How does glycemic index impact sugar digestion?

Natural sugars in foods such as bread and rice noodles can raise our insulin levels as much as the sugar that is in icecream or spooned into a cup of tea. This important revelation came to scientist Professor David Jenkins in 1981, after he conducted a series of tests on carbohydrates at the University of Toronto and recorded their impact on blood sugar.

The result was the glycaemic index (GI), which revolutionised our understanding of healthy eating by showing that some healthy complex carbs and starches can cause significant rises in insulin. The higher the GI in the foods you eat, the higher your risk of diabetes type 2 and coronary heart disease, according to several large studies from Harvard School of Public Health in the United States.

“Low-GI foods, by virtue of their slow digestion and absorption, produce gradual rises in blood sugar and insulin,” explains Professor Jenni Brand-Miller, an international expert in glycaemic index from the Human Nutrition Unit at the University of Sydney. “That’s why these foods have proven health benefits such as lowering blood fats and appetite, providing more sustained energy and reducing insulin resistance which can lead to weight gain.”

The GI lowdown

Every food on the GI has a number between 0 and 100, with foods rating as low (0–55), medium (56–69), or high (70–100). The higher the number, the faster the response on your blood sugar. Glucose scores a substantial 100, while maple syrup is rated at only 19. Honey, due to its high fructose content, which slows digestion, has a GI of 55, which is considered low to medium.

To enjoy maximum benefit from the GI, you should stick to a diet of mostly low to medium GI foods in most meals with the occasional high source thrown in. Most fruit and vegetables are low to medium GI, so they are healthy choices. You can fine-tune those choices by minimising starchy vegetables such as pumpkin/parsnip and eating more low-GI fruits such as apple rather than pineapple/apricot, which rate higher. Wholegrains, nuts and seeds, eggs, meats and dairy foods generally boast a lower GI, unlike refined grains such as crispbreads, pastries, sweet biscuits and rice crackers, which are high GI.

There are some surprising entries, though. You’d think white pasta would have a high glycaemic index, but not so. The starchy components of the flour in pasta are in such a form that they are “entrapped” so take longer to break down, leading to less of a spike in your blood sugar. Basmati rice is another interesting example: studies show it has a lower GI than both wild rice and brown rice because it contains more amylose, a starch that slows the rate of digestion.

Considered through the GI lens, foods once thought of as healthy such as potatoes and short-grain white rice have been shown to have a higher GI than white sugar and therefore cause many of the same biochemical effects in your body. Many health professionals now advise minimising the intake of high-GI foods and also taking into account the glycaemic load, which gives more information about carbohydrate amount.

What is glycaemic load?

When you tuck into a sandwich, sushi or spaghetti, the carbohydrate you’ve just eaten influences the rise of your blood sugar. “How high it rises and how long it remains high depends on the quality of the carbs (the GI) and the quantity,” says Brand-Miller. “The glycaemic load, or GL, combines both the quality and quantity of the carbohydrate in one number. The formula is as follows:

GL = (GI x the amount of carbohydrate) divided by 100.

Confused? Then consider the following comparison of an apple and a potato…

Apple — GI: 40; carbohydrate: 15 grams

Glycaemic load = 40 x 15/100 = 6g

Small baked potato — GI: 80, carbohydrate: 15g

Glycaemic load = 80 x 15/100 = 12g

“From these results we can predict that potato will have twice the metabolic effect of an apple,” says Brand Miller. “It helps to think of GL as the amount of carbohydrate in a food ‘adjusted’ for its glycaemic potency.” Understanding the glycaemic load of a food is a helpful tool to ensure that you’re not overeating too many starchy foods in your diet — the reason many GI booklets now include a GL count as well.

The trouble with GI

Glycaemic index and glycaemic load do not take into account the kilojoules or nutritional value of a food. Snacks such as crisps, chocolate or instant noodles are low to medium GI because their fat content slows the rate at which they are absorbed in your stomach. Yet, obviously, a low-GI snack of popcorn, banana or raw almonds is a far healthier choice because it contains more fibre and nutrients.

This points to one of the biggest problems with the GI — that a low reading doesn’t always mean a healthier food choice. To ensure GI foods work in your favour, you need to apply common sense and take into account the big picture of the foods’ total nutritional value such as kilojoules, salt content and food group rather than relying on the GI rating alone. Otherwise, you could be misled to think a diet high in red meat and fats is OK because it relies on low-GI foods, when in fact the oils could be increasing your kilojoule intake and the red meat could be bumping up your risk of bowel cancer.

Remember, too, that many food manufacturers are jumping on the GI bandwagon but often adding unhealthy ingredients such as damaging trans-fats and high-fructose sweeteners such as corn syrup to lower GI. These ingredients may overload your liver, increasing the risk that these unhealthy oils and sugars will be converted to fat. Artificial sweeteners are also used to lower the GI of processed foods. There is still much debate over the safety of these sweeteners, including sucralose, saccharin and aspartame. More natural low-GI sweeteners such as xylitol seem to be better options.

So where does that leave you? Using the GI/GL system to guide your diet is certainly a healthy habit that can help you maintain your weight and ward off disease in the future, but only if good health sense is applied to your low-GI food choices. Remember that your body is a complex machine with an intricate number of metabolic processes and processing carbohydrates is only one of them.

Though GI is one aspect of healthy eating, other considerations including fibre, vitamin intake and your individual biochemical makeup should always be taken into account.

Lowering the GI of food

The way a food is prepared, how much it is processed or refined and how it is formed in nature can lower or raise its glycaemic index through the following mechanisms:

  • Acidity: Slows starch processing and stomach emptying. That’s why foods like sourdough bread are low GI. However, you can lower the GI of a meal further by adding vinaigrette to a salad, some lemon juice to a chicken stew, a little balsamic to a marinade or drinking a dash of apple cider vinegar in some water after your food.
  • Fat: Fat slows stomach emptying, so the more fat a meal contains, the lower its GI. That doesn’t mean shortbread biscuits, deepfried pretzels or oiled and salted cashews suddenly become healthy choices. Ensure that the oils you cook with are always healthy (and cold-pressed), such as virgin olive, canola, flaxseed, sesame, almond, macadamia and sunflower oils. When buying processed foods check that they don’t contain unhealthy oils such as palm or palm kernel, coconut, cottonseed or hydrogenated oils, which damage your heart and can raise your levels of LDL (bad) cholesterol.
  • Particle size: Stone-ground flours have lower GI than refined white flours (the smaller a particle, the faster it is digested). For this reason, some finely milled wholemeal or rye flours may have a medium to high GI because they are rapidly digested.
  • Cooking: The longer you cook a food, the higher its glycaemic index. Keep some crunch in your veg and you’ll help keep GI levels lower, and add fruits like banana to your porridge or rice in the bowl, not the saucepan.
  • Fibre: Foods that are high in soluble fibres, such as rolled oats, apples and chickpeas, boast a lower GI rating and keep your digestive system happy and functioning well.

Know your numbers

When choosing foods for their GI value, remember:

  • High GI = 70–100
  • Medium GI = 56–69
  • Low GI = 55 or less

GI food swaps

Reducing your intake of foods with a high glycaemic index, is easy by simply changing the first list below for the second:

High GI

  • Sugar
  • Cornflakes
  • Short-grain white rice
  • Fine-milled wholemeal/rye bread
  • Rice cakes
  • Watermelon
  • Potato (Desiree/red etc)
  • Parsnip

Low GI

  • Honey or maple syrup
  • Porridge (not the fast-cook variety)
  • Basmati or long-grain white rice

  • Wholemeal sourdough/stoneground rye
  • Buckwheat crispbread
  • Strawberries
  • New potato
  • Sweet potato

Rowena York is a naturopath, herbalist and nutritionist.

For more information go to the University of Sydney Glycemic Index website at

The WellBeing Team

The WellBeing Team

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