GI (the non-military type)

Low GI meals from BBC Good Food - Italian Butterbeans
Low GI recipes from BBC Good Food – Italian Butterbeans

References to high or low ‘GI’ foods are often banded about with little, or no explication. Indeed, I was guilty of this myself in my previous post. But how much do we really know about the term ‘GI’, and what does the GI number of a food actually indicate?

What does ‘GI’ mean?

The glycaemic index (GI) ranks foods based on the rate at which the body is able to brake down the carbohydrates within them into glucose, and by the amount that this glucose then raises the glucose level of the blood.

It was originally developed for people with diabetes, whose bodies either do not produce insulin (type 1) or who have become insulin resistant (type 2), as a means of helping them to help keep their blood sugar levels under control.

Why does this matter?

When glucose levels start to rise, the pancreas releases the hormone insulin, which promotes the uptake of glucose by the cells for use as energy, bringing blood sugar levels back into a more manageable range.

Insulin removes the surplus glucose from the blood and this can then be used as energy within the cells.

Insulin acts on the cells in the liver, muscle and fat tissue in particular and stimulates them to:

  • Absorb glucose, fatty acids and amino acids
  • Stop breaking down glucose, fatty acids and amino acids
  • Start building glycogen from glucose; fats (triglycerides) from glycerol and fatty acids; and proteins from amino acids

Essentially insulin encourages the cells to make fatty acids into fat molecules and to store them, while also stopping the breakdown of extant fat in the cells for energy, instead using the ready supply of glucose for a quick energy fix.

Eating low-GI carbohydrate foods causes a steady rise in the level of glucose in the blood, which in turn leads to a small and gentle rise in insulin. Small increases in insulin keep you feeling full and energised for hours after eating and also encourage the body to burn fat.

Erratic rises and falls in blood glucose levels from low-GI foods however, can leave you feeling hungry and lethargic soon after eating.

While our bodies are well-designed to cope with changes in blood glucose levels, excessive amounts of glucose over an extended period can, in some cases lead to insulin resistance and later, the development of type 2 diabetes.

GI numbers

Low GI  recipes from BBC Good Food - Red lentil, chickpea and chilli soup
Low GI recipes from BBC Good Food – Red lentil, chickpea and chilli soup

The ranking system is based by comparing the rate at which glucose is released from an item of food against glucose. For this reason glucose has a GI number of 100.

Foods with a GI of 70 or more are classified as ‘high GI‘ as they trigger a rapid increase in blood sugar levels. Foods with a GI of 55-69 are ‘medium GI‘ as they trigger a moderate increase. Foods with a GI below 55 are ‘low GI‘ because they have a minor impact on blood sugar.

What makes a food low rather than high GI depends on the proportion of amylose to amylopectin within them. Foods with a greater proportion of amylose such as lentils have lower GIs than those with more amylopectin, like potatoes, which have a high GI.

Why do GI scores vary?

A food’s GI is not fixed however; it will vary depending on a number of things including how the food has been prepared, whether it has been cooked, how hydrated it is and, in the case of fresh produce like fruit, how ripe it is.

An average serving of raw carrot, for example, has a GI of 16 but once peeled, diced and boiled this rises to 49.

Moreover, your glycaemic response to a food also depends on the other foods you eat with it. When a meal includes proteins and fat the impact of the carbohydrate foods is minimised. This is because, by combining foods in a single meal, the overall impact is to slow down the rate at which your body releases sugar from any single ingredient.

Are there any health benefits of eating low GI?

Low GI recipes from BBC Good Food - Whole wheat pasta with broccoli and almonds
Low GI recipes from BBC Good Food – Whole wheat pasta with broccoli and almonds

A positive side effect of choosing low GI foods is that you may lose weight since these sorts of foods tend to keep you feeling fuller for longer. However, it’s worth remembering that low GI doesn’t necessarily mean healthy or low fat.

A low GI eating plan can also be helpful if you’re worried about your risk of type 2 diabetes and heart disease since a low GI diet improves blood sugar and insulin control and helps manage cholesterol levels.

The effect of stabilising blood sugar levels should also mean you feel improvements in energy, mood and concentration levels.

However, using the glycaemic index to decide whether foods or combinations of foods are healthy can be misleading. Foods with a high GI are not necessarily unhealthy and not all foods with a low GI are healthy. For example, watermelon and parsnips are high GI foods, while chocolate cake has a lower GI value.

High or low?

The below table shows the GI of some common foods, taken from Harvard Medical School Health Publications

Food GI Serving (g)
Muesli, average 66 30
Oatmeal, average 55 250
Pearled barley, average 28 150
Sweet corn on the cob, average 60 150
Couscous, average 65 150
Quinoa 53 150
White rice, average 89 150
Quick cooking white basmati 67 150
Brown rice, average 50 150
Bulgur, average 48 150
Apple, average 39 120
Banana, ripe 62 120
Dates, dried 42 60
Grapefruit 25 120
Grapes, average 59 120
Orange, average 40 120
Peach, average 42 120
Pear, average 38 120
Prunes, pitted 29 60
Raisins 64 60
Watermelon 72 120
Baked beans, average 40 150
Blackeye peas, average 33 150
Black beans 30 150
Chickpeas, average 10 150
Chickpeas, canned in brine 38 150
Navy beans, average 31 150
Kidney beans, average 29 150
Lentils, average 29 150
Soy beans, average 15 150
Cashews, salted 27 50
Peanuts, average 7 50
Spaghetti, white, boiled, average 46 180
Spaghetti, wholemeal, boiled, average 42 180
Green peas, average 51 80
Carrots, average 35 80
Parsnips 52 80
Baked russet potato, average 111 150
Boiled white potato, average 82 150
Instant mashed potato, average 87 150
Sweet potato, average 70 150
Yam, average 54 150
Hummus (chickpea salad dip) 6 30
Honey, average 61 25

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