[kimi85]

Does anybody know, why in the test is preceded by ≥3 days of normal, unrestricted diet?

I can't find the answers here in the internet. 

[JGK]

Since the aim of the test is to see if the removal of glucose from the system is less than normal it is necessary for the patient to have a normal Basal glucose level at the start of the test.

Any carbohydrate restriction of the diet prior to the test may invalidate the results as an abnormal tolerance will be shown as metabolism may be set into the "fasted state" favouring gluconeogenesis.

From "Clinical Chemistry in Diagnosis and Treatment" J.F Zilva & P. R. Pannall, Lloyd-Luke