Serving size refers to how we measure the amount of food. It can be listed either in grams, ounces or common household measures such as cups, tablespoons or teaspoons. The sizes or measures are standardized so it is easier to compare similar foods for their nutritional composition. The serving sizes are smaller than usual portion sizes.
How are the serving sizes determined?
Servings of foods, as our main source of energy (energy dense), such as those in the Protein, Dairy and Cereal Grains groups, are standardized roughly by using the amount of energy they provide. Each serving of foods in these group has roughly (120-143Cal) (500-600kJ). (1)
For example, if we take 2 different foods from the Protein Group such as chicken meat and nuts, we can say that 3 ounces (85g) of chicken meat is roughly equivalent to 1 once of hazelnuts in terms of the amount of calories. We could consider 6 ounces of meat, but it would not be practical since the equivalent amount of nuts is 2 ounces, which is a little too much just for one snack. (2)
For the two remaining groups, Vegetables and Fruits, the servings are not only based on calorie content but also on the practical serving size as well. It would be unrealistic to compare a lettuce serving with a meat serving using either the same amount of calories or weight.
The standard serving of the fruit group is based on the recommended amount per day, equivalent to two fruit (e.g. apples) of medium to large size, of about 150g each. On average, fruit have similar energy nutrient amounts of 84Cal/550kJ. (1)
The average servings of vegetables are a little harder to determine since they include vegetables such as energy dense potatoes (loaded with carbohydrates) and lettuce, which has few calories and a large volume.
The average serving size of vegetables is 75g and represents an energy range of (24-84Cal)(100-350kJ). (1)