"Glucose intolerance" is the inability to effectively control blood glucose as it enters the bloodstream from the digestive system. It results in elevated blood sugar after eating carbohydrate, which is not a good thing. In someone with normal glucose tolerance, insulin is secreted in sufficient amounts, and the tissues are sufficiently sensitive to it, that blood glucose is kept within a fairly tight range of concentrations.
Glucose tolerance is typically the first thing to deteriorate in the process leading to type II diabetes. By the time fasting glucose is elevated, glucose intolerance is usually well established. Jenny Ruhl talks about this in her wonderful book Blood Sugar 101. Unfortunately, fasting glucose is the most commonly administered glucose test. That's because the more telling one, the oral glucose tolerance test (OGTT), is more involved and more expensive.
An OGTT involves drinking a concentrated solution of glucose and monitoring blood glucose at one and two hours. Values of >140 mg/dL at one hour and >120 mg/dL at two hours are considered "normal". If you have access to a blood glucose meter, you can give yourself a makeshift OGTT. You eat 60-70 grams of quickly-digesting carbohydrate with no fat to slow down absorption and monitor your glucose.
I gave myself an OGTT tonight. I ate a medium-sized boiled potato and a large slice of white bread, totaling about 60g of carbohydrate. Potatoes and bread digest very quickly, resulting in a blood glucose spike similar to drinking concentrated glucose! You can see that in the graph below. I ate at time zero. By 15 minutes, my blood glucose had reached its peak at 106 mg/dL.
My numbers were 97 mg/dL at one hour, and 80 mg/dL at two hours; far below the cutoff for impaired glucose tolerance. I completely cleared the glucose by an hour and 45 minutes. My maximum value was 106 mg/dL, also quite good. That's despite the fact that I used more carbohydrate for the OGTT than I would typically eat in a sitting. I hope you like the graph; I had to prick my fingers 10 times to make it! I thought it would look good with a lot of data points.
I'm going to have fun with this glucose meter. I've already gotten some valuable information. For example, just as I suspected, fast-digesting carbohydrate is not a problem for someone with a well-functioning pancreas and insulin-sensitive tissues. This is consistent with what we see in the Kitavans, who eat a high-carbohydrate, high glycemic load diet, yet are extremely healthy. Of course, for someone with impaired glucose tolerance (very common in industrial societies), fast-digesting carbohydrates could be the kiss of death. The big question is, what causes the pancreas to deteriorate and the tissues to become insulin resistant? Considering certain non-industrial societies were eating plenty of carbohydrate with no problems, it must be something about the modern lifestyle: industrially processed grains (particularly wheat), industrial vegetable oils, refined sugar, lack of fat-soluble vitamins, toxic pollutants and inactivity come to mind. One could make a case for any of those factors contributing to the problem.