The glycemic index or GI is a measure of the effects
of carbohydrates (laden foods) on blood sugar levels. Carbohydrates (laden
foods) that break down quickly during digestion and release glucose rapidly
into the bloodstream have by definition a high GI; carbohydrates (laden foods) that
break down more slowly have by definition a low GI.
The concept was developed by Dr. David J. Jenkins and colleagues
in 1980–1981 at the University of Toronto. The purpose of their research was to
find out which foods were most suitable for subjects with Diabetes Miletus.
A lower glycemic index suggests slower rates of digestion and
absorption of the foods' carbohydrates and may also indicate greater extraction
of carbohydrate digestion. A lower glycemic response usually equates to a lower
insulin demand but not always, and may improve long-term blood glucose control.
Glycemic Index
Defined
The glycemic index of a food is defined as the area under the two
hour blood glucose response curve (AUC) following the ingestion of a fixed portion
of carbohydrate (usually 50 g). The AUC of the test food is divided by the AUC
of the standard (glucose) and multiplied by 100. The average GI value is
calculated from data collected in 10 human subjects. Both the standard and test
food must contain an equal amount of available carbohydrate. The result gives a
relative ranking for each tested food in comparison to 50g of pure glucose.
Glycemic Indexes Of Foods
|
Classification
|
GI range
|
Examples
|
|
Low GI
|
55 or less
|
most fruits and vegetables, whole-grain breads, pasta,
legumes/pulses, milk, yogurt, fructose
|
|
Medium GI
|
56–69
|
whole wheat products, basmati rice, sweet potato, table sugar
|
|
High GI
|
70 and above
|
corn flakes, puffed rice, baked potatoes, watermelon,
croissants, white bread, white rice, glucose (100)
|
The glycemic effect of foods depends on a number of factors such
as the type of starch (amylose vs. amylopectin), physical matrix of the food,
fat and protein content of the food and organic acids or their salts content.
Adding vinegar, for example, will lower the GI of a meal. The presence of fat
or soluble dietary fiber can slow the gastric emptying rate, thus lowering the
GI
The glycemic index can be applied only to only foods with reasonable
carbohydrate content, as the test relies on subjects consuming enough of the
test food to yield about 50 g of available carbohydrate. Many fruits and
vegetables contain very little carbohydrate per serving, and the average person
is not likely to eat 50 g of carbohydrate from these foods. Fruits and
vegetables tend to have a low glycemic index and a low glycemic load.
Related Glycemic Indexes
Glycemic load
The
glycemic load (GL) is a ranking system for carbohydrate baring food
portions based on their glycemic index (GI) and the portion size. Glycemic load
or GL combines both the quality and quantity of carbohydrate into one ‘number’.
It is one of the better best ways 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.The
usefulness of glycemic load is based on the idea that a high glycemic index
food consumed in small quantities would give the same plasma glucose effect as
larger quantities of a low glycemic index food. By way of example white rice
has a somewhat high GI, so eating 50g of white rice at one sitting would give a
particular glucose curve in the blood, while 25g would give the same curve but
half the height. Since the peak height is considered by many experts the most
important parameter for diabetes control, multiplying the amount of
carbohydrates in a food serving by the glycemic index gives an idea of how much
effect an actual portion of food has on blood sugar level.
Many
have questioned the value of using glycemic load as a basis for weight loss
programs; however, glycemic load is generally a useful concept in dietary
designs targeting health maintenance. Studies have shown that elevation in
blood sugar and insulin levels may lead to increased diabetes risk. Studies
concluded (repeatedly) that women whose diets had the highest glycemic index
were 21 percent more likely to develop type 2 diabetes than women whose diets
had the lowest glycemic index.
Insulin
index
The Insulin Index is a measure
used to quantify the typical insulin response to various foods. The index is
similar to the Glycemic Index and Glycemic Load, but rather than relying on
blood glucose levels, the Insulin Index is based upon blood insulin levels.
This measure can be more useful than either the Glycemic Index or the Glycemic
Load because certain foods (e.g., lean meats and proteins) cause an insulin
response despite there being no carbohydrates present, and some foods cause a
disproportionate insulin response relative to their carbohydrate load.
Holt
et al. have noted that the glucose and insulin scores of most foods are highly
correlated, but high-protein foods and bakery products that are rich in fat and
refined carbohydrates "elicit insulin responses that were
disproportionately higher than their glycemic responses." They also
conclude that insulin indices may be useful for dietary management and
avoidance of non-insulin-dependent diabetes mellitus and hyperlipidemia.
Explanation
of Index
The insulin index shows how much
insulin is present in a subject’s blood as a result of particular food
consumption, the glucose index shows how much glucose is present in the blood
as a result of a particular food, and the satiety index shows how much a
particular food decreases one's propensity to eat more.
Glucose
(glycemic) and insulin scores were determined by feeding 1000 kilojoules (239
kilocalories) of the food to the participants and recording the area under the
glucose/insulin curve for 120 minutes then dividing by the area under the
glucose/insulin curve for white bread. The result being that all scores are
relative to white bread. The satiety score was determined by comparing how much
food was eaten by participants at a buffet after being fed a fixed number of
calories of a particular food while blindfolded (to ensure food appearance was
not a factor), then dividing that number by the amount eaten by participants
after eating white bread. White bread serves as the baseline of 100. In other
words, foods scoring higher than 100 are more satisfying than white bread and
those under 100 are less satisfying.
Limitations And Criticisms
- The
glycemic index does not take into account other factors other than
glycemic response, such as insulin response, which is measured by the
insulin index and can be more appropriate in representing the effects from
some food contents other than carbohydrates.
- The
glycemic index is significantly altered by the type of food, its ripeness,
processing, the length of storage, cooking methods, and its variety
- The
glycemic response is different from one person to another and even in the
same person from day to day, depending on blood glucose levels, insulin
resistance, and other factors.
- The
number of grams of carbohydrate impacts blood sugar levels more than the
glycemic index.
- Lowering
glycemic index leads to small (momentary) improvements in blood sugar
levels, but consuming fewer total carbohydrates would benefit the blood
glucose profile much more.
- Carbohydrate
impacts glucose levels most profoundly, and two foods with the same
carbohydrate content are generally comparable in their effects on blood
sugar. A food with a low glycemic index may have high carbohydrate content
or vice versa; this can be accounted for with the glycemic load.
- Most
of the values on the glycemic index do not show the impact on glucose
levels after two hours.
- The
GI of foods is determined under experimental conditions after an overnight
fast (with only 10 test subjects), and might not apply to foods consumed
later during the day because glycemic response is strongly influenced by
the composition of the previous meal.
Relevance And Application
The Glycemic Index holds extraordinary relevance to
a diabetic subject (or anyone using exogenous insulin). The ability to
comparatively rate the release rates of dietary glucose holds life saving value
to the clinical diabetic.
For non diabetic and non
insulin using subjects the value of such data is questionable at best. The
Glycemic Index holds almost no valuable info for the non insulin using athlete.
Food volume and meal frequency has a far greater “GI” slowing effect than the
carbohydrate bearing matrix or starch chemistry.
As the above overview
illustrates dietary carbohydrate quantity dictates the total daily plasma
insulin volume. The simple conclusion is that over consumption of carbohydrates
not the comparative GI of said carbohydrate is the larger problem.
