The High-Glycemic Foods Nobody Suspects

Most people know that sweets raise blood sugar. Almost nobody expects it of a rice cake, and that is the problem: the foods with the steepest glucose response are very often the ones sitting in the basket precisely because they seemed harmless.

The glycemic index measures how fast and how far a food raises blood glucose compared with pure glucose, which is set at 100. Anything above 70 is high. What follows is the ordinary supermarket list — nothing exotic, nothing anyone would flag as a treat.

The ones that beat table sugar

Sucrose sits at about 65. Every item here is higher, which means it raises blood glucose faster than eating sugar from the bowl.

• Rice cakes, 82–91 — the diet food. Puffed starch with nothing to slow it: no fat, no protein, barely any fibre. The most misleadingly positioned product in the shop
• Cornflakes, 81–93 — higher than most breakfast cereals marketed to children, and it is the plain unsweetened version that scores this
• Baked potato, 85, and instant mash around 87 — potato without fat behaves like a fast carbohydrate. Boiled and cooled it drops considerably, which is a real and useful difference
• Parsnip, up to 97 in some tables — a root vegetable, and among the highest readings recorded for any whole food
• Pretzels, 83, and rice crackers, 87 — the savoury snacks people choose over crisps because they are low in fat
• Puffed rice cereal, 82 — puffing is a process that exists to make starch more available, and it succeeds
• Rice milk, 86 — the plant milk with the highest glucose response by a wide margin. Oat milk is lower but not low
• Glucose syrup, rice syrup and maltodextrin, 85–110 — maltodextrin scores above pure glucose and appears in protein powders, sports products and a great many bars sold as natural

The ones that only look better than white

• Wholemeal bread, 71–74 — barely below white bread at 75. The wholegrain in most commercial loaves is milled to flour, and flour is flour whatever colour it is. Genuine whole-kernel and sourdough rye are meaningfully lower
• Gluten-free bread, often 70–90 — higher than the wheat bread it replaces, because rice flour, potato starch and tapioca do the work the gluten used to. Removing gluten is not the same as removing the glucose load
• Instant oats, 79, against steel-cut at about 55 — the same grain, cut finer and pre-cooked. Processing is the whole difference
• Granola and muesli with dried fruit, 60–80 — the dried fruit is the reason, and it is usually sweetened on top of being concentrated
• Couscous, 65, and instant noodles, 47–75 — fast in every sense

Fruit, honestly

Fruit is where the index misleads most often, and the correction runs in both directions.

• Watermelon scores 72–80 but a normal portion carries so little carbohydrate that the actual effect is small. This is the standard example of why the index alone is not enough
• Dates, 62–103 depending on variety and ripeness, and eaten in quantity because they are sold as a natural sweetener. They are a natural sweetener, which is the point
• A ripe banana is around 62 and a green one around 42. The same fruit, days apart
• Fruit juice and smoothies, 50–75, but drunk in minutes and stripped of the structure that would have slowed them. Blending is not neutral: a whole apple and a blended apple do not behave the same way

The ones sold as the healthy option

• Sports drinks, gels and recovery powders, 75–100 — designed to raise blood glucose quickly, which is honest engineering for an endurance athlete mid-race and nothing else
• Low-fat yoghurts and low-fat anything: removing fat removes flavour, and sugar is what usually replaces it. Read the carbohydrate line rather than the fat claim
• Organic and 'no refined sugar' products sweetened with rice syrup, date syrup or concentrated apple juice. The adjective changed; the glucose did not
• Popcorn, around 65 — better than most snacks, worse than it feels, and rarely eaten in a measured portion

The pattern is not hard to see. Almost every item on this list is a starch that someone has already broken down for you — puffed, milled, instantised or dried. Processing is not a separate concern from the glycemic index. It largely is the glycemic index.

What the number does not tell you

It would be easy to read the list above and start memorising numbers. That is the wrong lesson, and this site would rather say so than let a tidy figure do the work of thinking.

Glycemic load matters more than glycemic index

The index describes the speed of the response to 50 g of carbohydrate from a food. The load accounts for how much carbohydrate a real portion actually contains. Watermelon is high index and low load; a plate of mashed potato is high in both. Where the two disagree, the load is the one describing your afternoon.

It is measured in isolation, and you do not eat that way

• Fat and protein eaten alongside slow gastric emptying and blunt the peak substantially
• Vinegar or acid with a meal lowers the response measurably
• Cooling cooked starch converts some of it to resistant starch — cold potato and cooled rice are genuinely lower than they were hot
• Eating carbohydrate last in a meal rather than first produces a lower peak from identical food

Your number is not the published number

The most useful finding in this whole area is that individual responses to the same food vary enormously. Two people eating identical bread can produce opposite glucose curves, and the published index — an average across a small panel — predicts neither of them well. Which is why a continuous glucose monitor worn for two weeks tells you more about your own metabolism than any table ever will.

What to do with this

• Stop treating rice cakes, puffed cereals and low-fat snacks as neutral. They are the sharpest spikes in an ordinary shop
• Prefer the whole kernel to the flour, the intact fruit to the juice, the cooked-and-cooled to the hot and fluffy
• Never eat a fast carbohydrate alone. Fat, protein and fibre alongside it change the curve more than swapping the carbohydrate would
• Watch frequency, not just choice. Three spikes a day keeps insulin elevated in a way one spike does not, and it is the elevation rather than the peak that drives insulin resistance
• If you want to know rather than guess, wear a monitor for a fortnight. It costs less than a year of getting it wrong

None of this makes any single food forbidden. It makes the label unreliable, which is a more useful thing to know — the products most likely to be marketed as the sensible choice are, on this measure, frequently the worst one in the aisle.