The glycaemic index, explained: Why food doesn’t always follow the numbers
An apple is considered a low-glycaemic-index food. And yet, if I eat one on an empty stomach, there’s a good chance I’ll feel distinctly jittery half an hour or so later.
I’ve always found that a little puzzling.
My partner lives with type 1 diabetes, so carbohydrates, glucose levels and the way food affects the body are familiar topics in our household. But the glycaemic index? It’s not something we’ve particularly relied on.
I’ve looked at the lists before, of course. Some foods are ranked low, others high, and the implication seems straightforward enough: choose the lower numbers for a gentler rise in blood glucose.
Except real life isn’t quite so straightforward.
Why does an apple have a low GI if I sometimes feel uncomfortable after eating one? Why can chocolate rank surprisingly low? And does knowing the GI of a food actually tell us how our bodies will respond to it?
To answer those questions, it helps to understand what the glycaemic index measures — and, perhaps more importantly, what it doesn’t.
What is the glycaemic index?
The glycaemic index, usually shortened to GI, is a way of ranking carbohydrate-containing foods according to how they affect blood glucose levels.
When we eat carbohydrates, our digestive system breaks down digestible carbohydrates into sugars that can be absorbed into the bloodstream. Glucose is one of the body’s main sources of energy, and insulin helps regulate how that glucose is used and stored.
But not all carbohydrate-containing foods affect blood glucose in the same way.
Some are digested and absorbed relatively quickly, producing a faster rise in glucose. Others lead to a slower, more gradual increase.
The glycaemic index helps describe those differences.
Foods are assigned a value using a scale on which pure glucose has a reference value of 100.
| GI classification | Value |
|---|---|
| Low | 55 or below |
| Medium | 56–69 |
| High | 70 or above |
A lower number generally indicates a slower or smaller rise in blood glucose under standard testing conditions, while a higher number indicates a faster or greater response.
Importantly, the GI measures the response to a standardised amount of available carbohydrate, rather than an ordinary serving of food.
And that’s where the limitations begin.
What foods have a low or high GI?
You might expect the rankings to follow a simple pattern: healthy foods at the bottom, unhealthy foods at the top.
But that’s not quite how it works.
Some familiar examples include:
Generally lower-GI foods
- Apples, pears and many berries
- Lentils and beans
- Coarse oats and traditional porridge
- Many types of pasta
- Certain wholegrain and rye breads
- Unsweetened milk and yoghurt
Generally medium-GI foods
- Some varieties of rice, including basmati
- Couscous
- Pineapple
- Certain breads and breakfast cereals
- Some varieties of potatoes
Generally higher-GI foods
- White bread and baguettes
- Cornflakes and some other processed cereals
- Rice cakes
- Many instant potato products
- Glucose-containing drinks
These are broad examples, not fixed classifications for every product or preparation.
The GI of a food can vary depending on its variety, ripeness, processing and cooking method.
And here’s an interesting detail: chocolate can have a relatively low GI because its fat content slows digestion.
That doesn’t make chocolate nutritionally superior to an apple.
It simply demonstrates that the glycaemic index isn’t a measure of how healthy a food is.
So why does an apple sometimes make me feel jittery?
This is the question that originally made me curious about the limitations of GI.
Apples are generally classified as low GI, yet I sometimes feel shaky or jittery within 30–60 minutes of eating one on an empty stomach.
Does that mean the glycaemic index is wrong?
Not necessarily.
A GI value describes an average response observed under controlled conditions. It doesn’t predict exactly how one individual will feel after eating a particular food.
And feeling jittery doesn’t automatically mean that blood glucose has risen sharply or fallen too low.
There are several possible explanations for those sensations, including factors unrelated to glucose. Without measuring glucose at the time, I couldn’t say whether my experience reflects a genuine glucose change.
What it does illustrate, though, is that our experiences of food aren’t always captured by a number on a chart.
The GI can tell us something useful about a food’s general characteristics.
It can’t tell us everything about the person eating it.
If you regularly experience shaking, sweating, dizziness or similar symptoms after meals, it’s worth discussing them with your GP rather than assuming they’re caused by a blood sugar spike or crash.
Why the amount of carbohydrate matters
One of the biggest limitations of the glycaemic index is that it doesn’t account for how much of a food we actually eat.
Consider watermelon.
Watermelon has a relatively high GI, which might make it sound like a poor choice for someone trying to manage their blood glucose.
But watermelon also contains a great deal of water and relatively little carbohydrate in an ordinary portion.
Compare that with a generous serving of pasta.
Pasta generally has a lower GI than watermelon, yet a large serving can contain considerably more carbohydrate.
That means the pasta may have a greater overall effect on blood glucose despite its lower GI ranking.
This is why the quantity of carbohydrate matters so much, particularly for people managing diabetes.
It also introduces another useful concept: glycaemic load.
Glycaemic load: Looking beyond the GI number
Glycaemic load, or GL, combines two pieces of information:
- How a carbohydrate-containing food generally affects blood glucose, represented by its GI.
- How much available carbohydrate is present in the portion being eaten.
The calculation is:
Glycaemic load = (GI × grams of available carbohydrate per serving) ÷ 100
For example, imagine a portion of food with a GI of 50 containing 20 grams of available carbohydrate.
Its glycaemic load would be:
(50 × 20) ÷ 100 = 10
Now imagine eating twice as much of the same food.
The GI remains unchanged, but the glycaemic load doubles because you’re consuming twice the carbohydrate.
This makes GL a potentially more useful way of thinking about a real portion of food.
Still, it isn’t something most of us need to calculate every time we prepare a meal.
The practical lesson is simpler: both the type and the amount of carbohydrate influence what happens to blood glucose after eating.
And what is glycaemic response?
There’s one more term worth understanding: glycaemic response.
While GI is a standardised measurement assigned to a food, glycaemic response describes the actual change in blood glucose after eating.
That response is influenced by far more than the GI of the food itself.
It can depend on:
- The amount of carbohydrate eaten
- Whether the food is eaten alone or as part of a meal
- The presence of fat, protein and fibre
- Cooking methods and food preparation
- Physical activity, stress, illness and hormones
- Individual digestion and metabolism
- Insulin production, insulin sensitivity and, where relevant, diabetes medication
This is why two people can eat the same meal and experience different glucose responses.
It’s also why the same person might respond differently to a familiar meal on different days.
For someone living with type 1 diabetes, the timing and action of insulin add another important consideration.
A meal containing fat and protein, for example, may affect how quickly carbohydrate is absorbed, sometimes leading to a more prolonged glucose response.
None of that is visible in a simple GI ranking.
Is the glycaemic index useful for people with diabetes?
It can be.
Research suggests that choosing lower-GI carbohydrate foods may help improve glucose management for some people, particularly those living with type 2 diabetes.
There is less conclusive evidence for its benefits in type 1 diabetes, where carbohydrate counting and appropriate insulin management remain particularly important.
For people with gestational diabetes, choosing lower-GI carbohydrates can also be one useful part of dietary management.
But GI is best understood as an additional consideration, rather than the foundation of every food decision.
Someone living with type 1 diabetes may find that understanding carbohydrate quantities, insulin timing and their own glucose patterns is more practically useful than memorising GI values.
Others might find the index helpful when choosing between different breads, cereals or starchy foods.
Neither approach is inherently better.
The value of GI lies in whether it helps someone make informed, manageable choices — not in following a ranking system perfectly.
Does that mean we should avoid high-GI foods?
No.
A high GI doesn’t automatically make a food unhealthy, just as a low GI doesn’t guarantee that a food is nutritious.
A baked potato, for example, can have a relatively high GI while still providing fibre and useful nutrients.
And there are situations where rapidly absorbed carbohydrate is exactly what’s needed.
For someone experiencing hypoglycaemia — a hypo — fast-acting carbohydrate such as glucose tablets or an appropriate sugary drink can be essential.
In those circumstances, choosing a slow-release carbohydrate instead may delay treatment.
People at risk of hypos should follow the treatment guidance provided by their diabetes team.
More broadly, there’s no need to approach every rise in blood glucose as something to fear.
Blood glucose naturally changes throughout the day, including after meals. For people living with diabetes, the important considerations are individual glucose targets, overall patterns and appropriate treatment — not the elimination of every increase.
How to use GI without overthinking your meals
You don’t need to memorise a long list of numbers to make use of the glycaemic index.
In fact, some of its most practical lessons are things you may already do when preparing balanced meals.
Choosing a less processed, higher-fibre carbohydrate can sometimes produce a gentler glucose response.
Eating carbohydrates alongside protein, fat and fibre can change how quickly a meal is digested.
Paying attention to portions can be more useful than focusing on GI alone.
And considering the nutritional value of a food matters just as much as its ranking.
For example, you might choose traditional porridge instead of a highly processed breakfast cereal, or enjoy an apple alongside yoghurt or a handful of nuts.
Not because the apple is somehow problematic on its own, but because combining foods can influence digestion and make a snack more satisfying.
These are options to experiment with, not rules everyone needs to follow.
For people managing diabetes with insulin or other glucose-lowering medication, significant dietary changes are best discussed with their healthcare team.
A useful guide, not a set of food rules
The more I’ve learnt about the glycaemic index, the more I appreciate both its usefulness and its limitations.
It’s interesting to understand why some foods affect blood glucose more quickly than others. And for some people, choosing lower-GI carbohydrates can be a helpful part of managing diabetes.
But the index doesn’t account for everything that makes eating such an individual experience.
It doesn’t know whether you’re eating breakfast before a long walk, enjoying dinner with friends or grabbing something quick between appointments.
It doesn’t know how much insulin is active in your body, how well you slept or how your glucose levels behaved yesterday.
And it certainly can’t explain, on its own, why an apple sometimes leaves me feeling jittery.
Perhaps that’s the most useful thing to remember.
The glycaemic index can offer information about food. It doesn’t have to become another set of rules about eating.
And understanding the difference might be more valuable than memorising the numbers themselves.
This article is intended for general information and does not replace personalised dietary or medical advice. If you live with diabetes, speak to your diabetes team or a registered dietitian before making significant changes to your diet or treatment.