The glycemic index, one of nutrition's 'it' girls for the last couple of years, has been cited to me frequently as a reason to not eat this or that food. I wouldn't be surprised if several of you out there have used the glycemic index to make food choices, especially when it comes to your sweetener choice (hello, agave.) But let's take a minute to really understand the glycemic index, I think you may find that it's not as straight-forward as it seems.
Some of you may not even know what the glycemic index (GI) actually is, so here’s a brief rundown. The glycemic index is a measure of how quickly a specific food raises blood sugar as compared to a control. The glycemic index is a measure of the increase in blood glucose level above the fasting level during a 2-hour period after eating a specific amount of carbohydrate (normally 50 g) when compared with the same amount of carbohydrate in a reference food. Foods that are considered to be “high-GI foods” are rapidly digested and absorbed causing a rapid rise in blood glucose levels, which the body tries to combat by a rapid release of insulin that can subsequently lead to a rapid fall in blood sugar, frequently to levels below the normal fasting level. Foods that are considered to be “low-GI foods” are more slowly digested and absorbed, leading to a more gradual and longer lasting rise in blood sugar with a lower peak level of blood sugar and insulin overall, which in turn leads to a more gradual fall back to normal fasting levels.
Seem relatively straight forward enough? Well, get ready for some complication.
What most people don't know is that there isn't just one glycemic index. Different studies use their own indices and they don't all use the same reference/control. The reference food can be white bread or it can be pure glucose. Which ever is used as the control is given a score of 100 on the index, so if white bread is the control it has a score of 100, but if glucose is the control white bread has a score of 71. Therefore the glycemic index of a food will be different based on which control is being used. Here are some examples:
Furthermore, the glycemic response provoked by a food is dependent on many, many, many variables. These variables include: the total amount of carbohydrate in the meal, the composition of the meal itself (especially the amount of fiber, protein and fat), the composition of the previous meal, how long it has been since the previous meal, the physical activity level of the person consuming the food, the individual’s glucose and insulin response, the form of the food (is it a whole apple or is it in a smoothie), the ripeness of the food, the food’s growing location, the variety of the food (is it a Bartlett or Bosc pear), the cooking method, whether or not it has been cooked and then cooled (fun fact: when using glucose as a control, a hot boiled red potato has a GI of 89.4 but a cold boiled red potato has a GI of 56.2), and on and on and on.
To make things even more complicated, the glycemic index is actually only one side of the coin, with the other side being made up of the glycemic load. The glycemic load (GL) takes the total amount of carbohydrate in a typical serving of the food into account. The glycemic load is measured by multiplying the glycemic index by the total amount of carbohydrate in a typical portion of the food (instead of the standard 50 grams used for the glycemic index), all divided by 100 – GI x portion size/100. Carrots provide a nice example: when compared to glucose, carrots have a GI of 71, but as stated previously the glycemic index is calculated based on the consumption of 50 grams of carbohydrate; given that it would take roughly 1.5 POUNDS of carrots to get 50 grams of carbohydrate, the GI measure is not exactly realistic. Instead let’s calculate the glycemic load of carrots: assuming that a typical serving of carrots would be about two medium sized carrots, which have roughly 10 grams of total carbohydrate, the math would look like this: 71 (GI) x 10 (total grams of carbs in a portion size)/ 100 = 7.1 GL. That is a massive difference between glycemic index and glycemic load.
The fact that there can be such a large difference between the glycemic index and glycemic load of a food is part of the reason why the glycemic index can be a very dangerous tool and lends itself to being used in the promotion of unsound dietary advice, such as avoiding all foods that are high-GI foods, including fruits and veggies.
Based on all of the above reasons I think it’s pretty safe to say that the glycemic index is a useful tool in understanding how food can affect our system, but it cannot be divorced from the many other variables that also affect how our body responds to food. It also cannot be divorced from the very way in which it is measured, using a 50 gram load, which is why the glycemic load might be a better tool since it uses a scalable “typical portion size.” But glycemic load is also subject to all of the other variables that affect the glycemic index – the composition of the meal, the soil the food was grown in, etc.
One last thing, I also want to be clear that just because a food has a low glycemic index that does not mean it is 'better' for you and should be eaten with abandon. When compared to glucose, fructose has a GI of 19, but anything that has a higher proportion of fructose without much else, including agave, will still have a big impact on the body, especially the liver, even if it doesn’t raise blood glucose. Remember, a rise in blood glucose is not the only marker of how a food can negatively impact health.
At the end of the day, both the glycemic index and glycemic load will show you that eating highly refined, highly processed, highly sweetened food will spike your blood glucose, and if that is the majority of your diet it could have a negative health impact. But is that really new information?