Why is it when you eat a bunch of carbs you feel tired and lethargic? If they are energy why don't they make you feel more energized?
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In most folk the cause is often referred to as "sugar crash". This typically occurs roughly one hour after eating a meal laden with simple carbs, (white bread, biscuits, pancakes with syrup, you get the idea) the net effect is an "overshoot" of serum blood glucose which results in "excessive" secretion on insulin and a dramatic fall in serum blood sugar (<70mg/DL).
So in effect the "sugar crash" is the consequence of hypoglycemia. (Please DO NOT misconstrue the hypoglycemia which may occur in this instance with a DX of being "hypoglycemic" they are entirely different)
The best treatment is prevention IMO. First and foremost just avoid simple carbs or get some whole grain in there to delay absorption. Another means is to increase the amount of "good fat" and/or protein when you eat.
Regs
Jim