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GLYCEMIC INDEX (GI)

Glycemic index is a method of assessing and classifying the blood glucose response to carbohydrate containing foods. A rating between 0-100 is given by comparing each food’s response to glucose (100). Foods below 50-60 are considered to be low or moderate GI and those above as high GI.

High GI foods are digested quickly and supply glucose more rapidly into the blood stream. However blood glucose levels remain elevated for a short period of time.
Low GI foods take longer to digest and supply a slower more sustained blood glucose response.

FACTORS INFLUENCING THE GI OF FOODS

•    Glucose vs. Fructose content
Fructose is the sugar found naturally in fruit; it produces a low blood glucose response. Glucose however has a GI of 100.
•    Type of starch
The structure is important. Glucose units can be strongly bound together or highly branched. Amylose is tightly bound and has a lower GI, whilst amylopectin is more open and has a higher GI rating.
•    Cooking methods / processing
Milling, grinding etc, breaks cell walls and allows a faster rate of digestion. Cooking also increases digestibility of starch. E.g. rice bubbles have a higher GI than rice.
•    Fibre
Viscous (water-soluble) fibre found in fruits, vegetables and oats lowers the GI response.  Digestion is also slowed by the physical structure of insoluble fibre (bran, whole grains)
•    Fat
Fat lowers the GI of food as it slows gastric emptying but this is not advantageous due to an increased kJ intake and the other disadvantages of high fat diets. This shows that low GI foods are not always the healthiest choice.
•    Acidity
Vinegar / lemon added to carbohydrates will lower their GI response. Similarly  sourdough breads have a lower GI than white bread.

HIGH GI FOODS

Include: glucose, dextrose, corn syrup, brown & white rice, baked potato, white bread, banana, bagels, wafers, rice cakes, crumpets, watermelon, sports drinks, lollies, gnocchi, pineapple, pretzels, puffed crispbread, pumpkin.

Good for:

•    Quick release of glucose during exercise or when blood glucose is low.
•    Replenishment of glycogen levels after strenuous exercise. This is believed to be particularly beneficial in the first 2
      hrs after training.
•    First thing in morning to ensure that liver glycogen stores are replenished after an overnight fast.

LOW-MODERATE GI FOODS

Include: low fat yoghurt, multigrain bread, skim milk, fruit bread, dried apricots, apples, pears, oranges, fructose, basmati & doongara rice, legumes, lentils, fresh fruit generally, soy & linseed bread, sourdough bread, traditional oats, cherries, grapefruit, peaches, soya beans, sweet potato, nuts, lactose (milk sugar)

Good for:

•    Slow continuous release of glucose into the blood (sustained energy).
•    Maintaining even blood sugar levels (energy levels).
•    Higher satiety value (feel fuller longer).
•    Typically higher fibre / less processed foods.
•    Less of a ‘glucose challenge’ and therefore less insulin production - important for diabetics but additionally - high
     insulin levels have been shown to be a risk factor for atherosclerosis.
•    As part of a training diet and pre-event to increase muscle glycogen and enhance endurance capacity.
•    Low GI diets generally show improvements in blood glucose control, decreased triglyceride and cholesterol levels in
     diabetics, as well as the general public.

MIXED MEALS / OTHER CONSIDERATIONS

Most studies have found that the GI of a meal of mixed foods can be predicted from the (weighted) GI of its constituent foods. i.e. Low GI foods eaten together generally result in a low GI meal. High & low GI foods consumed together, result in a moderate GI meal.

A recent variation on the GI concept is that of ‘glycemic load’ (GL) of a food or mixed meal. The GL takes into account both the quality (GI) and the quantity (total amount) of the carbohydrate in a food or mixed meal. For more information, visit the nutritionaustralia.org website.

Protein and fat both slow digestion and can therefore reduce the GI of meals that include them. For example, white bread has a GI of around 70-80, however eaten with peanut butter the ‘meal’ is of moderate/low GI, due to its added fat and protein content.

There has also been a proven ‘carry over’ effect of eating low GI meals. Research shows that by even including 2 low GI meals in the course of the day; a person’s overall blood glucose response to carbohydrates is improved.


Article contributed by:

Australian Fitness Academy
701 Glenhuntly Road
Caulfield South VIC 3162
03 9532 7800
www.fitnesseducation.com.au