You exercise. You do this because it’s good for your health, can help stabilize your blood sugars and makes you look and feel better. It provides you with exhilarating, character-building challenges whether you run in the Boston Marathon or increase your walking distance from two to three miles per day.
If you take insulin, you may also have experienced the negative side of exercise. With a miscalculation, exercise can wreck your glycemic control and send your blood sugars plummeting. After eating a candy bar or two your sugars can skyrocket, followed by a struggle to bring them back down to a level of optimum performance and safety.
Exercise for people with diabetes who take insulin has a dark side because factors like insulin doses, training, wind resistance, effort and stress hormones vary from person to person and day to day. Unfortunately, no set rules exist to simplify this situation.
However, your exercise will produce fewer problems when balanced with the ExCarb System. This system starts by giving you an understanding of your internal fuel supplies and how they relate to your insulin level as you exercise. This insulin/fuel relationship changes depending on the intensity and duration of your exercise. The system provides specific guidelines that help you determine when to eat replacement carbohydrates for fuel, when to adjust insulin levels and when to combine these two approaches.
What Are Your Fuel Sources For Exercise?
Mild exercise, such as a leisurely 15 minute walk, draws fuel from fat and glycogen stores and causes few problems for BG control. (Unfortunately it also burns so little fat that it won’t help you lose much weight.)
When you start moderate or strenuous exercise, the first fuel source tapped is the glucose present in the blood, followed by glycogen stored in muscle and the liver. Glucose is a fast-acting, easily accessible source of fuel, but the body’s supply is limited.
During strenuous exercise, the sugar in the bloodstream can be used up in only four minutes compared to the normal 30 minutes at rest. Your liver plays a critical role in supplying additional glucose for exercise through its release of glucose from glycogen stores into the bloodstream. But your liver’s stores are also relatively small and can be depleted after another 20 to 30 minutes of very strenuous exercise.
Other sources of glucose are also tapped, including nearby muscle glycogen and new glycogen production by your liver. The “wall” often encountered by marathon runners at 20 to 24 miles into the race comes from the total depletion of all the glucose and glycogen stores in the hardest working muscles and the liver.
Glycogen stores are largely depleted following endurance events and need to be rebuilt. Athletes assist this process by consuming a large carb meal after the exercise. Generally, this rebuilding process pulls extra glucose from the blood for 24 to 36 hours after major events. This rebuilding of liver and muscle glycogen stores is the major cause for post-exercise hypoglycemia experienced by people with diabetes.
Fat stores are used as well, but are tapped more gradually. These stores are about 2000 times as large as those of glucose. Fat stores are accessed during any long or strenuous exercise and make good performance and stable blood sugars possible.
Is the Insulin Level Important?
The amount of insulin in the blood determines whether carbohydrate or fat will be used as fuel. For the nondiabetic, fuel delivery is facilitated by rapid changes in the blood insulin level.
In the first 15 minutes of strenuous exercise, the insulin level of a person without diabetes drops to half of its pre-exercise level. With moderate exercise, about an hour passes before the same drop is seen.
This insulin drop triggers the release of internal stores of glucose and fat. It also allows the body to switch to other fuel sources, instead of depending on the relatively small supplies of glucose. The blood sugar remains steadier when the fuel used in exercise comes from these other sources. Accessing these fuels becomes more important as exercise stretches beyond 40 to 60 minutes.
Lower insulin levels mean more exercise can be done without blood sugars falling. Since insulin levels are not adjusted automatically in people with type I diabetes, some thought and care has to be given to adjusting insulin properly for the best results.
If insulin levels are not adjusted and are too high, sugar exits from the blood into exercising muscles quickly, while glucose release from glycogen stores in the liver is reduced. The result is a rapidly dropping blood sugar. An insulin reaction results unless the blood sugar was high at the start of exercise or extra carbohydrates are eaten during exercise.
On the other hand, if insulin levels are too low, both glucose and free fatty acids are readily released into the bloodstream, but sugar has trouble entering working muscles. This results in a rising blood sugar during the exercise.
For those of you with type II diabetes, your pancreas will automatically drop your insulin production as you exercise. But if you take insulin, you should drop your doses as well. For those on oral medications, exercise lows will only be experienced in the occasional person on sulfonylureas.
One of the great health benefits of exercise is that it increases insulin sensitivity in the muscles, thereby counteracting the cause of insulin resistance in type II diabetes. In fact, exercise is the only relatively easy lifestyle change found to prevent and at times reverse type II diabetes. It has been found to greatly reduce heart attacks as well.
Should You Exercise If Your Blood Sugar is High?
It depends on what has made the blood sugar high.
Situation 1. If your blood sugar is over 250 mg/dl first thing in the morning, moderate or strenuous exercise at that time is not recommended. Usually a high blood sugar in the morning means that the insulin level is truly low. Exercising in the morning when the blood sugar is high is likely to raise the blood sugar even higher.
Situation 2. Your normally good blood sugar spikes to 250 mg/dl after you’ve eaten too many carbs and not covered them with enough insulin or sulfonylurea. Mild to moderate exercise would likely bring this blood sugar down. Be sure to check your sugars often and carry fast-acting carbohydrates (glucose tablets or dextrose candies) in case you go low.
Situation 3. You are an athlete preparing for a long athletic event by lowering your mealtime Humalog or Regular insulin and by eating extra carbohydrate. In this case you may start the event with a blood sugar that has rapidly risen to over 250 mg/dl. But a quick drop in blood sugars will occur shortly into the event because you have enough insulin to move this glucose into exercising muscles.
In this last situation, the alternative to spiking the blood sugar is to eat fewer carbohydrates and reduce both the mealtime insulin and the long-acting insulin before the event. This will lessen the chance of a high blood sugar and won’t require as much eating during the event to prevent an insulin reaction.
How Do Intensity And Duration Affect Fuel Use and The Blood Sugar?
At rest or during light exercise free fatty acids supply most of our fuel. For this reason, a drop in the blood sugar is less likely during less-strenuous exercise.
But as exercise intensity increases, so too does the use of glucose as fuel. Whether you walk or run a mile makes little difference in the amount of energy used.
However, in a one-mile walk, as little as 20 percent of the calories may come from glucose, whereas as much as 80 percent of calories may come from glucose during a strenuous mile run. Strenuous exercise, therefore, has to be balanced with more carbohydrate intake or a larger reduction in insulin than mild exercise.
The duration of exercise also influences the blood sugar. Activities that last longer are more likely to drop the blood sugar. For example, a 30-minute walk might not affect the blood sugar, but walking for 60 minutes may require extra carbohydrate, or a lower mealtime insulin dose.
The duration of exercise also plays a role in which fuel is used. As exercise continues over time, the body switches from using its limited stores of glucose and glycogen to using the very large stores of fat for fuel. Graph I on page 13 shows this shift in a person without diabetes over several hours.
In this example, the person is getting almost 80 percent of his or her energy from glucose at the start of fairly strenuous exercise. After three hours an equal amount of energy is coming from glucose and fat, but after six hours of exercise almost 80 percent of the energy comes from fat.
Remember, though, this person has a normal pancreas that automatically lowers insulin levels. If someone that requires insulin does not reduce his doses for this same exercise, he will have trouble accessing fat for fuel. As a result, it will be necessary to eat extra carbohydrates to supply much more of this energy.
The numbers on the far right of this graph show the total amount of energy coming from each source (1657 calories from fat and 1690 from carbohydrates). If someone with diabetes kept his or her insulin level high during this 6 hours of exercise, the excess insulin would blunt access to internal fat and glycogen stores. With these fat and glycogen stores largely blocked, 3,347 calories, mostly as carbohydrate, would have to be eaten to keep the blood sugar from falling. This is equivalent to eating almost two pounds of pure sugar, or drinking 20 12-ounce cans of regular soda. Obviously, lowering insulin levels helps to prevent these low blood sugars – and stomach aches!
The ExCarb System
ExCarb is a system designed to help you estimate the right amount of extra carbohydrates and the right insulin reductions needed for a variety of exercise activities. Because intensity and duration of exercise affect the quantity and type of fuel used, recommendations can be based on these factors.
Eat Extra Carbohydrates
The simplest way to offset exercise and keep blood sugars normal is to eat extra carbohydrates. To do this, find your planned exercise on the left hand side of Table I on page 13. The middle of the chart shows the estimated grams of carbs used per hour for each exercise. The estimates are given for different weights because energy expenditure increases as weight increases, especially in weight-bearing exercises.
The next step is to determine the amount of carbohydrates you will need to eat for this planned exercise by counting grams of carbohydrate in food. For example, if you weigh 150 pounds and walk briskly at 4.5 miles an hour for one hour, you will burn approximately 45 grams of carbohydrates. Two average-sized apples or a cup of milk and three graham crackers will provide 45 grams of carbohydrate. (See Stop The Rollercoaster by John Walsh, PA, CDE; Ruth Roberts, MA, and Lois Jovanovic-Peterson, MD, for a complete version of this table and a chapter on carbohydrate counting.)
Lowering Insulin (or Sulfonylurea) Doses
The second way to offset exercise is to lower insulin doses. For those on medications only, an occasional reduction in sulfonylureas may be necessary. (Metformin, acarbose and troglitazone do not cause lows.)
As exercise increases in length and intensity, a reduction in insulin doses becomes more and more necessary. Long, intense periods of exercise require that insulin levels (or sulfonylureas) be lowered before the exercise starts. They may also require lowered doses for many hours following the exercise.
Those who use multiple injections of insulin or an insulin pump can rapidly lower their blood insulin levels to better match long or strenuous periods of exercise. Reductions can be made in the pre-meal injections or in pre-meal bolus for pumpers before exercise begins. Reductions may also be made in the long-acting insulin dose the morning of the exercise, and possibly the night before.
Giving less Humalog or Regular for a meal is ideal for moderate or strenuous exercise that occurs within one to two hours of eating. A reduction in the long-acting insulin is ideal for moderate or strenuous exercise that is going to last 90 minutes or longer. However, when the long-acting insulin doses are lowered, it takes four to eight hours before the blood insulin level actually starts to drop, so any reduction in the long-acting insulin has to be planned well in advance of exercise. In addition, taking less long-acting insulin may be necessary for up to 36 hours following longer exercise.
Since lowering the insulin dose allows more fuel to be obtained from internal stores of glycogen and fat, you don’t need to eat as much additional food. This helps those who want to lose weight and those participating in long periods of exercise who don’t want to consume the large portions of carbohydrates that would otherwise be required.
General recommendations for combined carbohydrate and insulin adjustments are given in Table II on page 14. The table details how the intensity and duration of exercise can be balanced with ExCarbs or reduced insulin doses.
Let’s try using the tables to plan the adjustments necessary for Jane, our hypothetical exerciser. Jane has type I diabetes, weighs 100 pounds and uses 38 units of insulin a day. She is planning a 30-minute run at eight miles per hour, a strenuous run for her. From the table we see she will need approximately 48 grams of carbohydrate. Looking at the intense exercise section of Table II we see she can balance her exercise by eating 40 grams of carbohydrate (without extra insulin) and reducing her pre-meal insulin by 30 percent.
Why combine adjustments here? By reducing her insulin beforehand, she can get by with less carbohydrate intake. Combining lower insulin doses with eating extra carbohydrates is the safest approach for long and strenuous exercise, especially as you are learning to use the system.
Note: Adjustments of insulin and carbohydrates vary greatly from individual to individual. The reasons for these variations are complex and not completely understood. Some people may need only slight reductions in insulin doses for exercise; others may find that a large insulin reduction is the only way to control blood sugars. For many, the Humalog or Regular taken for breakfast does not need to be lowered for morning exercise, but when they do the same exercise later in the day, an insulin reduction is required.
Before you begin using the system, discuss all insulin and sulfonylurea adjustments with your physician/health care provider. Then cautiously experiment in order to learn your own responses to adjustments. Record your results, and discuss these with your physician or health care team.
Remember: The best control, whether you exercise or not, comes from planning ahead and from frequent monitoring and recording of your blood sugars. The ExCarb system will work for you only if you have good control before you exercise – that is, your insulin doses or sulfonylureas are set up correctly and you count carbohydrates. The benefits of exercise are well worth the difficulty of controlling your blood sugars while you exercise. Try the ExCarb system for blood sugar control with exercise and let us know how it works for you.