Key Takeaways
Glycaemic Index (GI)
The glycaemic index is a numerical scale that ranks carbohydrate-containing foods by how quickly they raise blood sugar (glucose) levels after eating. Foods are scored from 0 to 100, with pure glucose set as the reference point at 100. A lower score means a slower, more gradual rise in blood sugar; a higher score means a faster spike.
GI values are measured under controlled laboratory conditions using a standard portion containing 50 grams of available carbohydrate, tested on a group of participants — results can vary between individuals and testing conditions.
How the Glycaemic Index Is Actually Measured
The GI of a food is not estimated from its ingredients — it is measured directly on human participants. In a controlled test, volunteers consume a portion of food containing exactly 50 grams of available (digestible) carbohydrate, then have their blood glucose tested at intervals over two hours. Those results are compared against the response from 50 grams of pure glucose eaten on a separate day. The ratio produces the GI score.
This method sounds precise, but it comes with real limitations. Individual blood sugar responses vary widely depending on a person's gut microbiome, insulin sensitivity, and metabolism. Published GI values are averages from small groups, so a single number on a table may not reflect what happens in your body specifically. Testing conditions — time of day, previous meals, stress levels — also influence results.
GI Values Can Vary Between Sources
You may notice that different tables or apps list slightly different GI values for the same food. This is normal — values depend on which variety was tested, how it was prepared, and which study's participants were used. Treat published GI numbers as approximate guides rather than precise facts.
Why Real-World Eating Complicates the Picture
The GI system measures foods in isolation, but people rarely eat a single food at a time. Adding fat, protein, or fibre to a meal slows digestion and blunts the blood sugar response — meaning a high-GI food eaten alongside eggs, avocado, or a salad behaves very differently than it does in a lab test. Portion size is another major variable the basic GI number ignores entirely.
This is where glycaemic load (GL) becomes more useful. GL is calculated by multiplying a food's GI by the actual grams of carbohydrate in a realistic serving, then dividing by 100. Watermelon illustrates the gap well: it has a high GI (around 72) but a low GL per typical serving because a slice simply doesn't contain that many carbohydrates. GL gives a more grounded sense of real dietary impact.
Cooking method and ripeness add further nuance. Dietary fibre plays a key role here — foods higher in soluble fibre tend to digest more slowly, contributing to lower GI scores and more stable blood sugar responses.
Practical Way to Lower a Meal's GI
You don't need to memorise GI scores. Simply pairing starchy foods with vegetables, a protein source, and a small amount of healthy fat will naturally slow digestion and moderate blood sugar responses. Adding vinegar-based dressings or lemon juice may also help — acidity has been shown to slow gastric emptying slightly.
When GI Information Is Most Useful
For people managing type 2 diabetes or prediabetes, the glycaemic index can be a genuinely helpful tool — though it works best as part of a broader eating strategy guided by a registered dietitian or healthcare provider, not as a standalone rule. Research suggests that consistently choosing lower-GI foods may support better blood sugar management over time, though individual responses vary and other dietary factors remain important.
For the general population without blood sugar conditions, GI is less critical as a day-to-day metric. Most nutrition researchers and public health bodies point to overall diet quality — emphasising vegetables, legumes, whole grains, and adequate protein — as a more reliable framework than tracking GI scores. The good news is that a naturally varied, whole-food diet tends to be lower-GI by default.
~72
GI score of watermelon
Despite its high GI, watermelon's glycaemic load per typical 120g serving is approximately 4 — considered low — because the serving contains few total carbohydrates.
0–100
GI scale range
Pure glucose anchors the scale at 100; most whole foods fall well below this, with legumes typically scoring between 20 and 40.
~32
Typical GI of lentils
Lentils are among the lowest-GI carbohydrate sources commonly measured, partly due to their high fibre and protein content slowing digestion.
This article is for general informational and educational purposes only and is not a substitute for professional medical or dietary advice. If you have a health condition or concerns about blood sugar, please consult a qualified healthcare provider.