Glycaemic Index – What is it and why is it important?

The Glycaemic index, of GI is a rating give to foods, which contain carbohydrates. The GI is a comparison to glucose, a sugar and determines how fast a food raises your blood sugar level.

Your brain can only use glucose for energy and your digestive system is optimised for extracting glucose from your food, therefore anything which can be quickly converted into glucose, such as other sugars and simple starches, will lead to rapid rises in blood sugar. Glucose is given a GI rating of 100, which is the highest.

High GI foods, weight gain and diabetes

Low GI foods release sugars much more slowly, so help to reduce wild fluctuations in blood sugar. Why is this important? While we need sugar in our blood, we want to keep the levels fairly constant. Rapid increases are dangerous, so the body reacts by producing insulin, which converts the sugar into a substance called glycogen, which can be safely stored for later use. If blood sugar rises too rapidly, the body over-reacts, produces too much insulin, blood sugar drops and we feel hungry again. This is why eating foods high in sugar, or white bread, white pasta, biscuits, etc. will often leave us feeling hungry again.

In addition to this, the body can only store a limited amount of glycogen. Once these stores are full, the excess sugar is converted to fat. This is why when we eat more calories than our body needs, we put on weight – we literally get fat. If our body is constantly having to create insulin and store excess energy, it can also lead to insulin resistance and type-2 diabetes, which is why diabetes is often symptomatic of obesity.

The Glycaemic Index of a food is affected by a number of factors, including the type of starch the food contains, how much fibre is present, particle size and so on. White bread flour has had virtually all the fibre removed and is a very fine powder, compared with wholemeal flour, which has lots of fibre and a bigger particle size. The end result is that white flour has a very high GI, while wholemeal is much lower.

Rice is a slightly different case. Interestingly, brown rice, while lower GI than white is still quite high. Quick-cook Basmati rice is medium GI, while Basmati that takes longer to cook (20-25 minutes) is the lowest GI. Changing from standard quick-cook white rice to Basmati rice is something that most people can easily do and which has straight-forward benefits.

Knowing which foods are low GI

Foods which have lower a lower GI and therefore release their sugars more slowly are often those that have more fibre. Wholemeal bread and whole wheat pasta have much a much lower GI than their processed, white counterparts. Oats (so long as they are not ultra-processed quick oats) have a low GI, whereas cornflakes and puffed rice type cereals have a very high GI and often have sugar sprinkled on top, which makes them even worse!

All beans, lentils, nuts and pulses have low GI. Boiled Potatoes eaten with the skin have a low GI. However, baked (even with the skin) and mashed potato are very high GI. Most fresh fruit is low to medium GI, however, watermelon, lychees and fruit juice all have very high GI.

All green veg and carrots are low GI. Beetroot, sweetcorn and sweet potato & butternut squash are medium GI, while parsnips have a high GI. Dairy products such as cheese, milk and plain yogurt are low GI, however, flavoured yogurts with added sugar have medium to high GI.

Interestingly, fat does help to reduce a food’s GI. For instance crisps and milk chocolate have lower GIs than you might expect. However, not many people are going to tell you that high-fat foods are healthy choices!

Glycaemic index isn’t of course the be-all and end-all of choosing what to eat, it simply indicates how quickly that food’s sugar will be absorbed. There’s nothing wrong with having a few high GI foods in your diet. However, they need to be balanced with a much larger proportion of low & medium GI foods, particularly those high in fibre, as fibre has an important role in regulating blood sugar levels.

2 thoughts on “Glycaemic Index – What is it and why is it important?”

  1. Pingback: Food Labelling – understanding the nutritional information – Revive Fitness Classes

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