Ask the RCW Dietitian: Glycemic Index

Is there a benefit in calculating the glycemic index of a food? I read that having cheese pizza, a low glycemic index food, would be good for weight loss. Is that true?

The glycemic index rates foods with carbohydrates based on their effects on blood glucose (also known as blood sugar). The index uses a 1-100 scale – the lower a food’s glycemic index is, the less impact it has on blood glucose. A 50 gram serving of pure glucose has a value of 100 and serves as a reference for the other foods on the list.

There are several problems with the use of glycemic index to evaluate the quality of food and its potential impact on weight. First of all, it uses a portion size for each food that provides 50 grams of carbohydrate. This means that a serving size of five cups of watermelon is being compared to just over one cup of rice. Watermelon’s glycemic index (72) is higher than that of white rice. Eating a typical serving of watermelon (one cup) actually has very little impact on blood sugar because watermelon – as the name suggests – is mostly water. It’s really like comparing apples to oranges; pun intended.

Another problem with the glycemic index is that it does not account for any nutrients in the food other than carbohydrates. Watermelon, once again, is a great example. It’s rich in vitamin C, vitamin A, lycopene and more. The glycemic index does not portray any of this, however, and actually rates watermelon as “worse” than table sugar, which doesn’t contain any of the above nutrients.

Cheese pizza is an interesting case. Pizza crust, by itself, would actually have a high glycemic index. The addition of cheese lowers the glycemic index because fat and protein slow the absorption of carbohydrates into the bloodstream. Including fat and protein at meals is important (and actually essential to good nutrition), but the way glycemic index is calculated means that the more of these you add, the lower the glycemic index of a food will be. After a certain point you will get more than you need and excess calories may lead to weight gain – even as the glycemic index of your meal drops.

Pizza, of course, isn’t something to totally avoid. There is certainly a role for foods like pizza in the diet. In fact, I don’t know a single dietitian who doesn’t eat pizza. But when people use the glycemic index as their measure of health of a particular food it can be both confusing and counterproductive to meeting nutrition goals. Eating lots of cheese pizza, for example, is very unlikely to lead to weight loss.

Glycemic load is another tool that is used in this way, and it actually does provide more usable information than glycemic index. This is because glycemic load calculates blood sugar impact based on realistic serving sizes for the foods in question. Using glycemic load, watermelon (one cup serving) only scores an 8, much lower than white rice (33). This is a more realistic depiction of these foods’ typical impact on blood sugar levels.

Ultimately, however, glycemic load still does not provide a full picture of a food’s health value. There are too many factors to consider to boil the health value of a food down to a single number. Our nutrition needs are fluid and vary based on age, gender, body type and size, activity level, digestive factors, what else we’ve eaten that day, and more. Good nutrition advice is individualized.