Glycemic Index (GI)
The Glycemic Index (GI) is a scale from 0-100 whereby foods are ranked according to their effect on blood glucose (sugar) levels. For the same amount of carbohydrate, foods with a lower GI raise the blood glucose level less than those with a higher GI - for example, pure glucose has a value of 100, since it causes the greatest and most rapid rise in blood glucose. All other foods, most importantly carbohydrates, are rated in comparison to glucose.
Foods with GI indices of greater than 70 are high GI, those with indices of 55% or less are low GI and medium GI for foods that fall in between. See GI charts.
This simple index has proven to be a robust measure for a large variety of nutrients which are now tested in a laboratory with a standardized methodology.
The glycemic index - a short history
Jenkins and Wolever, of the University of Toronto, Canada, first asked whether all carbohydrates were created equal. They did the nutritional experiments and found, to the surprise of most physicians at the time, that the answer was decidedly “No”. Depending on composition and method of preparation, isocaloric (i.e. with the same number of calories) carbohydrate servings raised blood sugar (and insulin, as found later) to different measure. A Glycemic Index (GI) was thus derived based on the rise in blood sugar after ingestion of carbohydrate.
At almost the same time as the University of Toronto team hit upon the notion of GI, Michel Montignac in France arrived at a cuisine to manage subjects with diabetes and obesity including himself. This cuisine in retrospect was low GI with a French accent. Despite remarkable results among its followers it was some time before “The Montignac method” found favour in the nutritional establishment.
Low GI eating has since been widely adopted across the world. Australia is a bastion of low GI eating and is actually marking food packaging with a GI rating to help consumers make an informed choice.
The GI of some common foods
| FOOD | GLYCEMIC INDEX |
| White, brown or wholemeal bread | 100 |
| Potato, mashed or baked | 100 |
| Pizza, cheese | 60 |
| Pretzel | 60 |
| Basmati rice | 58 |
| Honey | 55 |
| Banana | 55 |
| Oats porridge | 47 |
| Sweet potato | 46 |
| Milk chocolate | 42 |
| Apple | 38 |
| Pasta (cooked al dente) | 40 |
| Pizza, thin , crispy (13% fat) | 30 |
| Cheese | 0 |
| Beef | 0 |
A new way of eating
By following a ‘low GI, healthy fat’ eating regime, specific foods are chosen from each food group that create only a positive reaction in and for your body: low GI carbohydrates and fruits under most circumstances, mono- and polyunsaturated healthy fats, lean protein and fat-free or low-fat dairy.
It sounds simple - and indeed it is.
Eating low GI is the ultimate way forward when it comes to boosting health and energy levels on a day-to day basis, and preventing, controlling and even reversing disease in the long term.
Traditional eating turned on its head
Foods such as jacket potatoes, regular wholemeal bread and cereals, were previously regarded as healthy choices since they are low in fat, and high in energy-giving carbohydrates. These foods are, however, rapidly digested and heighten blood glucose rise, with proportionate increase in serum blood insulin levels. High insulin levels, stimulated by high GI carbohydrate diets, are conducive (in the genetically predisposed) to the epidemic of insulin resistance-related disorders. (Table 1)
The truth is that everything we have learned about nutrition over the years is currently being turned on its head. The traditional food guide pyramid offered the bulk of its diet in the form of carbohydrate (not specifying low GI choices) and fruits, accounting for 60-90% of our diet.
In today’s world of limited energy expenditure, we rarely have such great energy needs.
Low GI eating has turned this traditional food guide pyramid on its head. A clear advantage of low GI eating is a decrease in insulin secretion, with a consequent improvement in insulin sensitivity. Carbohydrate-containing foods now form a much smaller part of the total intake, with vegetables serving as the bulk of the diet. This suits our energy needs better. Besides enabling us to exercise better control over our insulin levels, this regime allows for a higher intake of antioxidants, lowering of serum lipids, pro-coagulant proteins, markers of inflammation and homocysteine levels.
The glycemic index is affected by various factors
The glycemic index of a food can be influenced by its inherent characteristics, its method of preparation and the mix it is in:
- Starch gelatinisation The less gelatinised the starch, the slower the digestion. al dente spaghetti, oatmeal and biscuits have less gelatinised starch.
- Physical entrapment The fibrous coat around beans, seeds and plant cell walls acts as a physical barrier, slowing down the access of enzymes to the starch inside. Examples are pumpernickel and grainy bread, legumes and barley.
- High amylose to amylopectin ratio The more amylose a food contains the less water the starch will absorb and the slower will be its rate of digestion. Basmati rice and legumes contain more amylose than other starches.
- Particle size The smaller the particle size, the easier it is for water and enzymes to penetrate. Finely milled flours such as white, brown and even wholemeal, have high GI values, whereas stone ground flours have larger particles and lower GI values.
- Acidity Acidity in food slows down the gastric emptying rate, thereby slowing the rate at which starch can be digested. Vinegar, lemon and limejuice, some salad dressings, pickled vegetables and sourdough bread are all good examples of this.
- Fat, protein and fibre Any of these food groups eaten either together with the carbohydrate or forming part of the carbohydrate will slow down the rate at which the carbohydrates is broken down. Eating wholemeal pita bread stuffed with tuna and roast vegetables will lower the total GI of the meal, when compared to pita bread on its own which will have a higher GI
- Cooking time Overcooking starches can increase the GI of the starch. For example pasta cooked al dente has a good low GI of 40, which will rise if left to become starchy.
The glycaemic index should not be used in isolation
Some foods, which are not apparently particularly healthful, may have a low GI value. This is because large amounts of fat (and protein) in food slow down gastric emptying, thus giving such foods a relatively low GI rating. High fat foods will therefore tend to have lower GI values than their low fat equivalents. For example potato chips have a lower GI value (54) than potatoes baked without fat (85). However, the potato chips will have a higher caloric as well as hydrogenated fat content.
Foods with high saturated or hydrogenated fat contents should be seen as treat foods and eaten sparingly. The jacket sweet potato with a low fat filling would be the alternative to the regular jacket potato, as it is both low GI and has no added fats. Using good simple algorithms and healthy choices from individual food groups, can be produce nutritious low to moderate calorie, low GI meals.
A criticism often levelled at low glycemic diets is that they may not maintain their efficacy when low GI carbohydrates make up part of a cooked complex mixed meal. This has recently been put to rest in a control study.
It is important to take into account both the glycemic index and glycemic load of a food
The glycemic load (GL) of a food provides a measure of the degree of glycemia and insulin output produced by a serving of the food. GL is calculated by multiplying the GI of a food by the amount of carbohydrate per serving divided by 100.
The GL is greatest for those foods which provide the most carbohydrate; particularly those eaten in large quantities e.g. potatoes, bread and rice. Epidemiological surveys from Harvard University School of Public Heath related the risk of Type 2 Diabetes in women and of cardiovascular disease to both by the GI as well as the GL of the diet consumed emphasizing that both quality and quantity of carbohydrate are of important for health.
A day in the life of low GI eating
Clients are encouraged to eat roughly every 3 hours, comprising 3 meals and 2-3 snacks per day. Follow this link for some sample meal plans.
The logic that propels the low GI eating regime is controlling insulin output that is key to weight control, disease management and sustained energy levels. Thus we have in our power far greater control over our metabolism, weight, and disease prevention and treatment through food.
This winning formula applies not only to adults with insulin resistance, but to all healthy adults who have not yet been plagued by high insulin levels, and are determined to maintain their good health. Low GI eating is in fact recommended for children from the age of 4 years on. In this way, today’s parents will be able to prevent insulin resistance from developing in this generation of children when they reach adulthood.