Many people with type 1 diabetes and other insulin users are engaged in physically demanding exercise programs. Some of them participate in and excel at the top level of competitive sports, including swimming, golf, football, baseball, basketball, ice hockey, soccer, marathons, triathlons, and many others. To perform competitively, you need a greater understanding of the effects of circulating levels of insulin on exercise responses, the physiological response to different types of activities, and the effects of nutrition on performance. To assist you, a diabetic athlete, in achieving optimal glycemic control and their athletic goals, you need to learn as much as possible about these areas.
For starters, unlike nondiabetic individuals whose bodies normally maintain blood glucose levels in a very tight range, if you’re an athlete with type 1 diabetes, you’ll have to constantly adjust your insulin and food intake to balance your blood glucose levels when active. Any type of physical activity promotes the uptake of glucose into active muscle cells additively with insulin, but insulin and muscle contractions oppose each other’s effect on what fuels are available for your body to use. You’ll have to have the correct timing and dosage of insulin to effectively manage your blood sugars during exercise, and the more you know about the pharmacokinetics of injected insulin (or how it’s absorbed), the better for your control.
You may frequently find yourself with relatively high insulin levels that can result in hypoglycemia during exercise, early fatigue, and compromised performance. In athletes with diabetes, the normal, physiological decline in insulin seen in the non-diabetic athlete is difficult to achieve due to sustained absorption of insulin from where you inject it under your skin. The combined blood sugar uptake caused by circulating insulin and muscle contractions can produce hypoglycemia, particularly when you exercise during the peak effects of your short-acting insulin or rapid-acting insulin analogs. When insulin is high, your glucose uptake into muscles is enhanced and the ability of your liver to produce glucose suppressed, often resulting in rapid-onset lows.
Not surprisingly, then, it’s easiest to control your blood sugars during exercise when your insulin levels are relatively low, such as during morning exercise before you take any insulin or more than 2-3 hours after your last dose of rapid-acting insulin. However, almost any time that you give rapid-acting insulin before doing extended exercise, your doses must be reduced to prevent hypoglycemia. For example, in anticipation of moderate exercise, doses may need to be reduced by 33 to 50%, with greater reductions required for longer-duration activities. Insulin pump users can more closely replicate a normal physiological decline in insulin levels by reducing or suspending their basal insulin as well, particularly if reduced prior to the start of exercise. If anticipated, if you use basal insulins like Lantus or Levemir, you may also choose to reduce those doses either the evening before or morning of such physical activity.
Conversely, doing vigorous exercise without having enough insulin can aggravate or cause high blood sugar (hyperglycemia). In fact, when your insulin levels are too low, the rise in certain hormones like adrenaline elicited by exercise can trigger hyperglycemia or, if ketones are being produced, diabetic ketoacidosis (DKA). Thus, skipping your normal insulin doses for moderate or greater food intake prior to exercise seldom works effectively. Also, monitor your sugars closely whenever you suspend your basal insulin on your pump for more than 60 to 90 minutes to make sure that sufficient insulin is available during exercise.
Short-term highs may or may not be limiting to athletes with diabetes, depending on whether ketones are present in moderate or higher amounts. However, athletes have reported that they feel less sluggish and perform better when their starting blood glucose falls in a more normal range, such as 80 to 180 mg/dl (4.2-10 mmol/L) depending on the sport. If you let yourself run higher for longer, though, it can impact your performance. In a study of younger individuals with long-standing type 1 diabetes, the maximal workload and oxygen uptake were markedly impaired in participants with long-standing high blood sugars.
In my next blog, read more about other factors that impact your insulin levels and control during exercise.