Calculating GI Values For Meals

Most of the time we don’t eat servings of individual carbohydrate-type foods: we eat meals with a variety of foods containing varying amounts of carbs, as well as protein, fat and fiber. Thus in practice, for optimum blood-glucose management, we need to know the glycemic value of a meal, rather than simply the GI of individual foods.

How to Measure the Glycemic Index Value of Meals

To calculate the GI value of a meal, you need to know two things:

– The total grams of carbohydrates in the meal.
– The percentage of the carb-total contributed by each food.

Once you know this data, which is typically obtainable from most food composition or nutrition tables, the GI calculation for a meal is simple:

Multiply the GI value of each food by their percentage of total carbohydrate. Add up the totals, to get the GI value for the meal.

Calculating GI For a Simple Meal

  • Suppose you eat 2 slices (2oz) of toast and a glass (4oz) of milk.
  • The toast contains about 26g of carbohydrate; milk about 6g of carbs.
  • The total carb content is 32g. (81% from bread; 19% from milk)
  • The GI of bread is about 70; the GI of milk is about 28.

Calculation = GI X Percentage of carb contribution

Toast: 70 X 81% = 57
Milk: 28 X 19% = 5
Total: 57 + 5 = 62

The GI value of the meal is 62.

Calculating GI Value of Meals Not Always Possible

If we eat meals with non-standard foods that either do not have a GI rating, or are cooked in ways that have not been tested for GI effect, it is not possible to calculate a precise GI value for them. Even so, by using food composition tables and GI rating lists, it is possible to arrive at ballpark GI values for most meals.