If I’m asking, you probably already know the answer. It’s counterintuitive, it’s downright illogical, but whole-wheat bread, not sugar, causes a greater rise in blood sugar.
It blew my mind, but it’s one of those things that makes you rethink what you thought you knew, and makes you wonder what other self-evident truths you are wrong about.
From the fantastic book Grain Brain, by David Perlmutter:
When I give lectures to members of the medical community, one of my favorite slides is a photo of four common foods: (1) a slice of whole-wheat bread, (2) a Snickers bar, (3) a tablespoon of pure white sugar, and (4) a banana. I then ask the audience to guess which one produces the greatest surge in blood sugar—or which has the highest glycemic index (GI), a numerical rating that reflects a measure of how quickly blood sugar levels rise after eating a particular type of food. The glycemic index encompasses a scale of 0 to 100, with higher values given to foods that cause the most rapid rise in blood sugar. The reference point is pure glucose, which has a GI of 100.
Nine times out of ten, people pick the wrong food. No, it’s not the sugar (GI = 68), it’s not the candy bar (GI = 55), and it’s not the banana (GI = 54). It’s the whole-wheat bread at a whopping GI of 71, putting it on par with white bread (so much for thinking whole wheat is better than white). We’ve known for more than thirty years that wheat increases blood sugar more than table sugar, but we still somehow think that’s not possible. It seems counterintuitive. But it’s a fact that few foods produce as much of a surge in blood glucose as those made with wheat.
Grain Brain is one of the best books I’ve read in a long time, and one that has opened my mind to the many possibilities of nutrition and the impact of modern methods of food sourcing on our health. The couple of paragraphs I’ve highlighted above are just two of many mind-blowing, counterintuitive findings he has in store in the book, many lamenting our focus on carbohydrate-heavy and fat-light foods.
Donn, fragments 
People who strictly follow the Glycemic Index are, in my opinion, very narrow minded. There are many other factors that affect your body besides glucose! Let’s take fructose for example, which wreaks even more havoc on health in terms of obesity than glucose, and sugar is made of both! How about calories, which an equal amount of sugar as whole wheat has twice of. Besides, sugar doesn’t have the fiber nor any substantial minerals nor protein that is found in whole wheat bread. If we’re talking about getting the biggest bang for the buck, then whole wheat wins.
This is a trick question. There is a difference between glycemic index and glycemic load. Some foods technically have a high glycemic index, but contain relatively small amounts of carbohydrate, so actually have a small glycemic load. This website explains the difference https://www.hsph.harvard.edu/nutritionsource/carbohydrates/carbohydrates-and-blood-sugar/