What is the Glycemic Index?


Most people with blood sugar problems have noticed that carbohydrates raise blood sugar, often followed by a crash. The various low carbohydrate diets- Atkins, South Beach, Anti-Inflammation Diet, and even the Zone- do somewhat better at weight loss compared to a low calorie diet, but really excel at keeping blood sugar levels stable, lowering triglycerides, lowering blood insulin, and even cholesterol.

But not all carbs are created equal:  a baked Idaho potato with a sprinkle of salt will send your blood sugar spiraling more than an equal weight of even ice cream or table sugar.

The difference is that different foods break down faster than others. Foods that are all fat don’t raise your blood sugar at all. Protein has to go through a significant change in order to convert to sugar. But both simple sugars and whole grains will break down fast, and certain forms of glycinated carbohydrates like those in potatoes break down even faster.  Fast breakdown of carbohydrates sends your blood sugar, and even worse blood insulin, spiking in a curve that can destabilize your blood chemistry and even consciousness.

Diabetics, people with Metabolic Syndrome, insulin resistance or hypoglycemia all react badly to foods that break down rapidly. So the University of Sydney in Australia calculated the glycemic index, based on the rate at which foods break down to glucose on a variety of foods. David Mendosa’s great website has lists of glycemic indexed foods and food by glycemic load.

What is the difference between glycemic index and glycemic load? Some food is very dense in carbohydrates while other food is less dense. Glycemic load takes the quantity of available carbohydrates into account. Available carbohydrates are those that provide energy, i.e. starch and sugar, but not fiber. The glycemic load measures the effect of the glycemic index of a food times its available carbohydrate content in grams in a standard serving. For example, watermelon, which is mostly sugar when dry, has a high glycemic index. But most of the watermelon you eat is water, so a single serving has less of a glycemic load than a piece of cake which might have the same glycemic index.

The glycemic index is a numerical system of measuring how much of a rise in circulating blood glucose a carbohydrate triggers, The higher the number, the greater the blood glucose response. So a low glycemic index food will cause a small rise, while a high one will trigger a dramatic spike. A glycemic index of 70 or more is high, 56 to 69 is medium, and 55 or less is low.

The glycemic load gives a fuller picture than does glycemic index alone. A glycemic index value tells you only how rapidly a particular carbohydrate turns into glucose. It doesn’t tell you how much of that carbohydrate is in a serving. Glycemic load measures the effect of the glycemic index of a food times its available carbohydrate content in grams in a standard serving.

When we say that “complex carbohydrates” break down slower than simple carbohydrates, it behooves us to look at how much slower. A slice of whole grain bread has a glycemic index of 73 and a glycemic load of 10.2 per serving. A slice of white bread has a glycemic index of 70 and a glycemic load of 73 and a glycemic load of 10.2. Both have 14 available grams of carbohydrates.  So while there may be some advantage in vitamins or fiber, both have the same effect on blood sugar.

While legumes are fairly high in carbohydrates, much of that carbohydrate is insoluble fiber. So black eyed peas have a glycemic load of 12.6 versus 9.4 for garbanzo beans, 5.2 for lentils, 5.3 for mung beans and chana dal, the bean with the lowest glycemic index (1/5 of most beans) about 1-2.

Chana Dal -Lowest Glycemic Bean
Chana Dal -Lowest Glycemic Bean

A glycemic index of 55 is low; a glycemic load of 10 is low and while people looking for weight loss should keep their glycemic load under 60-80, or only eat foods with a glycemic index under 55, it is probably more meaningful to count carbohydrates but select them from low glycemic sources. Neither glycemic index nor glycemic load look at the contribution of fats or protein, nor do they look at the nutritional density of foods (where flavonoid content, minerals, vitamins and desirable fats contribute to health versus empty calories or carbohydrates.)

One of the main drawbacks of the glycemic index and load is that it only  measures the changes in blood glucose.  Fructose, lactose, and other sugars that can increase blood insulin levels do not show as raising blood glucose.  The problem with diabetes is that sugars of all kinds can spike blood insulin, which increases both insulin resistance as well as inflammation in the body.  But it is not easy to measure blood insulin, so the information on low insulemic foods is less available.  In other words, just because agave nectar and fructose show up as low in glycemic values, it does not follow that they are good for blood sugar.

Neither the glycemic index or glycemic load take into account the sugars that come from the breakdown of protein- excessive protein can raise your blood sugar and blood insulin too.  Fat on the other hand will not raise your blood sugar, although an excess can prevent the breakdown of your body’s fat stores.berriesFlavonoids

And if you have fat along with carbohydrates in a meal, it lowers the sugar and insulin spikes. So people with blood meters- available cheaply and without a prescription- ought to check what their most common combinations of foods- including their favorite cheats- do to their blood glucose.

The glycemic index is most useful when deciding which high-carbohydrate foods to eat. But don’t eat more carbohydrates than your body can handle, particularly if you have diabetes. The number of grams of carbohydrate we consume is very important.

The total amount of carbohydrate, the amount and type of fat, and the fiber and salt content are also important dietary considerations.

Generally speaking foods with a low glycemic load (under 10) have a low glycemic index. Since we don’t eat foods in precise 100 gram portions, glycemic load is more important than glycemic index. But since the total day’s proportion of foods has other considerations, we should probably look at a low carbohydrate diet consisting of low glycemic load, high nutritional value foods.




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