That test uses glucose only after a long fast. So naturally there will be highs very early after you swallow it. If it remains high during the duration of the test, it shows, low insulin response and hence diabetes. That test was designed for that. Not for evaluating other diabetic states like type II. That is why the A1C is replacing the GTT today.

Regular food, if it does not have pure glucose in it will show a different pattern.

Doctors are always being told new things...about diabetes.
Some may be correct and some not. We are learning that as people age, their A1C may go up a little, and this is NOT a sign of diabetes in most. I put up some papers about this to a post of Glenn's not too long ago.
This post link has two complex links in it... you will want to try and read it. I gave the papers to my own doctor too!
http://neurotalk.psychcentral.com/post947907-8.html

The Accord study found that the "aggressive" arm of the patients included had an alarming death rate...so they stopped it before the end of the study. The more modest goals, did not have the elevated death rate.
http://diabetes.webmd.com/news/20110...ise-death-risk

One of the new things is the post prandial spike. It is thought that it should remain below 200 in the hour after eating. But I am rather a cynic now. It seems just about everything that doctors will tell you will be out of fashion, in a few years time. I've watched this unfold for 50 yrs! So I don't think there are really complete answers yet.