I'm Alli, your friendly, cheeky nutrition nerd. Ever wondered why some carbs send your blood sugar on a roller coaster ride while others barely make a ripple? That mysterious number you see on nutrition charts — the glycaemic index (GI) — promises to be your guide to better energy, weight management and chronic disease prevention. But does it deliver? Let's dive into what the glycaemic index is, how it influences your metabolism and whether you should care about it at all.
What is the glycaemic index and why should I care?
The glycaemic index ranks carbohydrate containing foods based on how quickly they raise your blood glucose. Carbs provide energy for your brain and muscles, but not all carbs are created equal. GI measures how quickly carbohydrates break down during digestion and enter your bloodstream.
Foods like white bread or baked potatoes break down quickly, causing a rapid spike in blood glucose, whereas oats or lentils release glucose more slowly and lead to a gentler, prolonged rise. Harvard Health points out that a food with a GI of 95 acts almost like pure glucose, while one with a GI of 28 raises blood sugar only about 28 percent as much.
So why should you care? Sharp spikes in blood glucose are followed by sharp drops, which can leave you feeling tired, cranky and ravenous. Over time, repeatedly spiking blood sugar can stress your pancreas — the organ that produces insulin — and may contribute to insulin resistance. Using the GI to choose foods is one strategy to stabilise your energy levels and support metabolic health. However, as we'll see later, the story isn’t that simple.
How is the glycaemic index measured and does it apply to my meals?
Scientists measure the GI of a food by giving volunteers a portion containing a fixed amount of carbohydrate (usually 50 g) and then checking their blood glucose over the next two hours. The rise in blood glucose is compared with pure glucose or white bread, which is given a GI of 100.
Foods are then classified as:
• Low GI (<55): examples include beans, soy products, most fruits, pasta and wholegrain breads.
• Medium GI (55–70): foods like orange juice, honey and basmati rice.
• High GI (>70): foods such as white bread, baked potatoes and short grain rice.
It's important to remember that GI scores are measured in isolation, not in the context of a meal. Combining foods changes digestion: a bowl of cornflakes (high GI) eaten with milk (low GI) produces an average response. Fat, protein and soluble fibre all slow gastric emptying, lowering the overall glycaemic impact. Cooking methods, ripeness and processing also matter — a ripe banana has a higher GI than a green one, and cold potatoes have a lower GI than hot ones. In real life we eat mixed meals, so the GI is a guide, not a rulebook.
What’s the difference between glycaemic index and glycaemic load?
Another useful concept is glycaemic load (GL), which considers both the quality and quantity of carbs. GI tells you how fast a carbohydrate raises blood glucose; GL takes into account how much carbohydrate is in a serving. GL is calculated by multiplying the GI by the grams of carbohydrate in a serving and dividing by 100. A high GI food eaten in a small amount can have the same GL as a low GI food eaten in a large quantity. Pasta has a low GI, but a huge plate will still raise your blood sugar more than a small serve of rice. Using GL helps you plan portion sizes while still prioritising lower GI choices.
Does choosing low GI foods help control weight or reduce disease risk?
This is where things get interesting. Observational studies suggest that diets rich in high GI foods are linked with higher risk of type 2 diabetes and cardiovascular disease. Picking lower GI sources of carbohydrates can help control blood sugar and may lower the risk of heart disease and certain cancers. Some cohort studies show that low GI diets are associated with reduced inflammation and improved lipid profiles.
However, randomised controlled trials paint a more nuanced picture. The OmniCarb randomised clinical trial compared a DASH style diet with high vs low glycaemic index and found that in the context of an overall healthy diet, lowering the GI did not improve insulin sensitivity or major cardiovascular risk factors. Surprisingly, lowering the GI of a high carbohydrate diet even decreased insulin sensitivity and increased LDL cholesterol. In other words, the benefits of a low GI may be modest if you’re already eating a balanced diet rich in whole foods. Glycaemic index is not a magic bullet.
Is the GI useful for athletes and exercise performance?
Some athletes swear by high GI foods to fuel intense workouts and low GI snacks to maintain steady energy during endurance events. Research shows that eating a low GI meal a few hours before exercise can improve endurance performance compared with a high GI meal which should be used within 1-1.5hours of exercise. The theory is that a gradual release of glucose spares glycogen stores and delays fatigue. On the flip side, high GI foods may be useful immediately before and after exercise to quickly top-up or replenish glycogen stores. As always, individual responses vary, and what works for you may not work for your training partner.
What are the limitations and criticisms of the glycaemic index?
The GI has several shortcomings that deserve attention: It ignores nutrient quality. Lollies and watermelon both have a high GI, but watermelon also provides vitamins, antioxidants and hydration. A low GI muffin loaded with butter and sugar is not a health food. GI should not be your sole criterion for choosing foods.
It does not consider mixed meals. As mentioned, the GI of a food changes when eaten with fat, protein or fibre. Unless you eat foods in isolation, GI tables may not reflect real world responses.
Individual responses vary. Your gut microbiome, insulin sensitivity and even time of day can influence how you process carbohydrates.
Continuous glucose monitoring reveals that people can respond differently to the same food. GI is an average, not a personalised measurement.
It can be manipulated. Food manufacturers can add fat or acid to lower the GI without improving overall nutrition. Some low GI products may still be high in calories, saturated fat or refined starches.
Should I ditch the GI altogether?
Not at all. If you’re struggling with energy crashes or managing diabetes, using GI tables can be a helpful reference. But don’t fixate on numbers.
Focus on overall eating patterns — plenty of plants, lean proteins, healthy fats and adequate fibre. The GI is a compass, not a GPS. It points you toward slower digesting carbs but doesn’t map your entire journey.
What’s the bottom line?
The glycaemic index is a useful concept for understanding how different carbs affect blood sugar. Low GI foods like beans, fruit and oats digest slowly, leading to steadier energy, while high GI foods like white bread cause rapid spikes. Glycaemic load adds another layer by accounting for portion size and meal composition.
Using GI can help guide food choices, but it shouldn’t replace common sense or personalised advice. Emerging research shows that simply lowering the GI does not necessarily improve cardiovascular risk factors or insulin sensitivity. Instead, focus on whole foods, balanced meals and listening to your body.
When in doubt, remember: I’m here for you. You don’t really need to think about this stuff while you have me — I’m your carb loving ally on this journey.
– Alli
Disclaimer: This article is for general educational purposes and does not constitute medical advice. Consult your health professional for personalised guidance.