This makes a lot of sense to me, because there is great variation between individuals in the glycemic index of different foods. It could also indentify foods that put your body under stress.
I went to a seminar last year given by someone who had done research contributing to the development of a glycemic index (GI) database, and she concluded that the interindividual variation was so great that the index is almost useless.
The GI compares the rise in blood glucose after eating a food to the rise in blood glucose after consuming pure glucose. Generally, the GI is expressed as some percentage less than 100 because the body takes more time to break starches down into glucose or convert amino acids to glucose than it does to simply transport glucose from the intestines into the blood. However, in one case the researcher presented on, an individual had a much stronger rise in blood glucose after consuming white bread than after consuming glucose! Clearly, this person was not just breaking the bread down into glucose but also releasing stress hormones into his blood that would further rise his glucose levels by breaking down stored glycogen and converting his own protein into sugar. He may have been unaware of his adverse reaction to white bread, but it may have been doing damage to his body all along.
The $20 investment in a blood sugar monitor that Dr. Davis recommends may thus prove a useful investment for helping to determine your ideal diet.