What is the Glycemic Index?

Not all carbohydrate foods are created equal, in fact they behave quite differently in our bodies. The glycemic index, or GI, describes this difference by ranking carbohydrates according to how quickly they enter the blood stream after ingestion, and how much they affect our blood glucose levels. Choosing low GI carbohydrates – the ones that produce only small fluctuations in our blood glucose and insulin levels – is the secret to long-term health reducing your risk of heart disease and diabetes and is the key to sustainable weight loss.

Remember, carbohydrates provide sugar to the muscles, and carbohydrates are ranked (indexed) based on their immediate effect on blood glucose (blood sugar) levels. Low GI carbohydrates more gradually raise your blood glucose levels and maintain them in the lower range. The rapid drop off from high GI carbohydrates is due to insulin’s release and effect on blood glucose levels in reaction to elevated levels observed initially.

High vs Low Glycemic Index

The faster a specific carbohydrate delivers sugar to the blood, the higher its rank or glycemic index. The key to determining whether it is a simple (high GI) or complex (low GI) carbohydrate is the size of its molecules. A simple carbohydrate molecule is small enough to pass through the lining of your stomach so it enters your bloodstream quickly. On the other hand, a complex carbohydrate is a larger molecule and is only able to enter your bloodstream after the food leaves the stomach and gets into your small intestine where it competes for absorption with the other large-molecule foods (proteins and fats). Because simple carbohydrates can pass through the stomach’s lining, they generally do. This leads to a rapid spiking of blood glucose levels which causes the brain to signal the pancreas to produce and secrete insulin in your bloodstream. Insulin’s job is then to remove some of the blood glucose from your bloodstream returning your blood glucose levels to desired levels while ultimately storing that which it removed as body fat. The bottom line here is that some of the simple or high GI carbohydrates get stored as body fat while nearly all of the complex or low GI carbohydrates get stored as glycogen in your muscles and liver where they are ready for quick access for energy while exercising.

Complex, Simple Carbohydrates Examples

This Glycemic Index ranking system compares foods gram for gram of carbohydrate. Carbohydrates that breakdown quickly during digestion have the highest glycemic indexes and are often called “simple” carbohydrates and are smaller in molecular size. The blood glucose response is fast and high. Carbohydrates that break down slowly, releasing glucose gradually into the blood stream, have low glycemic indexes and are called “complex” carbohydrates because of their larger molecular size. The blood glucose response is more moderate and gradual. What are examples of simple carbohydrates? All products made with non-whole-wheat flour, white rice and rice cakes, white and sweet potatoes (especially baked russets), Cornflakes, Fruit Roll-Ups, most sports drinks, etc. What are examples of complex carbohydrates? Nearly all vegetables including yams (except white and sweet potatoes), whole-grains, legumes (beans), brown rice, most fruits (except bananas, grapes/raisins, and watermelon), etc.

How the Glycemic Index relates to the 30 Minute Window

Read about the how the Glycemic Index relates to the 30 Minute Window. For more on the complexity of carbohydrates, visit Harvard Nutrition.