Managing Diabetes with Sport, Why is there any problem?

Recognised Challenges to Fuel Regulation during Exercise and the Implications for Athletes with Type 1 Diabetes
• Predisposition to hypoglycaemia after 20-30minuntes if insulin levels are inappropriately high.
• Overall predisposition to hypoglycaemia during and after exercise.
• Heart rates at the upper aerobic thresholds result in the highest risk of hypoglycaemia.
• Risk of Hypoglycaemia typically diminishes after several hours of endurance exercise.
• Predisposition to hyperglycaemia
• Competition stress, heat stress including high humidity, dehydration, high pre-exercise blood glucose and arm exercises may augment counter-regulatory hormone response to exercise predisposing to hyperglycaemia.
• Repeated episodes of exercise or hypoglycaemia may reduce/impair counter-regulatory hormone response to exercise predisposing to hypoglycaemia.
• Particular care should be taken to avoid hypoglycaemia if exercising after a recent (antecedent) hypoglycaemic episode.
• Cold temperatures increase risk of hypoglycaemia.
• Altitude increases the risk of hypoglycaemia.
1. Longer Duration Moderate Intensity Aerobic Endurance Activities
Fuel Regulation in Athletes without Diabetes
During moderate intensity aerobic exercise typical of endurance activities, after muscle glycogen stores are depleted hepatic glucose production is essential for homeostasis with insulin and glucagon the major regulators. During exercise a fall in insulin promotes hepatic glycogenolysis whereas a rise in glucagon is also required to achieve maximal hepatic glycogenolysis and gluconeogenesis. The effects of insulin and glucagon are additive, indeed without the presence of glucagon a decrease in insulin alone will not stimulate hepatic glycogenolysis. Increased levels of insulin independent GLUT 4 transporters then allow the glucose produced to be transported into muscle.
Implications for Athletes with Type 1 Diabetes:
• Predisposition to hypoglycaemia after 20-30minutes
In the individual with diabetes injected insulin does not reduce on commencing exercise, indeed its levels may even increase as exercise promotes blood flow to and absorption from depots in subcutaneous tissues. Without a fall in insulin levels individuals with diabetes may have relative over-insulinisation. Over-insulinisation then blocks hepatic glucose production and results in an increased risk of hypoglycaemia following 20-60 minutes of moderate intensity aerobic exercise when intramuscular glycogen stores are depleted and hepatic glucose is essential for glucose homeostasis.
• Overall predisposition to hypoglycaemia during and following endurance activities
Increased transport of glucose into muscle secondary to up-regulation of insulin independent GLUT 4 transporters during exercise further increases the risk of hypoglycaemia in individuals with diabetes. These transporters are thought to be key both during exercise to enable glucose transport into muscle cells and post exercise to help replenish muscle and hepatic glycogen stores. Their persistence during the post-exercise phase may however further potentiate relative over-insulinisation leading to increased risk of hypoglycaemia post exercise in the individual with diabetes5.
• Exercising at the upper aerobic threshold further increases the risk of hypoglycaemia
Heart rates at the upper aerobic thresholds (usually in the region of 70% Maximal HR) result in highest rates of aerobic glucose oxidation and consequently highest risk of hypoglycaemia.
• Potential for hyperglycaemia to develop after several hours of endurance activity
After several hours of endurance activities the risk of hypoglycaemia is reduced for two main reasons. Firstly, because fuel consumption shifts to free fatty acids providing the major fuel source and secondly because in the majority of individuals insulin activity from previous injection (now at least several hours ago) is likely to be outside of the period of maximum activity. Indeed, at this stage hyperglycaemia may occur if oral carbohydrate supplementation is continued at previous rates required earlier during exercise.
2. Short Duration High Intensity Anaerobic Activity
Fuel Regulation in Athletes without Diabetes
High intensity exercise includes activities above the lactic threshold with a greater reliance on anaerobic metabolism. Cathecholamines are thought to take over primary control of hepatic glucose production from insulin and glucagon during exercise of high intensity. This is recognised to cause hyperglycaemia even in individuals without diabetes, it is thought that dramatically increased cathecholamines trigger a relative overproduction of glucose. This can then be compensated for by an increase in endogenous insulin production.
Implications for Athletes with Type 1 Diabetes:
• Predisposition to hyperglycaemia
Unable to compensate by an increase in endogenous insulin production hyperglycaemia is common following short high intensity exercise in athletes with type 1 diabetes.
3. Intermittent High Intensity Exercise
Fuel Regulation in Athletes without Diabetes
During periods of moderate intensity aerobic activity regulation is thought to be similar to described above. However, the high intensity bursts result in increased levels of counter-regulatory hormones (including cathecholamines) and consequently increased glucose production.
Implications for Athletes with Type 1 Diabetes:
• Smaller risk of hypoglycaemia and could cause hyperglycaemia
Increased counter-regulatory hormones during high intensity bursts compensate for predisposition toward hypoglycaemia during lower/moderate intensity periods. Hence the individual with type 1 diabetes would be likely to require significantly smaller alterations to diabetes management for example smaller reductions in insulin or smaller amounts of carbohydrate supplementation to prevent hypoglycaemia.
4. The Impact of Counter-regulatory Hormones
The counter-regulatory ‘stress response’ promotes glucose production and antagonizes insulin action. A number of counter-regulatory hormones are recognised including glucagon, noradrenaline, adrenaline and cortisol.
For the individual with diabetes high counter-regulatory hormone levels will increase the risk of hyperglycaemia during exercise whereas low levels promote hypoglycaemia. The exact mechanisms are as yet not fully elucidated5 and the breadth of factors that can influence the counter-regulatory response provides one of the greatest challenges to predicting the effect of exercise on the individual with diabetes. Recognised variables that are known to affect the counter-regulatory response are illustrated in table 1.
In Type I diabetes there are several important variations in physiology that are likely to impact on mobilization of fuels and the control of blood glucose during exercise. In common with all people with diabetes, there will also be issues relating to blood glucose control following food ingestion, but which are exaggerated by increased food requirements to support increased exercise.
5. Antecedent Hypoglycaemia
Following an episode of hypoglycaemia in the 24hrs preceding exercise the degree of counter-regulatory impairment is thought to be dose-dependent relative to the degree of antecedent hypoglycaemia and starting at a level of only 3.9mmol/l20. Interestingly women show relatively preserved counter-regulatory responses following antecedent hypoglycaemia which may confer greater protection from further hypoglycaemia. In practical terms exercising following a recent episode of hypoglycaemia, particularly in men increases the individuals’ risk of hypoglycaemia during exercise, this risk is proportionally higher if the degree of hypoglycaemia preceding exercise was more severe.
6. Miscellaneous Variables
Although it is impossible to cover all variables that may affect glucose homeostasis during exercise, cold and altitude commonly pose challenges to the exercising individual with diabetes. Both lead to a predisposition towards hypoglycaemia. Cold by increasing glucose uptake to generate heat in muscles and altitude by increased reliance on carbohydrate fuel sources and anaerobic metabolism due to reduced oxygen tension.
Exercise Physiology: Conclusions
Consideration for the physiological challenges to glucose homeostasis, in particular the nature of exercise including intensity, duration and the potential impact of counter-regulatory hormones are critical in the management of type 1 diabetes and exercise. Typical responses are summarised in the table two. It is however important to recognise that this table does not include every factor that impacts on glucose homeostasis and individuals will vary in their responses. Our physiological understanding can help guide individuals but it cannot replace the importance of individuals monitoring their own blood sugars and learning to predict their normal responses to a particular exercise.

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