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Glycemic Index (GI)
The new carbohydrate-classification
system known as the Glycemic Index rates the carbohydrate quality in foods
according to its immediate effect on blood glucose level.
Thus carbs that break down quickly
into glucose during digestion, causing a rapid rise in glucose levels, have a
High GI value. Those carbs that break down more slowly, are given an
Intermediate or Low GI value.
Invented in 1981 by David Jenkins and
Thomas Wolever of the University of Toronto, as a tool for the treatment of
diabetics who need to maintain stable blood sugar, the Glycemic Index (GI) has
now replaced the older method of classifying carbohydrates according to their
"simple" or "complex" chemical structure.
GI Shows Us Which Foods Are
Best/Worst For Blood Sugar Levels
In order to "use" food, our body
first converts it into glucose. This glucose then enters our bloodstream and can
be used as needed.
We have a safety mechanism to ensure
our glucose level remains relatively balanced. It works like this. If levels
fall too low, the brain makes us feel hungry, so we eat food that is turned into
glucose. If levels rise too high, the brain tells the pancreas to release
insulin into the blood to "mop up" excess glucose.
So far so good. However, the human
body was designed in the Stone Age - long before the existence of modern
super-refined carb-foods. Some of these super-refined carb-foods are converted
into glucose very fast - too fast for comfort. As a result, our blood-glucose
shoots up (called a "sugar-spike"). This fools the body into releasing too much
insulin.
This mops up too much glucose and our
level falls so low that the brain makes us feel hungry again!
Thus, not only do we overeat, but
also we can experience excessively high levels of insulin which can over time
cause health problems.
The glycemic index helps us to
understand which foods are best and worst for controlling our blood glucose
levels.
Details of Glycemic Index (GI)
The GI Scale
The glycemic index uses a scale from
1 to 100, which indicates the rate at which 50 grams of carbohydrate in a
particular food is absorbed into the bloodstream as blood-sugar. The main
reference food (rated 100) is glucose.
GI Rating Categories
The glycemic index divides
carbohydrate foods into three categories:
-
High GI Foods (GI value 70+) -
These foods cause a rapid rise in blood-sugar levels.
-
Intermediate GI Foods (GI value
55-69) - These lead to a medium rise in blood-sugar.
-
Low GI Foods (GI value 54 or less)
- These lead to a slow rise in blood-glucose.
GI Food Testing is Ongoing
Not all foods have been given a GI
value, although most food-types are covered. However, due to the way GI is
measured using volunteer subjects, results can vary, so GI values for some
specific foods are not yet uniformly established.
GI - Diabetes and Weight Control
Although the glycemic index was first
designed to assist
diabetes patients manage their blood-sugar levels, dietitians and weight
experts now use it as a tool to help treat obesity, food cravings and appetite
swings, and improve eating habits.
GI Measurement
Measuring the glycemic index or GI
rating of a food is conducted under strict conditions, although sampling methods
can vary slightly.
-
Portions of GI Food Being Tested
- Typically, 10 or more volunteers are given a serving of the test-food
containing 50 grams of digestible (available) carbohydrate. The actual portion
or serving size of each tested food varies according to how much carbohydrate
it contains, based on official food composition tables.
The smaller the percentage of carbs
in the food, the larger the portion needs to be in order to provide the
standard 50 gram amount of digestible carbs. Thus in practice, if a food
contains a very small proportion of carbohydrate, necessitating a very large
test serving size, the test serving is typically scaled down.
Test Food Results Compared Against
Glucose Results
The volunteer's blood-glucose
reaction to the food being tested is compared with his/her response to 50g of
pure glucose. These comparative tests using pure glucose are typically conducted
several times, on several occasions, to provide a mean response.
-
Low GI Food - These are foods
with a GI rating below 55.
-
Intermediate GI Food - These are
foods with a GI rating between 55 and 70
-
High GI Food - These are foods
with a GI rating above 70
-
Glycemic Load -
Both the type AND quantity of
carbohydrate in our food influence the rise in blood glucose. But the glycemic
index only rates a standard 50 gram serving size of digestible carbohydrate in
a particular food, which may not be appropriate for all foods.
For example, foods whose serving
size contains only a small amount of carbohydrate may in practice be better
for blood sugar control than foods whose normal serving size contains a large
amount of carbs. Therefore, to provide a more meaningful GI-rating system,
researchers at Harvard University invented the term Glycemic Load,
which applies the glycemic index to normal food serving sizes.
GI Values: Determining Factors
As we have seen (How Carbs are
Digested) the speed at which carbohydrate is broken down in the intestinal tract
into simple sugars and ultimately glucose, is usually dependent on the specific
structure of the carb in question (meaning: how easy is it for the digestive
enzymes to gain access to the carbohydrate) but can also be affected by other
factors which interfere with the action of enzymes.
-
The physical characteristics of
the carbohydrate is a major factor in determining its likely GI value.
-
The small particle size of finely
ground flour gives digestive enzymes great surface area to attack and
metabolize the bread. By comparison, stone-ground flour has a larger
particle size and consequently a lower GI value.
-
A "puffed up" or "swollen"
carbohydrate is more rapidly metabolized to glucose and has a higher GI
value than less enlarged carbs. For example, fluffy white bread typically
has a much higher GI value than denser rye breads.
-
A carbohydrate surrounded by
fiber, like that in vegetables, beans, nuts and seeds, is better protected
from rapid attack by digestive enzymes and has a lower GI value than other
carbohydrates.
-
Very refined carbs (eg. many
breakfast cereals) typically have most of their 'natural' fiber and other
indigestible constituents removed. Result? These carbs tend to be rapidly
metabolized into glucose.
-
Starches - An interesting
example of how a carb's chemical structure can affect GI rating is seen in
various starches. Starch comes in two forms, amylase and amyl pectin. Starchy
carbs typically contain a mixture of the two types, but the molecular
structure of amylase is tighter and more compact, thus less easy to breakdown
than amyl pectin, whose structure is more extended and more vulnerable to
attack from digestive enzymes. Thus, carbohydrates like beans, lentils,
basmati rice with a high ratio of amylose-to-amylopectin tend to have lower GI
values.
Other Factors That Influence GI
Rating
-
Presence of Soluble Fiber in the
Gut - Soluble fiber in the stomach or small intestine inhibits interaction
between carbohydrate and the digestive enzymes, whose job is to break down the
carb into glucose. One reason why it's a good idea to include a high fiber
food with every meal. It helps reduce the GI value of the meal.
-
Presence of Fat - The same
applies to fat. Like acid, fat retards stomach emptying and thus glucose
metabolism. This explains why certain candy bars have relatively low GI
values, despite their refined nature and simple physical structures.
Glycemic Load Explained
-
Glycemic Index Rates 50 Gram
Portions of Carbs - As
we have seen (How GI is measured) the glycemic index rates how rapidly a
particular food raises blood-glucose. The test is performed using a
food-serving that contains 50 grams of digestible carbohydrate. While
providing valuable comparative data on the glycemic effects of different
foods, this test is not ideal. Why not? Because it doesn't take into account
the fact that some foods contain a lot more carbohydrate than others.
-
Different Percentages of
Carbohydrate - Take two
foods: assume both have GI values of 80, making them high GI foods. On this
basis, we should eat both relatively sparingly. But suppose one of them
contains 75 percent carbohydrate, whereas the other is only 5 percent
carbohydrate. These foods are now quite different. The 75 percent carb food
should be eaten more sparingly. The glycemic index does not allow for
different carb percentages.
-
Glycemic Load Applies GI to Food
Serving Sizes - In
response to this problem of carbohydrate percentage, researchers at Harvard
University invented the idea of Glycemic Load, which applies the glycemic
index to normal food serving sizes. The glycemic load of a particular food
rates the effect of a normal serving-size of that food on blood glucose
levels.
How Glycemic Load is Calculated
The formula for calculating glycemic
load is simple. Multiply the GI value of a food by the amount of carbohydrate
per serving and divide the result by 100.
Example No 1
Spaghetti has a GI value of 40
A serving (1 cup) contains 52 grams
of carbohydrate.
The glycemic load of spaghetti is:
(40 X 52) divided by 100 = 20.8
Example No 2
An apple has a GI value of 40
A serving (medium size apple)
contains 15 grams of carbohydrate.
The glycemic load of an apple is:
(40 X 15) divided by 100 = 6
Calculating GI Values For Meals
Most of the time we don't eat
servings of individual carbohydrate-type foods: we eat meals with a variety of
foods containing varying amounts of carbs, as well as protein, fat and fiber.
Thus in practice, for optimum blood-glucose management, we need to know the
glycemic value of a meal, rather than simply the GI of individual foods.
How to Measure the Glycemic Index
Value of Meals
To calculate the GI value of a meal,
you need to know two things:
Once you know this data, which is
typically obtainable from most food composition or nutrition tables, the GI
calculation for a meal is simple:
Multiply the GI value of each food by
their percentage of total carbohydrate. Add up the totals, to get the GI value
for the meal.
Calculating GI For a Simple Meal
Suppose you eat 2 slices (2oz) of
toast and a glass (4oz) of milk.
The toast contains about 26g of
carbohydrate; milk about 6g of carbs.
The total carb content is 32g. (81%
from bread; 19% from milk)
The GI of bread is about 70; the GI
of milk is about 28.
Calculation = GI X Percentage of
carb contribution
Toast: 70 X 81% = 57
Milk: 28 X 19% = 5
Total: 57 + 5 = 62
The GI value of the meal is 62.
Calculating GI Value of Meals Not
Always Possible
If we eat meals with non-standard
foods that either do not have a GI rating, or are cooked in ways that have not
been tested for GI effect, it is not possible to calculate a precise GI value
for them. Even so, by using food composition tables and GI rating lists, it is
possible to arrive at ballpark GI values for most meals.
Reduce GI of Your Meal
First of all, remember that GI values
only apply to foods containing carbohydrate. Therefore, if the only carbohydrate
in a meal has a high GI, then the GI of the whole meal is high. For example, if
the only carb in your meal is bread, the GI of that meal will be 70. Therefore,
to reduce the GI value of a meal, you must include lower-GI carbs. Here are some
healthy suggestions for lowering GI value.
How to Lower Glycemic Effect of a
Meal Even Further
As we have seen (What Determines
Glycemic Effect), the presence of certain substances in the stomach and
intestinal tract, during digestion (eg. fiber, acid or fat), can reduce the
speed at which carbs are converted to glucose and thus reduce glycemic effect.
By adding these items to our meal, we can lower it's glycemic impact on our
system.
-
Add Soluble Fiber to Your Meal -
Soluble fiber (eg. from
oats, apples, citrus fruits) in the stomach or small intestine inhibits
interaction between carbohydrate and the digestive enzymes, whose job is to
break down the carb into glucose.
-
Add Food Acids to Your Meal
- Acid (eg. from lemon juice or vinaigrette) in the stomach during digestion
slows down stomach emptying and thus the conversion of carbs to glucose.
-
Add a Little Healthy Oil -
Like acid, fat/oil retards stomach emptying and thus glucose metabolism.
However, all oil is high in calories, while not all oil is healthy. So choose
carefully, and add any oil sparingly. The best fats/oils are uncooked and
unrefined. Choose ones like extra virgin olive oil, wheat germ oil, canola,
flaxseed (linseeds).
Creating Lower GI Meals: Summary
The GI effect of high-GI foods can be
reduced by eating them together with low GI foods in a balanced meal. Adding
soluble fiber and/or food acids also helps lower GI. When using fats or oils,
choose carefully.
Low Carb vs. Low GI
-
Low Carb Diet Plans Vary -
There is considerable variation in the level of carbs permitted by popular
low-carb diet plans. Some, like Atkins Diet, are deliberately ketogenic - they
starve the body of carbohydrate in order to force it to burn fat for fuel. For
severely overweight or obese patients who prefer high protein eating, this
approach might be fine. Even for moderately overweight people, the Atkins
ketogenic method can be highly effective in reducing weight in the short term
and good for motivation. Other low-carb plans, like the Zone Diet or South
Beach, are not ketogenic like Atkins. They permit a wider intake of carbs and
may be regarded as healthier. See Recommended Low Carb Diet
-
All Carbs Are Not Bad For Weight
Loss Health - The low carb diet craze is fuelled by the idea that "all
carbs are bad for weight control". Therefore the less carbs we eat, the
better. But this notion is misleading, even dangerous. The truth is, the
leanest and longest living people in the industrialized world are the Japanese
whose diet is dominated by carbohydrates and is low in fat. High carb foods
like grains, rice, and vegetables are daily staples of the Japanese diet, and
intake of high protein, high fat animal products is minimal. Yet Japan has one
of the lowest rates of obesity, heart disease, cancer and diabetes in the
world. Promoters of the "all carbs are bad for weight loss" viewpoint know
this perfectly well - they know the problem lies in high-GI carbohydrates and
junk food - but this does not get attention-grabbing headlines.
-
Low-GI Diet Plans Tackle the
Real Issue - A low-GI eating plan does not seek to improve your health and
weight by restricting how much carbohydrate you should eat. Instead, it
advises what type of carbohydrate to eat. It recommends lower-GI carbs that
boost health and therefore weight reduction. Unlike the low-carb method, the
low-GI dietary method is backed by a wide range of clinical studies, conducted
all over the world, that demonstrate it is good for long term health.
Summary
A healthy type of low-carb diet is
fine for fast initial weight loss. Rapid weight reduction (even if largely
water) can boost motivation and diet compliance. However, significant
carb-restriction should be short-term.
Most experts consider a low-GI diet
to be healthier than regular low-carb diets, as it delivers all the benefits of
healthy carbohydrate without the blood-glucose problems and other health risks
of high-GI carbs.
The above opinionated
views and information serves to educated and informed consumer . The
information provided herein should not be used during any medical emergency or
for the diagnosis or treatment of any medical condition. .It should not replaced
professional advise and consultation. A licensed physician should be
consulted for diagnosis and treatment of any and all medical conditions
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