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Daniel J. West BSc, PhD, Newcastle University (right) and Matthew D. Campbell BSc, PhD, Leeds Beckett University (left) report on whether altering diet and insulin doses could help to prevent hypoglycaemia (study carried out at Northumbria University). This article focuses on evidence-based strategies for managing blood glucose (sugar) during and after aerobic/continuous based exercise, such as jogging and cycling.
Hypos during exercise
Hypoglycaemia (low blood sugar) occurs during exercise due to a mismatch between glucose (sugar) uptake by the muscles and new glucose entering the blood stream. When muscles contract during exercise, signals push the glucose transporters to the cell surface and increase glucose uptake from the blood and into the muscle.
However, when insulin interacts with muscle it does the same thing. This means that during exercise, if insulin is high when a muscle is contracting, there are two signals that are triggering glucose to be taken from the blood and into the muscle.
Normally, at the onset of exercise the pancreas beta-cells switch off, and insulin concentrations quickly decline, and turn the pancreas alpha-cells on, so that glucagon starts to increase. Adrenaline will also be released.
These changes have the effects of:
• reduced blood glucose burning by the exercising muscle;
• increased release of glucose from the liver;
• greater use of muscle energy stores (carbohydrate stores [glycogen] and fat [triglycerides]);
• release of fatty acids from the fat tissue.
All of these changes have an important role in providing energy for the exercising muscles but maintaining glucose for the brain. In people with diabetes, as insulin is taken and not controlled within the body, the first, and most important step, is lost.
Furthermore, the large increases in body temperature and changes in blood flow during exercise can make insulin be released more quickly than usual from the injection site, resulting in exercising under even higher insulin concentrations.
The second key step, the increase in glucagon, can also be defective.
This results in:
• increased uptake of glucose by the exercising muscle and a continued uptake into muscles which aren't exercising;
• reduced release of glucose by the liver;
• reduced release of fatty acids from the fat tissue;
• reduced burning of fat and a greater reliance on carbohydrate burning for energy.
This creates a situation where blood glucose concentrations will fall, and with an inability to regulate insulin and defective glucose-regulation hormones, hypoglycaemia will occur.
What is the answer?
In theory, the body has enough stored energy to sustain multiple back-to-back marathons; people with diabetes have muscle and liver glycogen (stored glucose) and fat stores to use just as someone who does not have diabetes does, as such it is important that strategies are implemented which can maximise the use of this stored energy without relying on constantly eating carbohydrates.
Insulin is a master-switch in terms of how energy is stored and burned. Insulin promotes both the storage and burning of carbohydrates and it promotes the storage of fat and limits fat being burned.
With this in mind, an obvious option is to exercise with reduced insulin. Research has shown that prior to exercise, reducing bolus insulin by 25-75% with a carbohydrate meal one to three hours before exercise is an effective way of getting through an exercise session without the need to eat more carbohydrates during exercise.
Although hyperglycaemia (high blood sugar) may occur, it will likely be short-lasting and without any problematic increases in ketones (acid that remains when the body burns its own fat).
A more simple option is to perform a short burst of highly intense exercise (e.g. a sprint) for a few seconds intermittently throughout the session, or for 10 seconds at the beginning or right at the end; intense exercise makes the body secrete hormones like adrenaline which make the liver release more glucose and reduce the muscles’ uptake of glucose from the blood.
However, this effect is likely short lasting (around one to two hours after exercise), when these hormones return to normal levels, blood glucose may start to fall. While there has been a lot of attention placed on the prevention of hypos during exercise, less attention is paid to the disruption in blood glucose control that is experienced for many hours after exercise. If exercise is to be something that all people with diabetes can engage in regularly, strategies which enable people to return to a 'normal' routine after exercise need to be focused on, i.e. with a reduced fear of hypos, without the constant need to eat carbohydrate, or glucose yo-yoing.
After exercise, changes occur inside the muscle that mean for many hours (around 48 hours, maybe longer) the body is far more sensitive to insulin. This means that despite being at rest, the potency of the insulin taken is increased and means that glucose uptake into the muscle is far greater.
Combining this with defects in the hormones which stop glucose from falling creates an issue of 'late-onset post-exercise hypoglycaemia' - this is a big fear for people with diabetes, particularly if it occurs during the night. This fear of a late hypo/night-time hypo is thought to put people with diabetes off regular exercise altogether.
For the first few hours after exercise, the muscle's glucose transporters are highly active, and this is a particular hot spot for a 'hypo,' as insulin sensitivity and the muscle's capacity to transport glucose from the blood is very high.
Research has shown that it is important that with the meal after exercise, bolus insulin is reduced (30-50%), and this preserves blood glucose and protects against hypoglycaemia for around eight hours after exercise. Whether exercise is in the morning or the evening, reducing bolus insulin with the meals before and after exercise can help protect against 'hypo' for around eight hours.
Experiencing a night-time 'hypo' is a real worry, and it has been shown that after evening exercise, despite people with diabetes consuming a large evening meal with reduced bolus insulin, and a carbohydrate snack before bed, 'hypo' still occurred during the night. Even if going to sleep hyperglycaemic (12-15 mmol/l), this will not guarantee protection against a night-time hypo. To protect against a late-onset hypo, combining a reduced bolus insulin dose with the meals before and after exercise, and a slow-releasing carbohydrate snack before bed, with a 20% reduction to basal insulin, can keep blood glucose concentrations in the normal range during the night, meaning 'hypo' can be prevented during and for 24 hours after exercise. Importantly, combing these strategies is unlikely to cause hyperglycaemia (high blood sugar) or increase ketones.
In the tables above we have provided a summary of evidenced-based strategies for combating hypo's during and after exercise. These strategies are based on small, controlled laboratory studies and as such we recommend that people with diabetes tailor them to their own individual exercise routines to see which strategies work for them/are preferred.
Frequently monitoring blood glucose levels and the use of a training log/diary to record your progress can help in adjusting to your own individual needs and help to accommodate exercise as part of a regular routine.