blood sugar

How do you check your blood sugar that it is balanced during the day. Would not getting enough sugar cause me to get tired and fatigued during and after a workout on certain days of the week. I'm not talking about I'm just feeling run down after work out but I'm talking about passing out and...

Okay so this is really weird. When I am fasting I noticed sometimes my blood sugar goes up like over 120 even to 140+ especially around the 8 hour mark of fasting. I was under the impression that the blood sugar is always going to go down. can anyone explain the science of why this happens...