Different carbohydrate-containing foods affect blood glucose differently — an effect quantified by measures known as the glycemic index and glycemic load.
The glycemic index (GI) assigns a numeric score to a food based on the rise in blood sugar after eating a standard amount (50 grams) compared with the rise after eating 50 grams of pure glucose. Foods are ranked on a scale of 0 to 100, with pure glucose given a value of 100.
The lower a food’s glycemic index, the slower and lower blood sugar rises after eating that food. A glycemic index of 55 or below is considered low; 70 or above is considered high. In general, the more cooked or processed a food is, the higher its GI, and the more fiber or fat in a food, the lower the GI.
The glycemic index tells just part of the story because it indicates only how fast a particular carbohydrate-rich food raises blood sugar. What it doesn’t tell you is how high your blood sugar could go when you actually eat the food, which is partly determined by how much carbohydrate is in a serving of a particular food. To understand a food’s complete effect on blood sugar, you need to know both how quickly it makes glucose enter the bloodstream and how much glucose it can deliver.
Understanding glycemic load
A separate measure called the glycemic load gives a more accurate picture of a food’s real-life impact on blood sugar. The glycemic load is determined by multiplying the grams of carbohydrate in a serving by the glycemic index and dividing by 100. A glycemic load of 10 or below is considered low; 20 or above is considered high. Watermelon, for example, has a high glycemic index (80). But a serving of watermelon has so little carbohydrate (6 grams) that its glycemic load is only 5.
Some nutrition experts believe that people with diabetes should pay attention to the glycemic index and glycemic load to avoid sudden spikes in blood sugar. A review by the Cochrane Collaboration (an international group of health authorities who evaluate evidence from randomized clinical trials) found that people who followed a low-GI diet lowered their HbA1c levels by 0.5 percentage point — and they were also less likely to experience hypoglycemia. To put that effect into perspective, the FDA considers a drop in HbA1c of as little as 0.4 of a percentage point to be meaningful when assessing drugs to treat diabetes.
The American Diabetes Association acknowledges the value of the glycemic index and glycemic load, saying that “substituting low-glycemic-load foods for higher-glycemic-load foods may modestly improve” blood sugar control. But it also says that the total amount of carbohydrate in a food, rather than the food’s glycemic index or load, is a stronger predictor of what will happen to blood sugar. Some dietitians feel that focusing on the glycemic index and glycemic load adds an unneeded layer of complexity to choosing what to eat. The bottom line? Following the principles of low-glycemic-index eating is likely to be beneficial, as it essentially encourages a high-fiber diet that is moderate in carbohydrates, but reaching and staying at a healthy weight is more important for your blood sugar and your overall health.
4 principles of low-glycemic eating
- Eat non-starchy vegetables, beans, and fruits such as apples, pears, peaches, and berries. Even tropical fruits like bananas, mangoes, and papayas tend to have a lower glycemic index than typical desserts.
- Eat grains in the least-processed state possible: “unbroken” grains such as whole barley, millet, wheat berries, brown rice, and whole-kernel bread; or traditionally processed grains such as stone-ground whole-grain bread, steel-cut oats, and natural granola or muesli breakfast cereal.
- Cut back or cut out white potatoes and refined grain products such as white breads, bagels, pastries, and regular pasta.
- Let concentrated sweets — including high-calorie foods with a low glycemic index, such as ice cream — be an occasional treat instead of a staple. Don’t drink more than one-half cup of fruit juice a day. Completely eliminate sugar-sweetened drinks.
— Adapted from Ending the Food Fight by David Ludwig with Suzanne Rostler (Houghton Mifflin, 2008).
To look up the GI values for a wide range of foods, visit www.glycemicindex.com.
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