What is the Glycemic Index?
The glycemic index (or GI) is a ranking of carbohydrates on a scale from 0 to 100 according to the extent to which they raise blood sugar (glucose) levels after eating.
Foods with a high GI are those which are rapidly digested, absorbed and metabolised and result in marked fluctuations in blood sugar (glucose) levels.
Low GI carbohydrates – the ones that produce smaller fluctuations in your blood glucose and insulin levels.
What makes blood glucose go up and down?
With respect to the major nutrients in foods and beverages, carbohydrates have the most profound effect on blood glucose and insulin levels, however, protein and fats also have an effect.
But as most people with diabetes know – particularly those who monitor their blood glucose levels regularly – it’s not just the foods and beverages we eat that affect our blood glucose levels. For example, blood glucose levels are often highest in the morning after an overnight fast, and after exercising, our insulin requirements typically decrease. The reasons why are relatively complex, and require a little knowledge of the way our bodies work.
Let’s start with the pancreas. It’s a small tadpole-shaped organ located in the abdomen, tucked behind the stomach. It secretes digestive juices into our gut, which digest the proteins, fats and carbohydrates from foods and drinks into their components (amino acids, fatty acids and sugars, respectively) so that they can be absorbed into our bodies through our intestines. In addition, it releases (secretes) hormones into our blood stream that regulate blood glucose levels. The hormone insulin is secreted by beta-cells, and glucagon is secreted by α-cells that are clumped together in little islands (islets of Langerhans) in the pancreas.
Most insulin is secreted by the pancreas after a meal containing carbohydrate, but it is also released in small amounts in-between meals at what’s known as the basal rate. Interestingly, our brain and nervous system uses about 80 per cent of the glucose utilised by our whole body, but glucose uptake by our brain and nervous system is not regulated by insulin. Also, while insulin stimulates glucose uptake by our muscles, they have other glucose receptors that do not require insulin to work.
Insulin lowers blood glucose levels partly by suppressing the release of glucose from the liver, by increasing glycogen (a kind of starch) synthesis and storage, and by inhibiting glycogen breakdown and the formation of glucose from other sources such as glycerol (from fat), lactate (from the metabolism of fuels like glucose and fructose by the liver and muscles) and amino acids (from proteins) by the process known as gluconeogenesis. It also enhances the uptake of glucose and some amino acids by our muscles and increases protein synthesis. In fat cells, it stimulates the synthesis of fatty acids from other fuels (e.g. glucose and fructose), and prevents the breakdown of fats into fatty acids.
Glucagon is secreted by the pancreas when blood glucose levels start to go below optimal levels, such as after an overnight fast, or during periods of increased energy expenditure (e.g., when exercising), to sufficiently induce a rapid, yet transient, rise in blood glucose. Glucagon raises blood glucose levels by having essentially the opposite effects of insulin by promoting the production (gluconeogenesis) and release (glycogenolysis) of glucose from glycogen stores in the liver and muscles. In fat cells, glucagon causes the breakdown of fats into free fatty acids which are released into the circulation to provide another source of fuel for our bodies.
So, while the foods and drinks we consume have a significant effect on blood glucose levels, they are not the only factors that make our blood glucose go up and down.
What is the difference between glycemic index (GI) and glycemic load (GL)?
Your blood glucose rises and falls when you eat a meal containing carbs. How high it rises and how long it remains high depends on the quality of the carbohydrates (the GI) and the quantity (the serve size). Glycemic load or GL combines both the quality and quantity of carbohydrate in one 'number'. It's the best way to predict blood glucose values of different types and amounts of food.
The formula is: GL = (GI x the amount of carbohydrate) divided by 100.
Let's take a single apple as an example. It has a GI of 40 and it contains 15 grams of carbohydrate. GL = 40 x 15/100 = 6 g
What about a small baked potato? Its GI is 80 and it contains 15 g of carbohydrate. GL = 80 x 15/100 = 12 g
We can predict from this that our potato will have twice the metabolic effect of an apple. You can think of GL as the amount of carbohydrate in a food “adjusted” for its glycemic potency.
Do I need to eat only low GI foods at every meal to see a benefit?
No you don't, because the effect of a low GI food carries over to the next meal, reducing its glycemic impact. This applies to breakfast eaten after a low GI dinner the previous evening or to a lunch eaten after a low GI breakfast. This unexpected beneficial effect is called the "second meal effect". But don't take this too far, however. We recommend that you aim for at least one low GI food per meal.
While you will benefit from eating low GI carbs at each meal, this doesn't have to be at the exclusion of all others. So enjoy baking your own bread or occasional treats. And if you combine high GI bakery products with protein foods and low GI carbs such as fruit or legumes, the overall GI value will be medium.
Why do many high-fibre foods still have a high GI value?
Many people think that if a food is rich in fibre it will automatically be low GI, but that’s not the case at all. To begin with, there’s not just one type of dietary fibre – there are many different kinds and is typically divided into two main categories: soluble fibre and insoluble fibre. In addition, processing makes a big difference to fibre’s digestibility.
Soluble fibre is the one with the reputation for helping to reduce blood cholesterol levels and modulate blood glucose levels – but whether it does so or not depends in part on the amount of food processing and of course how much of it you eat.
It is often thick and jelly-like (viscous) in solution (water) and remains viscous even in the small intestine. What it means is that soluble fibre thickens the mixture of food entering the digestive tract and therefore slows down the time it takes for the fibre to pass through the stomach and small intestine. Essentially soluble fibre makes it hard for the digestive enzymes to move around and do their job, which is why foods with more soluble fibre have low GI values.
Soluble fibres include gums (e.g. agar), fructans (e.g. inulin), mucilages (e.g. psyllium) and pectins and they are found in fruits, vegetables, legumes (beans, peas and lentils) and some cereal grains (oats and barley).
Insoluble fibre is the one that’s often described as “roughage” and is renowned for keeping us regular on the inside.
It is found in all wholegrain cereals and products made from them that retain the outer layer bran (nature’s packaging) of the grain (corn, oats, quinoa, rice, spelt, wheat), and in vegetables, nuts and seeds.
Insoluble fibre is not viscous and doesn’t slow digestion of a food unless it’s acting rather like a fence, delaying access of digestive enzymes to the starch. However, when it is finely milled, the enzymes have free reign, allowing rapid digestion. This explains why high-fibre cereals like Bran Flakes that look so healthy and are full of fibre, actually have a high GI. They are digested in a flash because the production process makes the starch very accessible. It’s the same with most ready-to-eat breakfast cereals you’ll find in the supermarket.
Does the GI increase with serving size? If I eat twice as much, does the GI double?
The GI always remains the same, even if you double the amount of carbohydrate in your meal. This is because the GI is a relative ranking of foods containing the “same amount” of carbohydrate. But if you double the amount of food you eat, you should expect to see a higher blood glucose response – i.e. your glucose levels will reach a higher peak and take longer to return to baseline compared with a smaller serve.
Why does pasta have a low GI?
Pasta has a low GI because of the physical entrapment of ungelatinised starch granules in a sponge-like network of protein (gluten) molecules in the pasta dough. Pasta is unique in this regard. As a result, pastas of any shape and size have a fairly low GI (30 to 60). Asian noodles such as hokkein, udon and rice vermicelli also have low to intermediate GI values.
Pasta should be cooked al dente ('firm to the bite'). And this is the best way to eat pasta - it's not meant to be soft. It should be slightly firm and offer some resistance when you are chewing it. Overcooking boosts the GI. Although most manufacturers specify a cooking time on the packet, don't take their word for it. Start testing about 2-3 minutes before the indicated cooking time is up. But watch that glucose load. While al dente pasta is a low GI choice, eating too much will have a marked effect on your blood glucose. A cup of al dente pasta combined with plenty of mixed vegetables and herbs can turn into three cups of a pasta-based meal and fits easily into any adult's daily diet.
Most breads and potatoes have a high GI. Does this mean I should never eat them?
Potatoes and bread, despite their typically high GI, can play a major role in a higher carb/lower fat diet, even if your goal is to reduce the overall GI. Only about half the carbohydrate needs to be exchanged from high to low GI to derive health benefits. Of course, some types of bread and potatoes have a lower GI and these should be preferred in order to lower the GI as much as possible.
The good news for potato lovers is that a potato salad made the day before, tossed with a vinaigrette dressing and kept in the fridge will have a much lower GI than potatoes served steaming hot from the pot. There are a couple of simple reasons for this. The cold storage increases the potatoes' resistant starch content by more than a third and the acid in the vinaigrette whether you make it with lemon juice, lime juice or vinegar will slow stomach emptying.
This information was taken from http://www.glycemicindex.com/index.php