. When we surpass about 80% of maximum intensity, the nervous system interprets this as very strenuous exertion. To help us complete such energy-demanding work, it releases the stress hormones so that the liver can supply plenty of glucose to the hard-working muscles
How might this information help me?
Knowledge of the unique responses of BG to different types of exercise may be quite helpful in BG control. It is important for someone with T1DM to know if a given exercise session is likely to raise or lower one's BG. Assuming that all exercise reduces BG is an overly simplistic and outdated concept, and one that may leave you scratching your head and frustrated when your BG rises instead of falls with certain types of exercise.
The hyperglycemic response typical of intense exercise is relatively unknown in those with T1DM. Consequently many with T1DM may find it difficult to regulate BG associated with intense exercise, and they may make inappropriate adjustments for exercise, such as eating a carbohydrate snack before any type of exercise. This may lead to periods of hyperglycemia that are contrary to the goals of diabetes management and good health. Hyperglycemia during exercise may also result in reduced physical performance and enjoyment from exercise and lower long-term exercise adherence. Some may feel that exercise worsens their BG control and A1c readings, and so may be discouraged from exercising.
If you use existing exercise guidelines for diabetes, such as having a carbohydrate snack before playing basketball or soccer, you are likely to find your BG after a match or practice might be elevated instead of reduced. Parents may be more likely to encourage their children to play sports and games if they realized that hypoglycemia is less likely to occur in many sports. In a sense, sports and games and vigorous aerobic exercise such as swimming and running may be a safer form of exercise from the perspective of hypoglycemia. One study reported that a single 10-second sprint limited the decline in BG in a session that consisted otherwise of moderate intensity. It is assumed the single sprint embedded in the workout stimulated the secretion of hormones that cause the liver to accelerate the release of glucose. So, from a practical viewpoint, a jogger might toss in a sprint or two in the middle of a moderate pace run to spike the BG a bit which would reduce the likelihood of hypoglycemia occurring at any point during the run.
One wonders how many kids return home from sports practices or from play in the backyard with elevated BG that makes it seem they didn't take enough insulin hours earlier, when in reality the insulin taken may have been adequate. Or they may assume too much carbohydrate was ingested in the last meal. Many parents and active children and youth are perplexed by the unexpected rise in BG. They may search for answers that have little to do with the cause of the rise in BG: vigorous exercise! Hopefully, this information will help those with T1DM enjoy exercise more fully, experience fewer highs and lows from exercise, and improve BG control (A1c readings).
What adjustments can I make in insulin and diet to improve my BG readings?
Vigorous exercise stimulates your adrenal and other endocrine glands to secrete hormones that help you to perform hard work. But, they also cause your BG to rise. Therefore, you may not need CHO before such exercise and you may not need to reduce your insulin dosage.
With moderate intensity exercise, the adrenal gland is only minimally stimulated and your BG will not tend to rise with exercise. It will instead tend to drop, so a pre-exercise snack and/or a decrease in insulin dosage is typically needed.
When in doubt about the effect a given type of exercise may have on your BG, do extra BG monitoring. Maintain records of each type of exercise, and you will soon have a plan best suited to your own individual response to exercise. For example, when I mow the lawn, which is moderate exercise intensity for me and takes about 50 minutes, I need about 15 grams of CHO beforehand to maintain my BG. Yet for two hours of vigorous singles tennis, I begin play with a small cushion just above normal (e.g., 100 to 110 mg/dL) and typically finish with a BG of about 125. During the session, I ingest no CHO in spite of sweating profusely and burning a lot of energy. When lifting weights, my BG varies very little over a 45-minute session, so I begin with a normal BG and finish at about the same level. Thus, a different plan is needed for each type of exercise to optimize BG and how you feel and perform during exercise.
Having had T1DM for well over 50 years and having been an athlete nearly my entire life, I've learned from direct experience that vigorous exercise protects me from hypoglycemia and that I really don't need to worry as much about my BG as when I do less strenuous exercise such as raking leaves, cutting grass, or taking a walk. It all seems paradoxical, but it is rather nice to feel less concerned about hypoglycemia when performing vigorous physical activity.