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The glycaemic index (or GI for short) uses a scale from one to 100 to rank carbohydrate foods based on how quickly and how much they raise blood glucose after eating.

How Is the GI of a Food Determined?

The GI of a food is determined by getting study volunteers to eat a portion of the test food (e.g. white rice) containing 50 grams of carbohydrate after an overnight fast. Their blood samples are then taken at different time intervals to check their blood glucose levels.

On a separate day, the same volunteers are given glucose (simple sugar) or white bread also containing 50 grams of carbohydrate to drink or eat. The same blood glucose tests are taken. The two sets of measurements are then compared.

Glucose or white bread, being the reference food, is given a maximum GI of 100.

All other carbohydrate foods score a GI anywhere from one to 100. Foods that cause the most rapid rise in blood glucose are given higher GI values, while those that cause a more gradual rise in blood glucose are given lower GI values.

Scientists used to think that sugary foods cause a more rapid rise in blood glucose compared to starchy foods but this is not true. While many sugary foods have high GIs, some starchy foods like white bread and potato score even higher than honey or table sugar.

Related: A Guide to Carbs

Why Is the GI Important for Your Health?

Eating too much high GI foods causes repeated spikes in your blood glucose.

Studies[1],[2],[3],[4] show that this can lead to an increased risk for Type 2 diabetes, heart disease, obesity and colorectal cancer. For persons with diabetes, eating lower GI foods helps to control blood glucose levels[5] and improve weight loss.

Related: Blood Glucose Monitoring

What Affects the GI of a Food?

There are many factors[6] affecting the GI of a food and its impact on your blood glucose. Take note of the following when you choose your carbs:

Portion Sizes

The total amount of carbohydrate you eat is as important as the GI of the food.

For example, watermelon has a high GI but for the portion size you normally eat (one slice), the actual glycaemic load (GI of the watermelon multiplied by the amount of carbohydrate in one slice of watermelon) is low.

Compare this to a bowl of white rice. Although white rice has a GI value comparable to watermelon, it has a higher glycaemic load. This is because a bowl of white rice has more carbohydrates than a slice of watermelon.

Side Dishes

The impact of a food on your blood glucose is different when it is eaten on its own and when it is eaten together with other foods.

For example, eating a bowl of rice on its own produces a more rapid increase in blood glucose levels than if it is eaten with meat and vegetables.

Cooking

Cooking time affects the GI of a food too. This is because cooking breaks down the cellular structure of a food, making it easier to digest and raise blood glucose levels.

For example, porridge has a higher GI than plain rice, soft-cooked pasta has a higher GI than al dente pasta, and mashed potato has a higher GI than a whole baked potato.

Processing

Processed foods tend to have a higher GI.

For example, white flour has a higher GI than the wholegrains from which it is ground. This is because grinding breaks the protective layers surrounding the grain, making it easier to digest. Similarly, fruit juice has a higher GI than whole fruit.

Ripeness of Fruit

An over-ripe fruit has a higher GI than under-ripe fruit.

Variety

Different varieties of a food can have different GI.

For example, short grain and long grain white rice have different GI.

Related: The GI Values of Common Foods

Should You Choose Carbs Based on GI Alone?

The GI measures the impact of carbohydrate foods on your blood glucose. It does not represent the amount of nutrients in the food. Many nutritious foods have a higher GI than foods with little nutritional value. For example, oatmeal has a higher GI than chocolate.

In addition, fat combined with carbohydrate tends to lower the GI value of the food since fat slows digestion. For example, deep-fried potato chips have a lower GI than boiled white potatoes. This does not mean it is healthier to eat potato chips.

That is why a food’s GI value should not be the only thing to consider when planning your meals. Choosing foods lower in saturated fat and salt, and high in nutrients is also important for your health regardless of their GI.

Related: Carbohydrates and Diabetes

So How Should You Decide What to Eat?

  • Treat the GI as one of many tools to help you make sensible food choices. Balance the use of the GI with My Healthy Plate when deciding what and how much to eat.

  • Keep track of how different foods affect your blood glucose levels. Each person with diabetes reacts to foods and drinks differently.

  • Find a meal plan that works for you and helps you achieve your goals for blood glucose, cholesterol, blood pressure and weight.

Gradually, you will be more sensitive to how different types of carbs affect your blood glucose. Choose well and eat wisely!

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References

  1. de Munter, J. S., Hu, F. B., Spiegelman, D., Franz, M., van Dam, R. M., (2007, Aug). Whole grain, bran, and germ intake and risk of type 2 diabetes: a prospective cohort study and systematic review. PLoS Med, 4(8), p. 261.
    Retrieved Oct 2016 from https://www.ncbi.nlm.nih.gov/pubmed/17760498

  2. Beulens, J. W., de Bruijne, L. M., Stolk, R. P., Peeters, P. H., Bots, M. L., Broggee, D. E., et al. (2007, Jul 3). High dietary glycemic load and glycemic index increase risk of cardiovascular disease among middle-aged women: a population-based follow-up study. Journal of the American College of Cardiology, 50(1), p. 14-21.
    Retrieved Oct 2016 from https://www.ncbi.nlm.nih.gov/pubmed/17601539

  3. Ebbeling, C. B., Leidig, M. M., Feldman, H. A., Lovesky, M. M., Ludwig, D. S. (2007, May 16). Effects of a low-glycemic load vs low-fat diet in obese young adults: a randomized trial. Journal of the American Medical Association, 297(19), p. 2092-2102.
    Retrieved Oct 2016 from https://www.ncbi.nlm.nih.gov/pubmed/17507345

  4. Higginbothan, S., Zhang, Z. F., Lee, I. M., Cook, N. R., Giovannucci, E., Buring, J. E., et al. (2004, Feb 4). Dietary glycemic load and risk of colorectal cancer in the Women’s Health Study. Journal of the National Cancer Institute, 96(3), p. 229-233.
    Retrieved Oct 2016 from https://www.ncbi.nlm.nih.gov/pubmed/14759990

  5. Anderson, J. W., Randles, K. M., Kendall, C. W., Jenkins, D. J. (2004, Feb). Carbohydrate and fiber recommendations for individuals with diabetes: a quantitative assessment and meta-analysis of the evidence. Journal of the American College of Nutrition, 23(1), p. 5-17.
    Retrieved Oct 2016 from https://www.ncbi.nlm.nih.gov/pubmed/14963049

  6. Harvard T.H. Chan School of Public Health. (n.d.). Carbohydrates and Blood Sugar.
    Retrieved Oct 2016 from https://www.hsph.harvard.edu/nutritionsource/carbohydrates/carbohydrates-and-blood-sugar/