The Glycemic Index of foods is a ranking system for carbohydrate-containing foods (starches, sugars, fruits, vegetables and dairy) based on their effect on blood sugar (blood glucose) levels. It was devised in 1981 by David Jenkins, M.D., Ph.D., D.Sc. at the University of Toronto whose research set out to investigate exactly what happens when the body digests carbohydrates. Subjects involved in the research were given a predetermined amount of some commonly consumed carbohydrate foods and then their resulting blood glucose levels were recorded after a designated period of time. Jenkins and his colleagues discovered that not all carbohydrates (even in the same gram to gram quantities) raised blood sugar in the same way. They observed that in equivalent quantities, carbohydrates in white bread (a “complex” carbohydrate-termed so by the longer chain length of the carbon, hydrogen and oxygen molecules) elevated their subjects’ blood glucose more than those in ice cream, despite its sugar (a “simple” carbohydrate) content. This research raised some specific questions about what factors effect a food’s ability to raise blood sugar and what happens in the body based on quick versus slow increases in blood sugar.
A low Glycemic Index (GI) food will release blood sugar more slowly and steadily into the blood stream which is more favorable in regards to helping the body respond effectively. A high GI food causes a rapid rise in blood glucose and is helpful for athletes recovering from endurance exercise or for a diabetic who is experiencing low blood sugar. There are many health problems associated with an overall high GI diet (a diet that is filled with High GI foods), which is why research continues in this area.
The metabolic consequences of eating too many high GI foods are many. Blood sugar and insulin levels are not the only things affected, so are cholesterol levels, CRP levels (a measure of chronic, low-grade inflammation), body weight and composition (how much fat versus muscle we have) and blood flow. High GI diets have been associated with heart disease and stroke, metabolic syndrome (which is characterized by 3 or more of the following: insulin resistance, high blood pressure, low HDL (good) cholesterol levels, weight gain around the waist, impaired fasting glucose and/or high triglycerides), macular degeneration, diabetes and obesity.
Many factors can affect the glycemic index of a food.
The Glycemic Load of a food is based on portion size. It is calculated by taking the glycemic index of a food, multiplying it by the total grams of carbohydrate per serving of that food and dividing by 100. Here are some examples:
Spaghetti (1 cup) GI = 40
1 cup spaghetti contains about 45 grams carbohydrate.
The glycemic load of spaghetti is (40x45) divided by 100 = 18
1 medium apple GI = 40
1 medium apple contains 15 grams carbohydrate.
The glycemic load of an apple is (40x15) divided by 100 = 6
1 medium slice watermelon GI = 72
1 medium slice watermelon contains 6 grams carbohydrate (it’s mostly water). The glycemic load of a medium slice of watermelon is (72x6) divided by 100 = 4.3
1 medium piece chocolate cake from box mix (111g) GI = 38
1 medium piece chocolate cake contains 57.9 grams carbohydrate (it’s mostly sugar and white flour). The glycemic load of that chocolate cake is (38x57.9) divided by 100 = 22
The long and the short of it is that not only the type of carbohydrate we eat matters to our overall health, but so does the amount.
Limitations of Glycemic Index and Glycemic Load
Although this method for understanding how carbohydrates affect our blood sugar and health is important, overall composition of the diet is complex. Glycemic Index and Load are good tools but there are limitations such as:
First, get familiar with the carbohydrate-rich foods you typically like to eat. Then you can intelligently assess the overall worth of your diet. Jot down the foods and amounts you eat in a day and assess the quality of the carbohydrate.
Once you have a list of the foods you eat, go to www.glycemicindex.com to determine the GI and GL of the carbohydrates you eat day-in and day-out. We all tend to eat the same foods over and over again, so put your list of foods to the test. Finally, if you find your overall diet is full of high GI foods (say you eat cornflakes every morning, a few rice cakes at 10am, a white bread sandwich with pretzels and a soda at lunch, a couple of cookies at 3pm and instant white rice with some ground beef and a soda at dinner and a large bowl (3-4 scoops) of ice cream at night) you can make a switch to low GI carbohydrates as soon as you’re ready. Eating mostly low GI carbohydrate will also significantly improve the level of antioxidants, fiber, vitamins and minerals in your overall diet and decrease your risk of heart disease, diabetes, metabolic syndrome and chronic inflammation.
Check out any of my meal plans for low glycemic menu ideas.
DJ Jenkins et al (1981). “Glycemic index of foods: a physiological basis for carbohydrate exchange.” Am J Clin Nutr 34;362-366
Balkau et al (1998) “High blood glucose concentration is a risk factor for mortality in middle-aged nondiabetic men. 20 year follow-up in the Whitehall Study, the Paris Prospective Study, and the Helsinki Policemen Study.” Diabetes Care 1998 Mar;21(3):360-7
Sheard et al (2004). “Dietary carbohydrate (amount and type) in the prevention and management of diabetes: a statement by the American Diabetes Association.” Diabetes Care;27(9):2266-71
Chiu et al (2007) “Association between dietary glycemic index and age-related macular degeneration in nondiabetic participants in the Age-Related Eye Disease Study. Am J Clin Nutr 2007;86(1):180-8.