Elevated glucose levels in the blood cause the body to try to lower them through excretion. The kidneys will do this by excreting glucose in the urine. Extra water is required to disolve the glucose molecules and this leads to dehydration and thirst. This is why diabetics drink more water than normals if they aren't in good blood sugar control. If I have had too many carbs I will notice it right away because my thirst levels skyrocket and then I'm in the bathroom every half hour.
Hope this helps.
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