When we eat a meal, our blood glucose levels rise. But glucose in our blood can’t help the nerves, muscles and other tissues to function unless it can cross into their cells. Glucose molecules are too large to cross the cell membranes of our tissues by themselves, so glucose needs assistance from the hormone insulin. The arrival of insulin stimulates glucose transporters to travel to the surface of the cell to transport glucose across the cell membrane and into the cell. When you haven’t eaten for a long period of time, your blood glucose levels decline. This decrease in blood glucose stimulates the hormone glucagon which causes the liver to convert its stored glycogen into glucose to be used for energy. Epinephrine and norepinephrine are both released when we need a burst of energy to respond quickly when our blood glucose levels also are too low. Cortisol and growth hormone decrease the use of glucose by the muscles and other body organs and increases the use of fatty acids stored. These hormones together balance each other to maintain blood glucose in a healthy range, however if the balance is altered, it can lead to diabetes.
The glycemic index is the potential of foods to raise blood glucose levels. Foods with a high glycemic index cause a sudden surge in blood glucose. This triggers a large increase in insulin which may be followed by a huge drop in blood glucose. Foods with low glycemic index cause low to moderate fluctuations in blood glucose. The glycemic index of a food is not easy to predict. Most people assume that foods containing simple sugars have a higher glycemic index than starches, however that is not always the case. Although instant potatoes are a starchy food, they have a glycemic index of 83 while an apple is only 36.
The type of carbohydrate, how the food is prepared and its fat and fiber content all affect how quickly the body absorbs it. Since we eat most of our foods combined in a meal, the glycemic index of the total meal becomes more important than the ranking of each food.
Some expert nutritionists believe the glycemic load is more useful than the glycemic index. The glycemic load of a food is the total grams of total carbohydrate. For example, carrots have a glycemic index of 68, but a glycemic load of only 3. This is because they only contain a small amount of total carbohydrate in a serving. The low glycemic load of carrots means that carrot consumption is unlikely to cause a significant rise in glucose and insulin.
Foods or meals with a lower glycemic load are a better choice for someone with diabetes because they will not have dramatic fluctuations in blood glucose. This may also reduce the risk of heart disease and colon cancer because they generally contain more fiber and it is known that fiber helps decrease fat levels in the blood. Studies have shown that people who eat lower glycemic index foods have higher levels of high density lipoprotein, or HDL, which is a healthful blood lipid, and low LDL which is associated with increased risk of heart disease. Diets with a low glycemic index and load are also associated with a reduced risk for prostate cancer.
Many believe that the concepts of glycemic load/index are too complex for people to apply to their daily lives and the topic is controversial. Until the controversy is resolved, people are encouraged to eat a variety of fiber-rich and less processed carbohydrates such as beans, fresh vegetables and whole wheat bread, because we know these forms of carbohydrates are lower in glycemic load also contain important nutrients!
Source: Thompson, Janice, Melinda Manore, and Linda A. Vaughan. The Science of Nutrition. San Francisco, CA: Pearson Benjamin Cummings, 2011. Print.