Glycemic Index Explained

What is the glycemic index?

You've probably heard the term "glycemic index" mentioned alongside carbohydrates or the foods that contain them. A lot of people aren't quite sure what it means, and that's what today's article is here to fix.

The glycemic index (GI) shows how quickly a given food releases energy into your bloodstream. Foods are grouped into low, medium, and high GI. Low-GI foods are absorbed slowly and don't cause a sharp spike in blood glucose — glucose still rises, but the rise is spread out over a longer period thanks to the slower absorption. High-GI foods, on the other hand, cause a fast spike in blood glucose. In most situations that's not ideal, because the body is likely to store that sudden surplus of energy as fat. There are situations where a high GI actually works in your favor, though — right after training, for example, when you need to replenish glycogen in your muscles quickly.

All of this sounds fairly straightforward, but there are two catches. The first is that GI only lets you compare foods that contain the same amount of carbohydrate. That means you can't directly compare, say, 50 grams of rice (79% carbohydrate) with 10 grams of sugar (nearly 100% carbohydrate) using GI alone. Even though rice has a lower glycemic index than sugar, eating that portion of rice raises your blood sugar more, simply because you're eating far more of it. This is exactly the gap that a second measure was created to close: glycemic load.

Glycemic load

Glycemic load is closely related to the glycemic index, but it also factors in how much carbohydrate you're actually eating. Take that rice as an example: long-grain cooked rice has a GI of 56. A 50-gram portion contains 39.5 g of carbohydrate (50 × 0.79). Multiply that by the GI (56) and divide by 100, and you get 22.12 — a high glycemic load, if only just. As a rule of thumb, a glycemic load of 0–10 is low, 11–20 is medium, and anything above 20 is high. Keep in mind, though, that this can't be taken too literally — strictly applied, you'd need to be eating every hour rather than every three.

What affects the glycemic index?

The second catch is that a long list of factors can shift a food's GI. Cooking, for example, increases GI — the food gets absorbed faster as a result — while fiber has the opposite effect and lowers it. GI is also influenced by the amount of fat and protein eaten alongside the food, by enzymes, and more.

So how do you actually find out a food's GI?

Put all of this together and pinning down the exact glycemic index of any given meal is close to impossible. If you want to use GI to guide your eating, treat it as a rough guide at best, not an exact number.