If Jane, who weighs 60 kg/132 lbs wants to do an hour of an aerobics class 90 minutes after a light lunch with not much carbohydrate we can assume she is going to use up 60 g of carbohydrate. If her carbohydrate ratio is 1:10 (i.e she needs 1 unit of insulin for every 10g carbohydrate) then she will need 6 units less of insulin with her meal. BUT if she has a meal of only 40 g carbohydrate she would only be taking 4 units.

What she can do is reduce her meal bolus by 2 units (so only take 2 units). This would cover some of the carbohydrate used by exercise ie 2 units X 10 (for her carbohydrate ratio) eg 20 g so she would still need to take 40 g carbohydrate (60 g-20 g) before and during the exercise. (This would be equivalent to about 2 lots of 250 ml of a sports drink containing 15 g carbohydrate/250 ml ie 250 ml before exercise and 250 ml during exercise).

Jane would then need to check her blood sugars and can adjust this change next time she exercises if she needs to!

John (72 kg/159 lbs) takes his dog for a walk 2 hours after dinner. He is going out for 30 minutes but walks quite hard up and down hills (moderate intensity). We can assume he will use up 36 g of carbohydrate (1 hour would be 72 g so 30 minutes is 36 g). For his dinner he is having 80 g carbohydrate with a carbohydrate ratio of 1:8 (10 units of insulin). John decides to reduce his meal time insulin by 2 units which equates to 16 g carbohydrate (2 x 8). His total carbohydrate use by the walk will be 36 g so he will need to take an extra 20g of carbohydrate as a snack before or during his exercise.

Sarah (68 kg) is going swimming 2 hours after breakfast. She will be swimming for 1 hour. According to the table she will use up 56 g of carbohydrate per hour. For breakfast she has 80 g of carb with a carb ratio of 1:8 so she would normally take 10 units of insulin.

She has several options:

1. Taking extra carbohydrate< – Sarah can take 56 g of carb before and during exercise.
2. General method of reducing bolus insulin – reduce bolus by 75% e.g. only take 2 units.
3. Individualized method for bolus reduction – She can reduce her meal time insulin by 7 units (56 divided by 8 – her carbohydrate ratio).
4. Combination bolus method – She could decide to reduce her meal time bolus by 4 units e.g. equivalent of 32 g carb (4×8). This will leave her with 24 g carb (56-32) to replace while exercising.
5. Combination basal method – If she is on a pump she is likely to remove the pump anyway during swimming so she could cut her basal rate by 50% 90 minutes before exercise, stop it completely for 1 hour the resume at 50% for 90 minutes. If we assume her basal rate is 0.8 units per hour this is a reduction of 2 units (0.4 x 3 +0.8). With her carbohydrate ratio of 1:8 this would be 16 g of carb so she would need to take in 40 g carb (56-16) before and during exercise.