Source: Brand-Miller, Prof Jennie. Foster-Powell, Kaye. Colagiuri, Prof Stephen. The New Glucose Revolution. Sydney: Hodder, revised ed. 1998.
Much of what we have been taught for many years about carbohydrates (eg sugar = bad, complex starch = good) has been based on a theory that the body would break down sugars more easily than starches. It seemed like a good theory – but has failed the test of actually working in real human beings.
The researchers who wrote this book set about testing what different carbohydrates did in the human body, by using actual human beings, getting them to eat different carbohydrate-containing foods, and measuring what happened to their blood sugar over time.
They then classified foods according to that affect, along what is called the Glycaemic Index (GI)– a numerical value to indicate what each individual food will do to blood sugar.
What happens when carbohydrate is absorbed by the human body varies according to the GI of a particular food. Foods with a low GI release sugar into the blood slowly – blood sugar rises at a slow, steady, rate, the body produces insulin at a slow and steady rate, and the blood sugar then releases at a slow and steady rate. Foods with a high GI release sugar into the blood quickly – blood sugar rises greatly and suddenly, the body produces a lot of insulin to control the blood sugar quickly, and the blood sugar drops suddenly.
This is important because: sudden spikes and drops in blood sugar just aren't good for the body and sudden spikes and drops in insulin can lead the body to “insulin resistence” - a body that just doesn't pay any attention to what insulin is doing anyway - a precursor to diabetes. (Uncontrolled blood sugar is even worse for people who already have diabetes.) It also affects weight loss – sudden rises and falls in blood sugar can make you feel hungry – keeping your blood sugar at a steady level can help keep you in control of your appetite, and helps your body burn the fuel you eat more efficiently.
The surprise is that the sugar we always thought was “bad” does not have an especially high GI, and the complex starches (eg flour) we were taught were “good” or at least “better” than sugar, can have a much higher GI.
It is important to note that all it takes to lower the overall GI of a whole meal, is that you have some low GI foods – so there is no need to avoid all high GI carbohydrates, just moderate their effect by having some lower GI carbohydrate foods at the same meal (for example, have half and half sweet potato and regular potato.)
So what things affect the GI of foods? Looking at the book, some things are inexplicable – that's why each individual food is tested.
(Full diagram pp42-43)
- Starch gelatinisation: the less gelatinised (swollen) the starch, the slower the rate of digestion.
- Physical entrapment: the fibre around beans and seeds etc acts a physical barrier, slowing down digestion
- High amylose to amylopectin ratio this is the ratio between different types of starch in the food - basically it affects how the starch can be gelatinised – it means things like Basmati rice is better than other kinds of rice.(It's the kind of thing you and I can't tell from looking at a food product, unless it has its GI written on the label.)
- Particle size The smaller the particle, the easier to digest – finer ground flours tend to be higher GI
- Viscosity of fibre soluble fibre slows down the rate of digestion
- Sugar Sugar, contrary to what we've been told for years, can slow down digestion.
- Acidity Acids in food slow down stomach emptying, and therefore slow the digestion rate
- Fat slows down the rate of stomach emptying, and therefore the rate of digestion (NOTE: this is not an excuse to binge on fat – this is only looking a the action of carbohydrate on the body – fat is a completely different issue. For optimum health, a diet has to be low in saturated fats as well as relatively low in GI.)
GI Values. High = 70 or more; Medium = 56 to 69; Low = 55 or less.
(Note, that for weight loss, the GI isn't everything we need to be aware of – we still have to look out for calories, fat, and the nutrient value of each of the foods we eat. Where there's a choice – the low GI option will help keep your body functioning at its best, and help control your appetite.)
We also need to be aware that some things with a high GI (eg carrots) actually have so little carbohydrate in them that the effect on blood sugar is negligable. Another term sometimes used alongside the GI is the Glycaemic Load (GL) which takes into account not just the measure of GI, but the amount of carbohydrate you actually eat in a serve.
There is page after page of listings of foods with their GI and GL in this book, as well as a variety of low GI recipes. And the authors have put together more books with more food listings, recipes. Each of the authors is a leader in their particular field: nutrition, diet, and diabetes and metabolic disorders.
Very brief reference for lower GI foods – from pp 108 – 114
- Burgen varieties (GI 31-74) Honey Loaf with Barley (GI 31) is the lowest GI of all mixed grain breads
- Fruit loaf (GI 44-54)
- Pita bread (GI 57)
- Sourdough bread (GI 54)
- Plougman's loaf Wholegrain (GI 47)
- Pumpernickel bread (GI 41)
- Custard (GI 35)
- Ice-cream (GI 36-80)
- Milk (GI 31)
- Yoghurt (GI 14-36)
- Apples (GI 38)
- Apple juice (GI 40)
- Apricots – canned (GI 64); dried (GI 30)
- Cherries (GI 22) This measurement is for European Cherries – Australian may be slightly higher.
- Grapefruit (GI 25)
- Grapes (GI 46)
- Kiwifruit (GI 58)
- Oranges (GI 42)
- Peach – Fresh (GI 42); canned in light syrup (GI 52)
- Pears – fresh (GI 38); canned (GI 43)
- Pineapple juice (GI 46)
- Plums (GI 39)
- Sultanas (GI 56)
- Barley (GI 25)
- Basmati Rice (GI 58)
- Buckwheat (GI 54)
- Bulghur/Burghul /Cracked wheat (GI 48)
- Doongara Rice (GI 56)
- Oat bran (GI 55)
- Rice bran (GI 19)
- Legumes (GI 10-70)
- Soya Beans (GI 14-20)
- Most don't have a GI value – containing very little carbohydrate
- Peanuts (GI 14) are actually legumes
- Pasta (GI 32-78)
- Spaghetti (GI 38)
(Most vegetables are low in carbohydrate, which makes it difficult to measure their GI, and means they have a negligable affect on blood sugar.)
- Peas (GI 48)
- Sweet corn (GI 54)
- Sweet potato (GI 44)
So, in terms of eating for better health, the main message is to be aware of the impact food has on our bodies, and try to incorporate some low-GI carbohydrate at every meal.