The general rules for blood sugar management in conjunction with exercise will vary depending on whether you are a type 1 diabetic or a type 2 diabetic. With that in mind this article, the first in a series of two articles, will discuss what is known about the basics of energy metabolism in our bodies, whether at rest or exercising and will consider the benefits and the risks associated with exercise for type 1 diabetics. Finally we’ll consider different approaches for managing blood sugars before, during and after exercise. Ultimately the goal is to allow the type 1 diabetic to exercise safely by understanding what is going on in their body and still achieve reasonable blood sugar readings before, during and after exercise. Next issue we will discuss exercise as it pertains to type 2 diabetes.
A Brief History
Did you ever wonder how type 1 diabetes was managed prior to the discovery of insulin? It turns out that diet and exercise were the main therapies although exercise was difficult because of the associated metabolic abnormalities. Once insulin therapy became available it was easier for type 1 diabetics to exercise but still blood sugar management was challenging since 1. acute or delayed hypoglycemia was common and 2. post-exercise hyperglycemia and ketosis could also occur. Keep in mind this was all prior to the home blood glucose (BG) monitoring devices that we take for granted today. Back then healthy active individuals with type 1 diabetes were often not permitted to participate in various sports because of the concerns regarding unknown blood glucose levels in conjunction with intermittent activities of varying intensity and duration. With the advent of simple portable BG monitoring and increased understanding of neural and endocrine factors regulating metabolism during exercise it is much safer but still challenging for individuals with type 1 diabetes to exercise.
At present many people with type 1 diabetes are achieving the same level of conditioning and success as non-diabetics, many becoming world class athletes. However, in the general population, type 1 diabetics are on average less fit and have lower aerobic capacity than their non-diabetic peers. Research to date suggests that type 1 diabetics respond normally to physical training and thus the primary obstacle is likely the manner in which type 1 diabetics are educated/encouraged with respect to the benefits and the safety of exercising. There are of course risks as well which need to be considered. When the risks and benefits are weighed, and these will vary for everyone, it is essential to keep in mind the reasons for exercising. They should be for the fun and the fitness primarily. In terms of blood sugar management for the type 1 diabetic, the research to date does not show a significant benefit of exercise and it can actually make blood glucose management poorer and certainly more challenging.
Benefits Of Exercise
Regular exercise has many benefits to health and these are listed in Table 1. These benefits hold true for everyone, not just type 1 diabetics. Most of these are self-explanatory but a few points are worth adding. Since people with diabetes are at higher risk for premature cardiovascular disease, retinopathy, neuropathy and nephropathy it may be particularly worthwhile to pursue some form of physical activity as a part of day to day living. There are also the psychological benefits including a better quality of life and improved self-esteem. This is especially important for someone who is faced with the challenges and restrictions of living with a chronic disease.
|Table 1. Benefits of exercise for patients with Type 1 Diabetes
Although exercise does lower blood sugars, at least temporarily, overall blood glucose control may not be improved. Some studies have failed to show any benefits while others have shown modest benefits. Post-exercise hyperglycemia as well as additional caloric intake during or following exercise could be two reasons why benefits may not appear. Those are the averages however and individuals who are meticulous about food intake, BG monitoring, and insulin adjustments before, during and after exercise may be able to improve their overall BG management. However for the majority of individuals Edward Horton said it very nicely:
Risks of Exercise
There are several risks associated with exercise as noted in Table 2. These have to be weighed against the benefits when deciding on whether an exercise program is appropriate and if so what type of program. Especially in adults and in particular when long-term complications of diabetes have begun to appear it is essential that these are properly assessed and appropriate precautions taken when planning an exercise program. Review the list in Table 2 and determine which warnings may apply to you. Ask your own doctor or diabetes specialist if you have any uncertainties about your status with respect to these risks. Although the list is long do not let that discourage you from initiating some type of an exercise program, however modest it may be.
|Table 2. Risks of exercise for patients with Type 1 Diabetes
Energy Production in Our Bodies
First let’s consider the fuels available for energy production and how they are regulated. The three possible fuels are carbohydrate (glucose), fat (fatty acids) and protein (amino acids). Since amino acids only make up 1-2% of our energy requirements in muscle tissue, whether at rest or exercising, we will not discuss them in any detail. With respect to glucose and fat use as a fuel, they will vary depending on the energy demands, whether at rest or exercising, exercise intensity and duration, the level of physical training, overall diet and of course meals taken before and/or during exercise which affect substrate availability.
At Rest In the body at rest in the fasting state the glucose concentration is kept relatively constant by the balance between 1. glucose utilization in all tissues and cells and 2. hepatic (liver) glucose production. In terms of a body budget for glucose utilization, approximately 50% is used by the brain, 30-35% by other tissues (blood cells, liver, kidneys, gonads etc) and only 15 to 20% by muscle tissues. In terms of potential energy sources (figure 1) you can see that metabolic fuels are made available by 1. breakdown of glycogen and triglycerides in the muscles 2. fatty acids released from adipose tissue (fat) and 3. glucose release from glycogen stores in the liver. The latter two are released into the blood circulation and delivered to where they may be needed, especially muscle during exercise.
If we consider just the muscles in the fasting state at rest only about 10% of the energy generated in skeletal muscle comes from glucose. That means that 85-90% is derived from the oxidation of fatty acids. This is an interesting fact and is often overlooked when discussing diet, weight loss and various forms of exercise. We tend to think only of glucose when we think of energy and foods while the potential energy reserves in fat tissue are not considered or even discussed. In terms of type 1 diabetes and exercise the reasons for this are the dangers of allowing the blood sugars to go too low (hypoglycemia) and hence the focus on blood glucose levels. In addition in type 1 diabetes there is also the inability of the bodies glucose/fat metabolism regulation to respond to the demands. The scenario for type 2 diabetics is different and will be addressed next issue.
During exercise. During exercise a number of cardiovascular, hormonal and neural responses occur in concert to ensure efficient delivery of fuels and oxygen to muscle tissue and removal of end products. The onset of exercise results in a rapid increase in glucose utilization but not as much as you might think. At 50% of our VO2 max (for most individuals this represents a brisk walk or what is called moderate intensity) our muscles are using about 50% fat and 50% glucose. Now you can see why walking, especially a moderate to brisk pace, is such a highly recommended form of exercise. It strikes a wonderful balance in terms of physical safety, cardiovascular fitness, burning fat and lowering blood sugars. And you can talk as much as you like while you’re doing it.
As the intensity of exercise increases to 70-75% of VO2 max carbohydrate becomes the main metabolic muscle fuel. This intensity of exercise is comparable to a moderate to brisk jog depending on your level of fitness. As the intensity increases to 100% VO2 max nearly all energy is derived from carbohydrate oxidation with a very small (1-2%) being derived from fats and amino acids. Typically the muscles use up their glycogen stores first and then start to draw on circulating blood sugars. Blood sugars are replenished by glucose production from liver glycogen breakdown and liver gluconeogenesis (glucose synthesis) and as a result the blood sugars are kept relatively constant (refer to figure 1). At a moderate intensity these proportions of energy utilization are reliable estimates. After 2-3 hours, however, the glycogen stores can become depleted and there is a shift back towards a higher proportion of fatty acids for muscle energy.
Highly trained individuals perform the same work at a lower VO2 max than less conditioned individuals and as a result utilize less carbohydrate and more free fatty acids. Even if the conditioned individual performs at the same VO2 max they will consume less carbohydrate and a greater percentage of FFA. This means that their muscle and glycogen stores will be depleted less rapidly, they’ll have greater endurance and burn more fat in the process.
Following exercise the body must replenish both muscle glycogen stores and liver glycogen stores. Therefore, even after exercise, the muscles continue to take up glucose and in this case convert it into glycogen. This will happen very slowly in the absence of food and more quickly with carbohydrate intake; normal glycogen levels being established within 12 to 24 hours. Muscle glycogen replenishment occurs more rapidly than liver glycogen replacement. This post-exercise demand for glucose is the cause of post-exercise hypoglycemia. The best solution is frequent BG testing and appropriate food intake. Some will recommend BG testing at 1-2 hour intervals following exercise in order to determine how your body responds to exercise. Accurate records of this information allows you to make the appropriate adjustments. A key piece of advice is to always err on the side of caution. That means taking in additional carbohydrate or reducing insulin and accepting a BG reading higher than target rather than going low. This is especially true for post-exercise in the evening and bedtime when a low BG can be more than just an inconvenience but can actually be lethal. Be particularly cautious if you are just beginning an exercise program or exercise irregularly since BG values and your body’s response are even more difficult to predict.
Regulation of all these metabolic processes during and after exercise is highly integrated and beyond the scope of this article. Even though we will focus primarily on insulin many hormones are involved and ensure that fluid and electrolyte balance is maintained in addition to fuel balance.
The Role of Insulin. As mentioned earlier hormones play a major role in the regulation of these metabolic processes. Insulin in particular has several roles. First of all, in the non-diabetic insulin secretion is actually suppressed during exercise thus reducing the amount of glucose uptake by non-exercising tissue. Secondly, since insulin suppresses liver output of glucose and inhibits lipolysis in fat tissue, falling insulin levels result in an increase in both liver glucose production and release of fatty acids into the circulation providing more energy for working muscles. Since little or no insulin is needed in exercising muscle a lower plasma insulin level does not adversely affect glucose utilization while exercising. A note of interest here is that there are glucose transporters in muscle tissue that do not require insulin which are activated during exercise. This partially explains the ability to exercise with reduced levels of circulating insulin.
The implications of this are immense in terms of exercise and type 1 diabetes. First of all, in type 1 diabetes this critical regulatory hormone is essentially absent yet is provided artificially by insulin injection. Hence decisions made regarding insulin injections associated with exercise must be done carefully. For example taking a normal dose of insulin just prior to exercising maintains suppression of liver glucose, inhibits fat tissue breakdown and would further increase glucose uptake in tissue that would normally reduce glucose uptake during exercise (i.e. Non-exercising tissue). All of these could contribute to a hypoglycemic condition and are counterproductive to successful exercising. The situation is further complicated by the fact that exercise will increase circulation and thereby increase the delivery of insulin to the peripheral tissues. This is most likely to occur if insulin is injected shortly before exercising and even more so if injected in a part of the body that is being exercised. Hence some degree of insulin reduction prior to exercise is always recommended.
How much should insulin be adjusted? There is no one correct answer since everyone is different and exercise goals also differ. What is provided here are guidelines that you can incorporate according to your fitness level and exercise intensity and duration. You should also document important information such as insulin dose, time and site of injection, time of day, prior food intake, exercise duration and intensity and pre and post-exercise blood glucose levels. This information can be used to learn and make adjustments where possible before, during and after exercise to safely maximize your performance and optimize your blood glucose control.
Some Guidelines As a rule, if blood glucose levels are below 5.6 mmoles/L then exercise should not be undertaken without first ingesting some carbohydrate. You should then make a record of the amount of carbohydrate and use the information for future adjustments of either food intake or insulin under similar conditions. An additional consideration is the rate of change in your BG. For example if a BG value of 9.7 is followed by a BG value of 6.3 one hour later just prior to exercise then you should recognize that your BG is rapidly dropping and the 5.6 rule does not apply. Take food before you exercise!
- Reduce your usual insulin dose
- Inject somewhere away from the exercising tissue if possible
- Inject mealtime insulin at least 2 hours prior to exercise
- If blood glucose is less than 5.6 mmoles/L supplemental feedings should be taken before and during exercise
- Take in carbohydrate during exercise if exercise is vigorous and of long duration (ie. Greater than 1 hour)
- Test blood sugars before, during and after exercise if needed and take in food when indicated to avoid low blood sugars
- Be sure to also take in adequate fluids while exercising
- Keep in mind that your blood glucose response will vary depending on the type, intensity and duration of exercise.
- Do your best to plan ahead if you know you will be exercising and incorpaorate as many of these suggestions as possible.
The timing of your injection as well as the type of insulin are factors to consider. It is safer to exercise immediately after eating when regular insulin has been injected since the insulin has a slower time of action. Fast-acting mealtime insulin has a faster and more intense onset of action. For this reason you should consider waiting at least 2 hours after eating before doing any exercise.
In terms of the insulin dose reduction, a short-acting mealtime injection can be decreased by approximately 30-50%. For the longer acting insulins, either individually or pre-mixed, you have to plan further in advance. For example you may want to reduce your morning injection of long-acting insulin if you plan to exercise that evening. During that day you may also consider moderating your food intake to accommodate the reduced insulin and still maintain reasonable BG control. You will have to determine what works best for you based on your pre and post-exercise blood sugar values.
The time of day is also critical. Studies have shown that the risk of hypoglycemia is reduced when exercise is done in the morning before the pre-breakfast insulin dose. This is because insulin levels are lowest at this time. In contrast, late afternoon or evening exercise can be trickier since insulin levels can vary depending on when lunch and/or dinner were eaten. Worse yet, since the post-exercise blood glucose lowering effects can last for 12 to 24 hours the risk of nocturnal hypoglycemic events is increased. This doesn’t mean you shouldn’t exercise in the afternoon or evening. Simply be aware of additional challenges associated with BG management and take precautions; frequent BG tests and snacks if needed.
Type of Carbohydrate. Carbohydrate in either a solid or liquid form work equally well. The liquid form has the advantage of providing fluid that is essential for optimum performance and recovery. You may also find the liquid form easier to digest especially if consumed while exercising. Depending on the nature of the exercise you may have to consume carbohydrate before, during and after. The type of carbohydrate can be chosen depending on the desired effect. For example, if you are trying to address a blood sugar you know is low and dropping you want a fast-acting carbohydrate. In this case you should choose a carbohydrate with a high glycemic index such as fruit juice,fruit, granola bar etc. There are energy drinks designed especially for this and in most cases you can safely dilute these sport beverages in half. As a reminder, glycemic index refers to the rate at a which a consumed carbohydrate appears as glucose in your blood (Quarterly Newsletter Vol.2 No.3). There are also tables listing the glycemic index for a wide variety of foods. For a meal prior to exercise that you know will be of long duration (e.g. a 2-3 hour hike or bicycle ride) you should consume a balanced meal with a relatively low glycemic index. This meal will continue to deliver glucose to your circulation over a prolonged period of time buffering the blood glucose lowering effect of exercise. Be sure your diet is balanced and nutritionally sound in order to optimize your performance and your recovery.
This is a common occurrence and is believed to result from the continued output of glucose from the liver and the absence of insulin. If you find this is an issue for you some people have success with a small (i.e. 2-4 units) post-exercise insulin injection, which can signal suppression of liver glucose production and enhance removal of glucose from the blood. Always test your BG and take appropriate action.
Everyone is different and will respond in a different way to food and exercise. Take this information and these suggestions and see what works for you. Always consider your safety first and be sure to enjoy whatever exercise activities you choose.
Eric Norman is a research scientist investigating diabetes and heart disease.
- Exercise For the Patient With Type 1 Diabetes Mellitus. Horton, E.S. Chapter 46 in Diabetes Mellitus: A Fundamental and Clinical Text. Second Edition. Editors Derck LeRoith, Simeon I. Taylor, Jerrold M. Olefsky. Lippincott Williams and Wilkins. 2000.
- Fuel Metabolism, Exercise and Nutritional Needs in Type 1 Diabetes. Franz, M.J… In Canadian Journal of Diabetes Care 22:4 pp 59-63.
- Dietary Carbohydrate in the Management of Diabetes: Importance of Source and Amount. Woelver, T.M.S. In Endocrinology Rounds, St. Michael’s Hospital, U. of Toronto. May, 2002.