A fasting period is required to do a glucose tolerance test.
The glucose tolerance test, also known as the OGTT, is one of the main methods for diagnosing diabetes, both in the general population and in pregnant women. The OGTT is done mainly after 12 hours of fasting, with blood drawn for fasting glucose, followed by 75 g of glucose by mouth, and then blood drawn for glucose values at 1 hour, 2 hours, and 3 hours, respectively. So you must be fasting when you do the glucose tolerance test.