Ever get your blood sugar tested? There's also a more accurate fasting test that can be done called the glucose tolerance test. Might not be a bad idea.
Dehydration can also be big factor in energy. There's some useful calculators and formulas out there.
0.5 ounces x Body Weight in Pounds = Daily Fluid Requirement in ounces
The other common way to calculate daily fluid needs is to base the fluid need on caloric intake. 1 milliliter of fluid for every calorie ingested. Converted to the household measurement of ounces, your body needs .034 ounces for every calorie that you ingest.
0.034 ounces x Daily Caloric Intake = Daily Fluid Requirement in ounces
This is a hydration calculator that factors in time of exercise and intensity;
^According to this a 250 lb guy doing an hour of high-intensity exercise needs to drink 132 oz of water over the course of that day.
If your drinking stuff or taking supplements with caffeine or other diuretics and stimulants in them, you have to take that into account as well.