"The aim is to lower your insulin levels by reducing your total carbohydrate intake. But we do not want your carb intake to go so low that your liver runs out of reserves and you become ketogenic. Sometimes this can happen if you are simultaneously exercising and cutting your daily carb intake.

If you feel particularly weak, or experience symptoms of ketosis such as headache, nausea or vomiting, then it might be that you have exhausted your liver’s reserves of carbohydrate (your “battery” is flat). You should be aware that exercise can drain your liver of carbs and tip you into ketosis if you are not replacing them. So here is the good news: you need to replace those carbs that you burn during exercise – you can add them to your daily allowance."

I've been eating keto-ish for 17 years and it does not cause any problems. I do much bicycle riding (10,000+ miles last year) and don't replenish any carb stores during or after my rides.