How to calculate the GI of a food

The Glycemic Index, or GI, was formulated to help diabetics control their blood sugar levels. It can also help people lose weight and maintain a healthy diet by pointing them toward carbohydrates that release energy slowly, balancing blood-sugar levels and preventing sugar lows and spikes which can lead to overeating. The diet can be complex to follow. Each carbohydrate-related food has a value relating to how it affects blood sugar levels. High-GI foods score more than 70 on the index and include white bread, potatoes and sugary baked goods. Medium-GI foods such as sweetcorn, raisins, pineapple and bananas rank between 56 and 69. Low-GI foods score below 55, like raw apples and carrots, kidney beans, skimmed milk and lentils. Aim to select foods from the low-GI group.

Research GI diets. Diet books and Internet resources provide helpful tips and recipes to follow to make selecting foods and embarking on the diet easier.

Locate a comprehensive glycemic index listing. Glycemic indexes can be found in diet books and on the Internet for those seeking to calculate the GI of a food. The Sydney University Glycemic Index Research Services in Australia maintains a database ranking the carbohydrate value of foods, according to the Mayo Clinic. Find an index that will be most useful to you. Easily portable pocket GI books are available which, like calorie counters, list foods and their glycemic index so you do not have to memorise complex lists of facts and figures. This is helpful when grocery shopping. Only carbohydrate-related foods will be listed in the index as these are the foods that directly affect blood sugar levels, and some foods with carbohydrates are not on the Sydney University chart, as manufacturers must pay a fee to have their foods listed.

Note that the same food may have a different glycemic value depending on how it is cooked, according to a list of GI values published in the American Journal of Clinical Nutrition. This is important when shopping and preparing foods. For example, boiled potatoes vary in GI ranking from roasted potatoes, as do raw and boiled carrots. Generally, the GI rating goes up depending on how long a food is cooked, although this may not always be the case, according to the American Diabetes Association.

Be aware of what you add to raw ingredients when preparing foods. Breadcrumbs, batters and sugar will alter the original GI ranking of a food, as will adding certain dressings, and sauces.

Look for a list of ingredients on the back of the packet when buying packaged and ready-made supermarket foods made from a variety of ingredients. It can be difficult, if not impossible, to judge their glycemic value. Many processed foods have added sugars and additives making it hard to determine the overall glycemic value of a food. In such cases, it is best to avoid these foods and cook from scratch using raw ingredients and low-GI substitutes. Alternatively, dieters can find ready-made diet products from manufacturers that list GI information on their packaging.

Select low-GI dishes and avoid high-GI bread baskets, dressings, sauces and desserts when eating in restaurants. Ask the chef or your server for information on the ingredients in dishes. Generally, if you keep to eating healthy foods, such as beans, whole grains, legumes, fruits, lean meat, dairy and vegetables, you will be adhering to a low-GI diet, the Mayo Clinic says.


The Glycemic Index is just a guideline to indicate the relative glycemic impact of certain foods. Different indexes may give alternative glycemic valuations for the same food. Many factors impact this value, making it impossible to be exact, especially after foods have been ingested. Factors include the health of the individual consuming the food, the other foods a person consumes, the ripeness of the item, and the way the food has been grown and prepared. The Glycemic Index is only a guideline as to the glycemic value of various foods. Remember to account for the glycemic value of drinks, especially fruit juices, soda and alcohol.


Consult a doctor before undertaking any extreme or restrictive diet, especially if you have certain medical conditions, such as diabetes.

Things You'll Need

  • Glycemic Index diet book
  • Glycemic Index listings
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About the Author

Lizzy Darnill began writing professionally in 1999, working for "Accountancy Magazine." She holds a Master of Arts in English and a Ph.D. in English from the University of Exeter.