The concept of the Glycemic Index (GI) was first developed from research into the control of diabetes carried out at the University of Toronto in the early 1980s. Using the initial research as a springboard, work on this subject has been developed since then to refine the GI system, which has application in the quest for good health for everyone.
What is GI?
GI is a method of comparing the effects of the carbohydrates in the food you eat on blood glucose levels. It is based on the rate at which carbohydrates break down to release glucose into the bloodstream. Foods that break down rapidly into glucose are said to have a high GI and conversely foods that break down slowly and release glucose gradually have a low GI.
The index ranges from 0 to 100, with glucose representing the upper limit of 100. All foods can be divided into the ranges low (0 – 55), medium (56 – 69), or high (70 -100), with a range of examples shown in table xx.
What is GL?
GL or glycemic load is an additional measurement which gives a fuller picture than does the glycemic index alone. A GI value tells you only how rapidly a particular carbohydrate turns into sugar but the GL indicates how much of that carbohydrate is in a serving of a particular food. Therefore the GL of a particular food will vary depending on portion size.
The GL of is calculated based on the quantity (in grams) of the carbohydrate content within the serving, multiplied by its GI and then divided by 100. A GL of under 10 is low, 11 to 19 is medium and 20 or more is high.
To illustrate, watermelon has a (high) GI of 72 and a carbohydrate content of 5%. Therefore the GL calculation of a 100g serving of watermelon is 5 x 72 / 100 = 3.6 (low). The GL of a 120g serving of banana with a GI of 52 (low) and a carbohydrate content of 20% would be 24 x 52 / 100 = 12.48 (medium).
Taking account of both the GI and GL is beneficial: the GL helps you select portion sizes; while GI helps you differentiate between good and bad carbohydrate choices.
What is the connection with Insulin?
The hormone insulin plays a vital role in our metabolic system. It is produced in the pancreas and its main function is to distribute glucose throughout the cells of the body and to facilitate storage of excess glucose, in the form of glycogen, in the liver and muscles.
When low GI foods are eaten, a steady stream of glucose is released into the bloodstream and the body produces a steady flow of insulin to distribute it effectively.
The problems arise with high GI foods. When these are eaten, blood glucose levels surge and the steady flow of insulin is unable to cope. To compensate, the pancreas goes into panic mode, quickly producing a torrent of insulin to deal with it.
In a bid to regain equilibrium, the glucose is rushed to the fat stores instead of being distributed to the cells. Over time, this can damage the metabolic system, leading to obesity and diabetes. Eating a low GI diet plays a major role in promoting health and preventing these insulin related problems.
What are the health benefits of a low GI diet?
There are numerous benefits to choosing to follow a diet rich in low GI foods, particular for those involved in strength training, sports and athletics.
Key benefits include:
So how do I know which foods are low GI?
There are a number of factors that determine the GI of foods.
There are four main types of sugar: glucose, sucrose, lactose and fructose.
Glucose, as outlined above, is the upper benchmark for the Glycemic Index (100) as it is immediately available and raises the blood sugar rapidly. It is often present in energy and sports drinks for this reason. Sucrose is found in processed foods and table sugar and falls within the medium GI range.
Fructose (found in fruit) and lactose (from dairy products) are more complex sugars and most foodstuffs containing these sugars are classified as low GI.
Carbohydrate rich foods are readily converted to glucose by the body and are generally classified as medium or high.
Foods that are high in protein (such as meat, poultry, fish and eggs) or high in fat (including butter, margarine and oils) contain no carbohydrate and do not convert to glucose. Therefore, these foods have a low GI rating.
Amount of Fiber
High fiber foods such as beans, lentils and other legumes are broken down very slowly and therefore the release of glucose is gradual. They are classified as low GI.
Level of acidity
Foods with a high level of acidity such as citrus fruits (citric acid) and dairy products (lactic acid) pass through the system slowly, retarding the rate at which glucose is converted.
Carbohydrates account for the bulk of a well-balanced diet, so the first step is to make sure that your carbohydrates fall into the lower GI ranges.
One of the easiest steps you can take is to replace white bread and white rice with wholegrain and brown varieties. These are richer in fiber and therefore are digested more slowly. Grains like barley, bulgar wheat, millet and buckwheat all have a low GI and are rich in a range of important minerals and B vitamins. These are versatile and quick to prepare and can make tasty alternatives to bread, rice and potatoes to add diversity to your diet.
Pasta is an excellent carbohydrate rich food with a low GI (in most cases). For maximum effect, pasta should be eaten “al dente” because if it is over-cooked the starch converts more readily to glucose and this raises it GI.
Potatoes are a staple carbohydrate rich food but unfortunately they have a high GI, with the exception of new potatoes which are classified as low. Surprisingly, sweet potatoes have a medium GI rating and are more acceptable than the standard.
Breakfast cereals tend to fall into the high GI range because most of them are highly processed with high sugar content. However, high fiber bran flakes and traditional oatmeal make satisfying breakfast options.
Foods which are pure protein (meat, poultry, fish, eggs, fats and oils) contain no carbohydrate and have a low GI.
However many high protein foods also contain carbohydrates and can fall into the medium or high GI ranges. These are mainly processed and refined foods which are best avoided. Many dairy products contain the sugar lactose. However this is converted slowly by the body into glucose and results in products such as cheese, yogurt and milk being categorized as low GI.
Beans, lentils and other legumes are again low on the GI scale because of their high fiber content, making them an ideal source of protein. Nuts and seeds are also high in protein and are rich in fats and oils. Consequently, they have a low GI and make versatile snacks and accompaniments.
Vegetables and Fruit
Most vegetables and fruits fall into the low GI range and are therefore vital elements of the diet.
Starchy vegetables such as parsnips, swede and beetroot, however, fall into the medium GI range with some, like pumpkin, being classified as high and should be eaten in moderation. Most fruit may be classified as low GI, despite the high levels of sugar they contain. The acidity, high fiber content and the dominance of fructose (as opposed to glucose) all contribute to this low rating.
There are though a few exceptions. Fruit juices (which contain little or no fiber) and canned fruit (which has undergone processing), tend to convert more readily to glucose and have a medium to high GI rating.
A few points to bear in mind
The beauty about a low GI diet is that it gives you freedom to eat a wide range of foods to make your eating plan as interesting as possible.
No foods are totally banned from your plan, but the amount of high and medium GI foods must be kept under control. As a rule of thumb, you should eat no more than one portion of a high GI food or two portions of medium GI foods a day. Maintaining blood sugar at an even level is at the heart of the low GI plan. Eating breakfast is an essential element to kick start your metabolism. Without it your mental and physical performance are impaired because you are running on an empty tank.
To maintain optimum levels, you should eat a number of small meals throughout the day instead of two or three larger ones, which impose greater demands on the body.
A low GI diet really can unlock your potential as an athlete or strength trainer. Start eating a low Glycemic Index diet today and enjoy the health and performance benefits right away!