Glycemic Index Explained: The Only Numbers You Need

Quick answer: The glycemic index is a 0 to 100 score for how fast a food’s carbohydrate raises blood sugar compared with pure glucose, with 55 and below counted as low, 56 to 69 medium, and 70 and up high. It’s a shortcut and not a rulebook, because the number ignores how much you ate, which is why two slices of high-GI bread can hit me harder than a whole bowl of something marked medium.

The first time I saw a glycemic index table I did what most people do. I looked for the foods I ate every day, found half of them in the high column, and concluded the table was stupid and I was fine. Then I used my meter for two weeks and found the table was mostly right and I was mostly wrong, and both of us missed the point, because the number assumes one food, eaten alone, in a measured amount, which is not how a Tuesday goes.

What does the glycemic index actually measure?

It’s measured, not guessed. Researchers give volunteers a portion of a food containing a fixed 50 grams of carbohydrate, then track blood sugar for two hours, and compare that curve to the curve from pure glucose, which is set at 100. One food, one number.

That single number describes speed. Not amount, not your body, not what else is on the plate. It’s not a health score, it’s a speedometer. A high number means the carbohydrate arrives fast, a low number means it arrives slow, and your body doesn’t experience a number, it experiences a curve.

For me a steep curve is the thing to avoid, not because the number is rude but because steep means a big insulin ask, and my pancreas has been doing this for years and doesn’t need the extra work at 43.

What are the low, medium and high ranges?

Low, 55 and under. Legumes, lentils, chickpeas, black beans at 25 to 35. Apples, pears, oranges, plums at 30 to 40. Pasta cooked al dente around 40 to 50. Barley, bulgur and steel-cut oats from 30 to 55. Plain milk and plain yogurt around 30. This is where most of my plate lives.

Medium, 56 to 69. Basmati and parboiled rice around 55 to 60, brown rice 65 to 70 depending on the bag, whole-grain bread 55 to 70, ripe banana, pineapple, couscous.

High, 70 and up. White bread and baguettes 70 to 80, short-grain and jasmine white rice 70 to 75, boiled potatoes 75 to 80, watermelon around 76, cornflakes and puffed cereals 80-plus.

Notice what’s in the high band alongside the cereals. Potatoes and watermelon, two foods that are not junk. The score measures one property, fast carbohydrate, not the moral quality of the food.

Why does the same food score two different numbers?

Because the number isn’t glued to the food, it moves with the food’s circumstances, and this is where GI thinking gets genuinely useful instead of just tidy.

Ripeness. A green-tipped banana scores around 40. Wait four days, until it’s brown and spotted and sweet, and it’s climbing toward 60. Same fruit, same peel, different sugar because the starch converted while it sat on the counter.

Cooking. The longer you cook a starch and the more you break it down, the faster it goes in. Boiled potatoes sit around 78, pasta cooked to mush well above pasta al dente, instant oats well above steel-cut. You’re doing the stomach’s work in the pot and getting billed for it later.

Cooling. Cook potatoes or rice, cool them overnight, reheat them, and part of the starch has turned into resistant starch, which your body can’t fully break down, so the curve flattens. That’s chemistry, not folklore. I’ve written the longer version in resistant starch, rice and potatoes.

Milling. Whole kernels digest slowly. The same grain ground into fine flour digests fast, because a mill did the chewing. Coarse stone-ground beats finely milled almost every time, which is why a dense bread can hit me harder than a pot of barley with the same carb count on paper.

What it’s eaten with. GI is measured on a food eaten alone, which almost nobody does. Add protein, fat or fiber and the whole thing slows down. Rice with chicken and beans is not the same event as rice by itself, even though the rice is identical.

That last one is the lever I actually pull, and the whole argument in eating your carbs last. Same meal, eaten in a different order, and my two-hour number comes down. The traffic pattern changed.

What is glycemic load, and why does it matter more?

Because GI ignores quantity, somebody smarter than me multiplied it by the actual carbs in a serving and divided by 100. That’s glycemic load, and it answers the question people actually have: what happens when I eat this food in this amount.

The example that fixes it in my head is watermelon. Glycemic index 76, which sounds like a warning label. But a slice is mostly water and holds about 6 grams of carbohydrate, so the load is around 4 or 5, which is nothing. Two slices of white bread are also about 75, but they carry 30 grams of carbohydrate, so the load is around 22. Same index class, completely different bill.

GI tells you how fast the money leaves. Load tells you how much is in the account. If you only learn one of the two numbers, learn load, because it’s the one that shows up on your meter an hour later.

Where does the glycemic index fail?

Three places, none of them your fault.

It knows nothing about portion, because it’s built on a fixed 50 grams of carbohydrate whether that’s one slice or five.

It varies between people and even in the same person on different days, which is why my meter beats any published table. The watermelon question is the classic example, high index, small load, and the only real answer is to test your own slice.

And high GI isn’t automatically bad. If I’ve been up and down stairs for six hours and I’m shaking in a basement, fast carbohydrate is exactly the tool. Context is everything.

So how do I actually use this at the grocery store?

I don’t carry a table. I carry three habits. Pick the intact version over the milled version. Put protein, fat and vegetables on the plate before the starch. Keep the starch to a portion I measured once. When I want to know whether a swap worked, I eat it, test at one and two hours, and compare, and my numbers are 4 to 5 fasting with usually under 6 at two hours.

That’s it. The tables are a map of the terrain. The meter is the terrain.

When to call your care team

If you’re getting readings above 10 mmol/L two hours after meals most days, or your A1C is moving the wrong way while you think you’re eating well, that’s worth a real conversation, not another chart. If you’re on insulin or a sulfonylurea, portion and timing changes can change what you need, so that’s a dosing question. If you have kidney disease, some low-GI staples are high in potassium and phosphorus, so the list needs a second pass with your dietitian.

I’m not on medication at the moment and I check everything against my own measured data, which is my data and not your target. Targets come from your care team.

General information, not medical advice. Glycemic values are averages that shift with variety, ripeness and cooking, so test your own response and set targets with your care team.

Full Guide

Educational content only — not a substitute for advice from your doctor or pharmacist. Always work with your care team.