Carbohydrates tend to be broken into two main categories; Polysaccharides (complex) and mono- / di- saccharides (simple). The distinction between the two was thought to be quite simple, complex carbohydrates release over time giving a steady supply or glucose whereas simple causes an instantaneous release followed by a “dip” or “drop” in blood sugar. But it’s not really that simple, you see a lot of trainers cut out pasta, rice and grains but push foods like baked potatoes, which are of course a complex carbohydrate source, BUT they cause an instantaneous impact on blood sugar, so how does this complex/simple system work? Truth is it doesn’t, because biochemistry is not that simple.
Enter the glycaemic index. The GI system changes the way we view CHO (Abbreviation for carbon, hydrogen and oxygen basic building blocks of carbs and easier for me to type from now on) and how our bodies use them. For years, complex CHO have been given the title of “good” and simple as “bad” but that is a vast oversimplification.
So, what is the glycaemic index? It’s is a simple method used to compare the rate of absorption of CHO based foods. It’s established by giving fasted subjects meals on separate occasions consisting of individual foods, to the value of 50g CHO. The control meal was either 50g of glucose or in later studies 50g of CHO from white bread. The blood glucose response to the respective meals was then measured. The GI is calculated by measuring the area under the curve of a graph which is plotted as a function of time against blood glucose levels. This is then compared to the reference graph of either 50g glucose or 50g CHO from white bread. Foods with a GI of between 80 and 100 are classified as ‘high GI’. Foods with a GI of between 60 and 80 are moderate GI, and finally foods below 60 are classified as low GI. These classifications are not fixed and you may find some tables that vary slightly in terms of the classification. It can be seen that with a high GI you get a quick response in blood sugar that peaks around thirty minutes and then quickly falls away. Whereas with the low GI food the response is slower and peaks at about 1 to 1.5 hours and then slowly falls away. Both types of GI food are useful in the diet and the manipulation of the timing of consuming specific GI foods has been widely studied in relation to both health and exercise performance.
What Factors influcence the GI of a meal?
Type of Starch – The type of starch in the CHO will have an impact on the GI. Amylose has a lower GI than amylopectin. This is because it absorbs less water, the molecules are tightly bound, and it has a slower rate of digestion. Want to lower the blood sugar impact of a baked potato? Let it cool down and then eat it cold and it has significantly less impact on your blood sugar.
Viscosity of Fibre – Viscous, soluble fibres transform intestinal contents into gel-like matter that slows down enzymatic activity on the starch and consequently slows down its rate of absorption. Apples (40) and rolled oats (51) have a high fibre content and so fall into this category.
Sugar content – sugar like sucrose are made up of glucose and fructose, these have a GI of 100 and 19 respectively and because of this the GI of sucrose is 60. Examples would be frosted flakes (55) and raisin Bran (61). However maltose has a GI of 105 and is made up of two glucose molecules, both with a GI of 100. Examples of this would be Rice Krispies (82) and Golden Grahams (71).
Fat and Protein Content – The higher the fat and/or protein content the lower the GI. As both these macronutrients slow down gastric emptying (the rate at which a substance empties from the stomach) they lower the GI of a meal.
Food processing – The more you process a food, the more you are likely to breakdown the fibre content this will raise the GI of the food. An example of this type of processing would be old fashioned rolled oats that have a GI of 51, whereas quick, 1-minute oats have a GI of 66. Conversely, some cooking processes will alter the degree of gelatinisation in the food and this will also affect the GI. As the starch molecules swell and soften this will speed up the rate of digestion, this will raise the GI.
The Glycaemic Load
One of the problems with using GI is that it is based upon a classification of a food based on 50g of CHO. This is problematic as most people would think of a food in terms of its portion size. Some researchers and authors argue that because of this limitation the GI lacks practical utility. To overcome this problem the glycaemic load (GL) was introduced. This captures the glucose raising potential of a dietary CHO by simultaneously incorporating both the quantity and the quality of the CHO consumed. The GL is calculated by taking the grams of CHO in a single portion and multiplying these by the GI. It makes the whole system a lot more practical especially for athletes or sporty individuals:
GL= GI/100 x grams CHO in given amount of food.
example. GL of an apple = 40/100 x 15g = 6
There is good agreement for calculating the GL using this method when compared to direct measures of GL. This advocates of the use of GL/GI based diets argue that a dietary prescription based around low GI/GL to foods will decrease the occurrence of hyperglycaemia and hyperinsulinaemia, both of which have been linked to insulin resistance and ultimately diabetes.
Exercise caution when it comes to simply following a low glycaemic diet as it is not necessarily healthy. As has already been shown a high fat content will lower the GI of a meal as will low fibre content. Neither of these scenarios is desirable when prescribing healthy eating advice.
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