When our bodies digest carbohydrates, they are converted into glucose, producing energy. But not all carbs are created equal—some cause a quick spike in blood sugar levels, while others release sugar evenly into the bloodstream over a longer period. The glycemic index is a measure of how quickly a certain carbohydrate will affect blood sugar levels.

When foods with a high glycemic index are eaten, blood sugar spikes suddenly. If eaten when blood sugar levels are low, that can be a good thing, but if it pushes blood sugar too high, it starts an unhealthy up-and-down cycle that places a lot of strain on the body. The pancreas responds to high blood sugar levels by producing lots of insulin, which has the effect of bringing blood sugar rapidly down. (This describes the process for the majority of the population—for the 8% who have diabetes, it requires much more careful control).

So what does it feel like to have high or low blood sugar? High blood sugar produces a variety of short and long-term reactions in the body, including symptoms like tension and trouble concentrating. Worse, high blood sugar causes the body to produce insulin and cortisol, hormones that, in large enough quantities, can decrease the immune system’s effectiveness and impair memory. Low blood sugar is no better: its list of symptoms include shakiness, dizziness, sweating and mood swings. Eating carbohydrates with a low glycemic index helps regulate blood sugar since they are absorbed slowly, avoiding the quick spikes and drops in blood sugar. Physical activity also helps.

Low glycemic index foods:
-Nuts and seeds
-Beans and lentils
-Bran cereals
-Most vegetables
-Whole grains/rye
-Pasta
-Quinoa

High glycemic index foods:
-Rice Cakes
-Wheat cereals
-Potatoes
-French fries
-Bagels
-Sodas
-Watermelon

These are just a few examples—do a web search for a complete list.

Source: http://www.mayoclinic.com/ and http://www.mendosa.com/gi.htm