Fast Carbs, Slow Carbs

The glycemic index can help keep your blood glucose in check and your life in balance

All foods affect your blood sugar levels, but some cause sharper spikes than others. By keeping an eye on the glycemic index levels of the foods you eat, you may be able to keep your glucose levels more stable. Beth Anderson, a registered dietitian and certified diabetes educator in Liberty Medical’s Diabetes Self-Management Program, answers some questions about how the glycemic index works.

Q: What is the glycemic index?
A: The GI of a food indicates how quickly and how high that food will raise blood sugar levels. It is arrived at by having 10 volunteers, who have been fasting overnight, consume 50 grams of carbohydrate within 10 to 20 minutes. Their blood sugar levels are then tested several times within the next two hours. If, for example, the average blood sugar response of the 10 volunteers is 43 percent that of glucose (assigned the value of 100 on the GI), the GI of that food will be 43. Because of the testing technique, the GI only measures foods that have a significant level of carbohydrates. A food such as lettuce (imagine having to eat a mountain of lettuce in 20 minutes!) has so few carbohydrates, it can be assumed to be low GI. Similarly foods without any carbs, such as meat, oil or cheese are also automatically low GI.

Q: What do the numbers tell me?
A: Foods with a lower GI level don’t cause large changes in your glucose levels two hours after a meal, so they tend to be better choices, especially for people with diabetes. The ratings for foods fall into three categories:

Low: 55 or less  
Medium: 56 to 69  
High: 70 or higher

The GI can be used to help fine-tune glycemic control, and it should be used in conjunction with a person’s current diet plan. That is, when choosing a starchy food, choose one with a low GI instead of a high GI. In addition to raising blood sugars less dramatically, foods with low GI levels tend to be more satiating, allowing one to eat less and feel full longer. People who eat a lot of higher-GI foods may be more prone to obesity, heart disease and diabetes.

Q: Are foods with low glycemic index levels healthy?
A: Not always. The GI does not indicate the nutrient composition or quality of foods; therefore, people should not necessarily eat foods just because they have a low glycemic index. For instance, a Snickers candy bar has a low GI of 41, but that does not make it a healthy choice. Conversely, seedless watermelon has a high glycemic index of 72, but is still a better choice nutritionally than the candy bar with the low GI.

Q: Where do some typical foods fall on the index?
A: That’s the tricky part about the glycemic index — it doesn’t necessarily make sense. For instance you would think that brown rice would be better for you than white rice, right? Well, sometimes yes, sometimes no. For instance, converted long-grain white rice has a lower GI than brown rice, but the GI of short-grain white rice is higher than brown rice. Which has a lower GI: a baked white potato, a baked sweet potato or potato chips? If you guessed the baked white potato, you’re wrong. A baked white potato (russet) has a GI of 77, a baked sweet potato has a GI of 46, and potato chips have a GI of 56.

Q: So how can you tell where foods fall on the glycemic index?
A: All foods that have very few carbohydrates will be low GI (like certain vegetables, proteins and fats). Foods higher in carbohydrates can only be determined by looking at a table of GI values of foods. We have provided a short list here, but you can also buy a book or go to a Web site with GI figures. It’s not something you can guess.

Q: Can I assume that a food has a low GI if it does not appear in the table?
A: No. Although the GI list contains many common foods and continues to test more foods, unless a company submits its brand to a testing lab (and pays for the procedure), there are no GI values assigned to it. As stated above, you cannot assume that a product is low GI just because a similar one is. Try to stick with products that are on the list or have very few carbohydrates.

Q: What else can affect the GI?
A: Foods aren’t usually eaten in isolation, so other foods you eat at the same time, fat and protein levels in the food, and how the food is prepared can affect how your body responds. Plus, no two foods are exactly alike, so the index assigned to them is an approximation. In general, two factors will lower the GI of a food. The first is added fat — not a great choice for someone watching their diet. That’s why potato chips have a lower GI than baked potatoes. This does not mean you should put butter on your baked potato, but rather use the GI in combination with the facts you already know about healthy eating. The second factor is added acid, so if you use lemon juice or vinegar in a dish, that will lower the GI of the dish, and probably the meal.

Q: What makes foods have different GI levels?
A: The most important factor in determining the GI level of a food is how slowly it is digested. Foods that have the same amounts of carbohydrates can have very different GI levels depending on the variety (such as long-grain and short-grain rice), how much they are processed, how ripe they are and how big their particles are. For example, pasta from the same box can have different GIs depending on how they’re cooked. If the pasta is cooked al dente (still chewy), the GI will be lower than pasta that is overcooked or soft. This is because it takes longer for the body to breakdown the slightly undercooked pasta, while the overcooked pasta barely has to be digested at all. Fiber makes a difference too. A sweet potato has a lower GI than a white potato because the fiber content in a sweet potato is higher. Also, the amount of fat consumed in a meal can slow down glycemic response (see above).

Q: How does the "glycemic load" compare to the GI?
A: The glycemic index indicates how high and long a food will raise blood sugar, while the glycemic load takes serving size into account and attempts to incorporate both the quality and the quantity of carbohydrate consumed. Like the GI, the GL has a scale:

Low: 10 or less
Medium: 11-19
High: 20 or greater

The GL of a food is calculated using the following mathematical equation: GL = (GI x amount of carbohydrate) divided by 100. Let’s look at that watermelon we talked about earlier. It has a high GI (72), but once you figure out the GL the picture changes. For example, a 100-gram serving of watermelon has a carbohydrate content of 5 grams. That makes the calculation:
72 (GI) x 5 (g carbohydrate) = 360 divided by 100 = GL 3.6

That gives watermelon a low glycemic load value. Many experts suggest that people use the GI over the GL. The GL doesn’t necessarily distinguish the foods that are just low carb or are good carbs, and that could lead to diets higher in fat or protein and too few carbs. One way to incorporate the glycemic index into your diet is to exchange one higher-GI food for a lower-GI food at each meal. If you are testing your blood glucose levels before and after meals, you will be able to see whether this change is helping to control your blood sugar.

Editor’s Note: The glycemic index is not an exact science. If you look at two different lists of glycemic index levels, you may see two different values for the same food. The list we have here (from diabetesnet.com) gives the GI of All-Bran^® cereal as 51, but the table in The New Glucose Revolution, 3rd edition gives the GI for the same cereal as 34. The moral of the story is, use the GI as a guideline — not an absolute authority.

Foods and Their Glycemic Index Levels