The glycaemic index (GI) — also written glycemic index — is a 0–100 ranking of how quickly a carbohydrate-containing food raises blood glucose compared with pure glucose, which scores 100. Low-GI foods (55 or under) release glucose slowly and steadily; high-GI foods (70 or above) cause faster, sharper rises. It rates carbohydrate quality, not quantity.
What does the glycaemic index actually measure?
The GI captures how fast the carbohydrate in a food is digested, absorbed and turned into blood glucose. Two foods can hold the same amount of carbohydrate yet behave very differently in the body. A food whose starch breaks down rapidly — such as white bread or a soft, over-boiled potato — produces a quick, steep rise in blood sugar. A food whose carbohydrate is protected by fibre, fat, protein or an intact cell structure releases glucose more gradually.
Crucially, the GI is about the rate and shape of the blood-glucose response, not the total energy or the healthiness of a food overall. It says nothing directly about vitamins, minerals, salt or added sugar. That is why nutrition scientists treat GI as one useful signal of carbohydrate quality rather than a complete verdict on any single food.
How is the glycaemic index calculated?
GI values come from controlled human testing, not from a laboratory analysis of the food alone. Volunteers eat a portion of a test food containing a fixed amount of available carbohydrate (usually 50 grams), and their blood glucose is measured at intervals over the following two hours. The rise in blood glucose is plotted as a curve, and the area under that curve is compared with the area produced by the same person eating 50 grams of pure glucose (or, in some studies, white bread).
The result is expressed as a percentage — the GI value. Because it is measured in real people, the GI reflects the whole food matrix: how tightly the starch is packed, how much fibre surrounds it, and how ripe or processed the food is. This is also why published GI figures can vary between studies and between brands of the same food.
What are low, medium and high GI foods?
Foods are grouped into three bands. The illustrative examples below are typical values; real figures shift with ripeness, cooking method, variety and processing, so treat them as a guide rather than fixed numbers.
| GI band | Score | Typical examples | Effect on blood glucose |
|---|---|---|---|
| Low | 55 or under | Most whole pulses (dal, rajma, chana), rolled oats, most whole fruits, milk and curd | Slow, gentle rise |
| Medium | 56–69 | Whole-wheat chapatti, brown rice, basmati rice, ripe banana | Moderate rise |
| High | 70 or above | White bread, cornflakes, instant/soft-boiled potato, most sugary drinks | Rapid, sharp rise |
Several everyday factors push a food up or down the scale. Ripeness raises GI — a very ripe banana scores higher than a firm one. Longer cooking of rice or potatoes tends to raise GI, while cooling cooked starch can lower it. Eating carbohydrate with fat, protein, acid (such as lemon or curd) or plenty of fibre generally slows the response and lowers the effective GI of a meal.
Glycaemic index vs glycaemic load — what is the difference?
GI has an important blind spot: it is measured for a fixed 50-gram carbohydrate portion, which may not match how much you actually eat. Watermelon, for example, has a high GI but relatively little carbohydrate per slice, so a normal serving has a modest real-world effect.
Glycaemic load (GL) corrects for this. It multiplies a food’s GI by the grams of carbohydrate in an actual portion and divides by 100. A GL of 10 or under is considered low, 11–19 medium, and 20 or above high. GL therefore combines quality (GI) with quantity (portion size), giving a more realistic picture of a food’s impact than GI alone.
Why does GI matter for health?
International expert reviews have concluded that dietary patterns lower in GI and GL are relevant to the prevention and management of type 2 diabetes and coronary heart disease, and probably to weight management. For people managing blood sugar, choosing lower-GI carbohydrates can help smooth the peaks and troughs that follow meals.
This aligns with mainstream dietary advice in India and globally, which emphasises whole grains, pulses, vegetables and fruit — foods that are typically lower in GI and higher in fibre. The World Health Organization’s 2023 guideline recommends that carbohydrate intake come mainly from whole grains, vegetables, whole fruits and pulses, and that adults consume at least 25 grams of naturally occurring dietary fibre a day. India’s ICMR-National Institute of Nutrition similarly urges more vegetables, legumes and whole grains and less refined starch, sugar and ultra-processed food.
Does the glycaemic index apply to Indian meals?
Indian meals are rarely eaten as single foods, and that changes the picture considerably. A plate of white rice on its own has a high GI, but the same rice served with dal, vegetables, curd and a little ghee behaves very differently, because the accompanying protein, fat and fibre slow down digestion. The GI of a whole thali is therefore lower than the GI of its highest-scoring component.
The type of grain also matters. Traditional Indian staples such as whole millets — ragi (finger millet), bajra (pearl millet) and jowar (sorghum) — and whole pulses generally sit lower on the scale than polished white rice or refined-flour (maida) products. Swapping some refined grains for whole grains and millets, and keeping dal and vegetables central to the plate, is a simple, culturally familiar way to build a lower-GI diet without exotic ingredients.
How can you lower the GI of a meal?
Several everyday habits reduce the blood-glucose impact of a meal without cutting out carbohydrates altogether:
- Choose whole over refined: prefer whole grains, millets and whole pulses to refined flour and heavily polished rice.
- Keep the skin and the fibre: eat whole fruit rather than juice, and leave edible skins on vegetables where suitable.
- Combine food groups: pair carbohydrates with protein, healthy fat or an acid such as curd or lemon to slow digestion.
- Mind the cooking: avoid overcooking rice and potatoes to a soft mush, which raises GI.
- Watch the portion: even a low-GI food eaten in a large quantity produces a high glycaemic load.
These steps also tend to make meals more filling and nutritious overall, which is why they align so closely with standard healthy-eating advice.
What are the limits of the glycaemic index?
GI is a helpful tool, but it should not be used in isolation. A high-fat, energy-dense snack can have a low GI yet still be a poor everyday choice, while some nutritious foods sit in the medium or high band. GI values also vary between individuals and even within the same person from day to day, and they are measured for single foods rather than mixed meals. Published figures can also differ between studies because of differences in the food tested and the people tested.
For most people, the practical takeaway is simple: favour whole, minimally processed carbohydrates, pair them with fibre, protein or healthy fats, and watch portion sizes. Used alongside overall diet quality — and not as a rule that labels foods simply “good” or “bad” — GI and glycaemic load can support steadier blood sugar and better long-term health.
This article is general information, not medical advice. If you have diabetes or another health condition, consult a qualified doctor or registered dietitian before making significant changes to your diet.