There are a number of reasons why it is a bad idea to exercise with high blood sugar. Some of them are health related, and the others are just related to problems associated with exercise performance and ability.
First the health reasons.
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As you know, keytones can be a byproduct of high BG and can also lead to possible diabetic ketoacidosis (DKA). One of the recommendations for treating keytones is drinking a lot of water to flush the sugar and keytones out of your kidneys. High BG and keytones can cause dehydration. On top of that, higher amounts of sugar in the urine causes more water to be drawn into the urine, resulting in even more dehydration. Put the dehydration that comes with exercise on top of that, and you are making it even worse. This is a bad thing to do!
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High BG increases your insulin correction factor. Many (most?) people find that there is usually a much lower correction factor for a BG of 150 than 300 (meaning you need more than twice as much to correct a 300 than a 150). It is non-linear as you get higher. Higher BG means the body becomes more insulin resistant, for numerous reasons. There are many reasons for the higher correction factor that comes with higher BG, such as the signaling that occurs with the protein tyrosine phosphatase 1B - which attenuates insulin action by dephosphorylating the insulin receptor proteins. The short of it - correcting a higher BG means more insulin is needed, and that is not what you want to do when exercising. (See reference here ===> Nonlinear Metabolic Effect of Insulin across the Blood Glucose Range in Patients with Type 1 Diabetes Mellitus - PMC)
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Recent studies have shown that higher BG levels change the behavior of blood vessels, making them contract more than normal, and can possibly result in higher blood pressure. Again, not something to play around with. See reference here ===> https://bpspubs.onlinelibrary.wiley.com/doi/abs/10.1111/bph.13399)
Some of the non-health reasons, simply from a perspective of sports performance.
- High BG feels crappy. You just don’t have the same energy level and won’t feel as good. A negative feeling associated with exercise is not helpful.
I think #5 is important…
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You can’t burn fuel for the exercise. You can’t even perform fat metabolism! In the fuel sources discussed above - obviously your body can’t use blood glucose for energy (no insulin to process it). It can’t use liver glycogen for the same reason. And it can’t even use fat metabolism. (The reason your body can’t use fat metabolism when you have extremely high BG - for fat metabolism to occur the complete oxidation of Acetyl CoA in the Krebs cycle requires a derivative of carbs - oxaloacetate. People often express this by saying, “fat burns in the flame of carbohydrate”. Without carb metabolism, there is no oxaloacetate, which means no acetyl CoA, which means no fat metabolism…) Exercising with high BG is like a big “screw you” to your body. Do all this work but I am not giving you any fuel for it…
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Stressing your body with exercise - and causing even more stress by trying to perform it when feeling bad with high BG and without fuel - means your body may respond by dumping stress hormones out. The body says, “You are stressed. Here is some epinephrine, norepinephrine, and cortisol.” And your liver responds to the stress hormones by? You guessed it - dumping out more sugar! Ugh. A bad cycle.
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From a performance perspective - you just can’t do the same thing with high BG. Your body will not fire the same way.
Anyway, just a few reasons I can think of. I would always advise people to aggressively correct a high BG before exercising. The actual BG number maybe depends on each person, but definitely not with any keytones in the urine.