Friday, August 8, 2014

The Glycemic Index ranking of dietary carbohydrates

High carbohydrate foods are the basis of diets around the world. Both the quantity and quality of carbohydrate intake are important in optimal nutrition. In Asia, where rice is a staple food, carbohydrates provide as much as 80% of the fuel in the diet. Carbohydrate makes up more than 50% of the energy in a typical American’s diet. Carbohydrate provides energy for the brain, central nervous system and muscle cells. The sugar glucose is the most important carbohydrate. It is as glucose that dietary carbohydrate is absorbed into blood stream. Normal blood glucose levels are about 80 to 120 mg per 100 ml of blood. This level is regulated primarily by the hormones Insulin and glucagon.  Controlling carbohydrate intake plays a central role in maintaining normal glucose level (normoglycemia). Eating too much carbohydrate at one time can raise blood glucose too high stressing the insulin – producing cells. Eating too little carbohydrate can lead to abnormally low blood sugar (hypoglycemia). The diets high in carbohydrates have been blamed for everything from obesity to diabetes. The reason for this is related to the types and sources of dietary carbohydrates.

The glycemic response

A rise in blood glucose levels can be detected shortly after eating carbohydrate – rich foods. The impact of different foods on the magnitude and duration of the rise in blood glucose after a meal is called glycemic response. A rating system known as the glycemic index (GI) is developed to classify foods according to their relative glycemic response.

The glycemic index (GI)

The glycemic index (also glycaemic index, GI) is a measure of the extent of the change in blood glucose content (glycemic response) following consumption of digestible carbohydrate, relative to a standard such as glucose. The glycemic index was developed by David Jenkins, Thomas Wolever and colleagues at the University of Toronto in 1981. GI ranks the quality of individual carbohydrate – rich foods on a scale of 1-100 by measuring how glucose levels rise in blood after someone eats an amount of that food containing 50grams of carbohydrate.

The GI measurement

The glycemic index is a numerical index that ranks carbohydrates based on their rate of glycemic response. In the first step about 50 grams of available carbohydrates have been consumed and the blood sugar levels are measured over a period of 2 hours. The changes in blood glucose over time are plotted as a curve (glucose response curve). In the second step about 50 grams of two control foods, either white bread or pure glucose have been consumed. Once again blood sugar levels are measured over a period of two hours. Another curve is plotted and the glucose AUCs or “area under the curves” are compared.  The glycemic index is calculated as the area under the glucose curve after the test food is eaten, divided by the corresponding area after the control food is eaten. The value is multiplied by 100 to represent a percentage of the control food. For example, if a person consumes a starchy vegetable like green peas and the glucose AUC is 48%. In this case the GI for green peas would be calculated at 48% of 100 or simply 48. Glycemic index uses a scale of 1 to 100 with high values given to foods that cause the most rapid rise in blood sugar. The glycaemic index is influenced by the amount of carbohydrate, nature of the sugar components, and nature of starch, cooking / food processing and other food components such as fats, proteins, dietary fibres, anti-nutrients and organic acids.

Definition of Glycemic index

For a given food, glycemic index is defined as the incremental area under the glucose response curve 2 hours after consumption of 50 grams of carbohydrate from a test food relative to the same amount of available carbohydrate from a control food (either white bread or pure glucose). It reflects the rate at which the carbohydrate in the food is digested and absorbed into the blood stream.

The Glycemic index ranking system

The glycemic index is a ratio of the blood glucose response to a given food compared to a standard (pure glucose or white bread).Glycemic index uses pure glucose as its control food and rates all other carbohydrates in relation to it. It measures the rate and degree of blood sugar elevation following the consumption of a single food carbohydrate. It provides a numerical, evidence -based index of postprandial (post-meal) glycemic response. The glycemic index’s ranking system is only for carbohydrates and not for fats or proteins.
Carbohydrates are broadly categorized by the glycemic index as high (70 or greater), medium(56 to 69)  and low glycemic (0 to 55) rating system. High glycemic carbohydrates enter the blood stream quickly and readily available for the body. Low glycemic carbohydrates enter the blood stream slowly and help maintain stable blood sugar levels in an extended period of time. Lower glycemic eating  improves overall health, maintain lean body mass and reduces the risk of cardiovascular disease, type 2 diabetes, metabolic syndrome, stroke, depression, chronic kidney disease, formation of gall stones and cancers of the breast, colon, prostate and pancreas.  The glycemic index helps as a useful guide for persons with diabetes to help keep their blood sugar levels in the healthy range. The GI index is also used for weight loss and weight management.

GI classification of food carbohydrates

High GI foods such as white breads, white rice, cornflakes, baked potatoes, chips, rice crackers, muffins doughnuts and popcorn score between 70 and 100. Medium GI foods scoring between 55 and 70 include whole-grain cereals, brown rice, quick oats, some cakes and cookies, table sugar and energy bars.  Low GI foods scoring between 0 and 55 include all vegetables, most fruits, oatmeal, barley, nuts and seeds and most whole-grain products. Eating food with a glycemic index of 75 or above causes irritability, mood swings and excess weight gain. Eating foods with a glycemic index below 60 tends to conserve insulin, energy and moods, balances hormones and adds to our anabolic capacity.  We should select our daily foods with a glycemic index of 60 and below.

The Glycemic load, GL

Walter Willett and colleagues at the Harvard School of Public Health created the concept of glycemic load (GL). Glycemic load combines both quality and quantity of a carbohydrate in one number. It is an excellent way to predict blood glucose values of different types and amounts of food.
Glycemic load = (Glycemic index value X no. of grams of carbohydrates per serving)/100.
The glycemic load of a food is calculated by multiplying the glycemic index by the amount of carbohydrate in grams provided by a food and dividing the total by 100. Dietary glycemic load is the sum of the glycemic loads for all foods consumed in the diet. The average range in the glycemic load is lower than the glycemic index.
High Glycemic load = 20 or more; Medium Glycemic Load = 11 – 19; Low Glycemic load = 10 or less.
A glycemic load of 0-10 is considered low (slow, steady conversion to blood sugar), whereas a high glycemic load is 20 and more (sudden spike in blood glucose, tough on health and body homeostasis).

How to follow glycemic index diet

The glycemic index of foods is of primary importance for keeping blood glucose under control. Our management of blood glucose over time appears to depend on our regular food choice and life style. Try to start the day with a great breakfast of low glycemic index foods. Eat a balanced diet with a variety of nutrient – dense foods every day. Try to choose whole – grain foods over processed foods. Try  to have at least one food from the lower glycemic index list at every meal. Do choose foods that are high in fibre to prolong your digestion time. Do choose a diet rich in fresh vegetables, fruits and beans. Don’t mix too many foods together. Don’t over-eat or take too large portions. Over eating encourages insulin resistance, which leads to weight gain. Choose foods with little added sugar or calorific sweeteners. Be cautious of the types of fats that you choose. Use in moderation of unhealthy items such as salt, caffeine or alcohol. Try your best to make small changes in your dietary habits. Remember even the small changes are still changes.

Benefits of GI awareness

Glycemic index helps anyone making the best choices of low glycemic foods.Knowing the glycemic index foods, one can virtually eliminate the risk of contracting diabetes. Eating   low glycemic foods protects our insulin response from becoming overtaxed, which in turn keeps our heart, brain and other organs healthier. Over consumption of high glycemic index foods encourages the risk of heart disease and strokes. Focusing on food from low glycemic index list will significantly improve one’s heart health. By following the glycemic index, one can choose low glycemic foods which helps to lose weight and reduce the risk of developing diabetes, will also help to lower one’s level of blood cholesterol, which in turn will eliminate the risk of several chronic diseases. The low glycemic index eating is life – style friendly and keeps our sugar and insulin levels on a healthy and constant plateau.

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