The Glycemic Index Diet for Dummies (10 page)

The next sections help you understand how scientists calculate the glycemic index and explain a few of the resulting caveats.

Comparing foods to pure sugar with human help

Because the glycemic index deals with your blood sugar's reaction to various foods, determining a particular food's glycemic index calls for the help of human test subjects. First, researchers feed 50 grams of available carbohydrates (that's total carbohydrate minus fiber) to ten or more volunteers to test how the food raises blood sugar levels at different intervals over a two-hour period after it's consumed. They then plot these changes in blood sugar on a graph and compare the volunteers' responses to the test carbohydrate to their responses to the same amount of pure sugar or white bread. The average blood sugar response of all ten volunteers compared to their response to sugar or white bread determines the glycemic index of that food item.

Researchers use pure sugar for comparison purposes because it's the simple form of energy used by the human body. However, because most people don't typically eat sugar all by itself, researchers have also been known to use white bread for comparison purposes.

Figure 2-1 demonstrates the sharp rise in blood sugar response when pure sugar is consumed. You can see the quick rise and the ensuing drop over time. Notice that the maximum blood sugar spike occurs around 45 minutes. After this peak, the blood sugar levels drop quickly.

Figure 2-2 shows what happens when a high-glycemic food is consumed. The rise is similar to what you see in Figure 2-1, but this high-glycemic food falls a little short of the curve set in the pure sugar test. The maximum blood sugar spike from a high-glycemic food occurs around 45 minutes after food consumption, with a fairly quick drop afterward.

Figure 2-3 shows the difference in the curve when a low-glycemic food is eaten. Notice thatthe maximum spike is much lower and also occurs much later, around an hour after consumption with a slow drop back to the base line. This type of blood sugar response results in lower levels of insulin being released and better control of food cravings, hunger, and mood.

Keeping a couple limitations in mind

As great as the glycemic index is, it does have a couple limitations due to how it's measured. I bet you notice these differences as you begin coming across the various glycemic lists that are out there.

The lists are limited.
Glycemic index testing has only been around about 20 years, and it isn't required by federal guidelines in the United States. The process is quite costly and time consuming because each variation must be tested. Also, only a small number of researchers actually conduct glycemic index testing, and they can't possibly keep up with the thousands of new food products that manufacturers develop each year. Consequently, many foods haven't been tested and are therefore missing from glycemic index lists.

The findings vary.
The glycemic index must be measured by observing humans (specifically their blood sugar's response to foods; see the preceding section), and no two humans are alike. That means the rate at which people digest carbohydrates, their insulin response, and even the time of day that they're tested can cause variation. Therefore, each food must be tested by a group of people, and the results must be averaged together.

Also, there's a lot of variability in the food world, which means inevitably all glycemic index charts aren't going to be the same. For example, you may find one long-grain rice with a glycemic index of 62 and another with a level of 68. These differences may result from the grains being grown in different regions, the length of time they were cooked, how they were cooked, and so on.

The important factor to focus on is whether the food generally ends up high, medium, or low on the glycemic index. Don't get too caught up in small number discrepancies among different glycemic index charts.

Defining Low-, Medium-, and High-Glycemic Foods

Determining whether a food is high- or low-glycemic is pretty straightforward. The glycemic index is broken into high-, medium-, and low-glycemic foods. High-glycemic-index foods have the quickest blood sugar response; low-glycemic-index foods have the slowest. Here are the measurements on a scale of 0 to 100:

Low glycemic index:
55 or less

Medium glycemic index:
56 to 69

High glycemic index:
70 or greater