Exercise and Physical Activity: the Impact on Blood Glucose

Contents

• What is the intensity of an activity?
• Measuring the intensity of an activity
• Blood glucose monitoring
• Blood glucose targets
• Low blood glucose
• High blood glucose
• Exercising with diabetes complications
• Useful resources

What is the intensity of an activity?

The intensity of a physical activity refers to how much energy your body uses when you do that activity. This is measured by the number of calories you burn.

Different types of activity can have a different impact on your blood glucose levels, depending on the intensity of the activity.

Lower-intensity exercise uses a larger percentage of energy from fat, as the body does not need to produce energy quickly and efficiently to maintain the activity. If you walk a mile, 20% of the fuel you need will be supplied in the form of glucose and 80% from fat.

On the other hand, higher-intensity exercise gets a larger percentage of its energy from carbohydrates, as the body is able to produce this quickly, making glucose the main source of energy in such activities. If you run a mile at a strenuous pace, as much as 80% of the energy you need will come from glucose.

The higher the intensity of the exercise, the greater the impact it will have on your blood glucose level.

Measuring the intensity of an activity

There are several ways to measure the intensity of a particular form of exercise:

The talk test

This is a very easy test to help you work out the intensity of whichever physical activity you are doing. You simply need to pay attention to how breathless you are.

In general, if you are doing a low-intensity activity you should be able to carry on a conversation during the exercise.

If you are doing a moderate-intensity activity you will be able to talk but not sing during the exercise.

If you're doing a high-intensity exercise, you will not be able to say more than a few words without pausing for a breath.

Perceived exertion

The Borg Rating of Perceived Exertion is another way of measuring the intensity level of a physical activity. The rate of perceived exertion is how hard you feel your body is working and is based on the physical sensations you experience during exercise. These include:

• Increased heart rate
• Breathing more heavily
• Sweating
• Muscle fatigue

Try to estimate your feeling of exertion as honestly as possible, neither overestimating or underestimating it. It is important to focus on your own feelings of effort and exertion and not how they compare to other people’s. Look at Table 1 below and its explanations and then work out your own rating when you exercise.

Table 1: The Borg Rating of Perceived Exertion scale                                               

The optimal target intensity level for a healthy adult is 12–16.

Blood glucose monitoring 

If you manage your diabetes with insulin, you may find that exercise can cause high blood glucose (hyperglycaemia) or low blood glucose (hypoglycaemia). It is important that you become familiar with how different types of activity affect your blood glucose levels.

To help with this, if you have type 1 diabetes you should check your blood glucose levels:

• before an exercise session
• during an exercise session (every 30 minutes)
• after an exercise session.

Frequent blood glucose testing allows you to work out when you need to make changes to your insulin doses or your food intake. This in turn means that you can help prevent your blood glucose levels from getting too high or too low.

You should not make any changes to your diabetes medication until you have discussed it in detail with your diabetes care team. Any change should be gradual and should only be done after your blood glucose readings have been thoroughly assessed.

Blood glucose targets

The following guidelines have been developed for people who manage their diabetes with insulin.

If you have a starting blood glucose level of less than 7 mmol/L:

• Eat 10–15 g of carbohydrate and recheck your blood glucose after 10 minutes.
• Repeat this process as necessary until your blood glucose is 7 mmol/L or more.
• If you have low blood glucose before you exercise, it may be best to postpone the activity for that day.

If you have a starting blood glucose level of 7–12 mmol/L, you can go off and enjoy your exercise session.

If you have a starting blood glucose level of more than 12 mmol/L, you should not exercise unless you have previously worked out an exercise management plan with your diabetes care team and have been advised on what you should do in this situation, as recommendations will depend on the type of diabetes you have.

Type 1 diabetes

• Check for ketones. If they are present do not exercise and treat according to the instructions in the sick day guidance.
• If no ketones are present, it is fine to exercise, but you should not eat any additional carbohydrate until your blood glucose level has dropped below 12 mmol/L.

Type 2 diabetes

• Go ahead and exercise. It is unlikely that you will need any additional carbohydrate.

Low blood glucose

During the activity, the body needs additional oxygen and energy. The main source of energy comes from glucose, which is initially released from stores within the body’s cells, but as you continue to exercise is taken from the blood and eventually the liver.

Exercise-related low blood glucose (hypoglycaemia) is common if you have type 1 diabetes and, to a lesser extent, if you have type 2 diabetes and use insulin or tablets which increase insulin production, such as Gliclazide. Hypoglycaemic episodes (‘hypos’) can develop during or after the exercise, in some cases even as much as 24 hours later.

It is always best to plan ahead and, where possible, reduce your insulin dose, increase your carbohydrate intake or both. If you are unable to plan ahead, you will only have the option of eating additional carbohydrate if you need to. If you have not had a chance to reduce your insulin you will require 1 g of glucose per kilo of your body weight after each hour of activity to prevent you from having a hypo. You should make sure that you always carry treatment for hypoglycaemia on you when you exercise.

When planning ahead, it is important that you consider why you are exercising. If you are doing so in order to lose weight, for example, it would be best to reduce your insulin dose in advance so that you don’t have to eat additional calories in the form of carbohydrate. 

Before you exercise, think about the following factors so that you can work out how best to prevent a hypo:

• Your reason for exercising
• The duration and intensity of your exercise
• The impact of your exercise on your blood glucose levels (by monitoring before, during and after)
• The time of day
• The timing and action of any recent insulin
• The injection site of any recent insulin
• Any previous hypos within the past 24 hours

Before you make any adjustments to your insulin regimen, it is very important that you work out an exercise management plan with the help of your diabetes care team. 

High blood glucose 

Often the biggest concern of people with diabetes who take regular exercise is that they will develop low blood glucose (hypoglycaemia) during or after their exercise session. This often makes them eat additional carbohydrate which is not actually required, which in turn can result in high blood glucose (hyperglycaemia).

Remember, you are only at risk of exercise-related hypoglycaemia if you manage your diabetes with insulin, or with tablets which increase insulin production, e.g. Gliclazide. If this applies to you, checking your blood glucose levels before, during and after exercise will alert you to any need for additional carbohydrate.

If you are exercising to lose weight, you should remember that you will be consuming extra calories when you eat unnecessary carbohydrate. In addition, making unnecessary and inappropriate adjustments to your insulin before and after you exercise may also lead to hyperglycaemia.

If you have type 1 diabetes, having a high blood glucose level before you exercise may lead to an even higher reading afterwards. Your high blood glucose indicates that there is not enough insulin in your body, and since you need the insulin to transport fuel (i.e. glucose) to your muscles when you are exercising, this can trigger your liver to release glucose from its stores, which can lead to even higher blood glucose after you finish exercising.

Remember too that when insulin is lacking, the body will access other fuel sources, namely fat. A side-effect of this process is the production of ketones. It is essential to check for ketones if your blood glucose level is more than 12 mmol/L. If ketones are present, you should not exercise until your blood glucose has stabilised.

A further factor which can result in a rise in blood glucose levels during exercise is the type of activity in question. Short, sharp, high-intensity activities, such as sprinting and weightlifting, increase the production of the stress hormones adrenaline and cortisol, which can, in turn, have a significant impact on blood glucose readings.

If you notice that your blood glucose readings rise during exercise, you should talk to your diabetes care team who will be able to give you advice on how to keep your blood glucose stable.

Exercising with diabetes complications 

The table below gives you information about how to exercise safely when you have specific complications from your diabetes.

Table 2: Exercising with diabetes complications

If you have any complications associated with diabetes, you must always discuss any new activity with your diabetes care team before you start.