1 August 2013

GI Update with Prof Jennie Brand-Miller

Prof Jennie Brand-Miller answers your questions. 


I don’t understand how you can bake sweet foods like cakes or desserts that don’t have sugar in them (I'm not talking about artificial sweeteners here). What makes them sweet? Can you explain? 
I can see why it is a confused and confusing area. We have to go back to carbohydrate 101 for this. Sugar is a carbohydrate. So is starch. They are nature’s reserves created by energy from the sun, carbon dioxide and water. The simplest form of carbohydrate is a single-sugar molecule called a monosaccharide (mono meaning one, saccharide meaning sweet).

  •  Glucose is a monosaccharide that occurs in food as glucose itself and also as the building block of starch. Fructose and galactose are also monosaccharides.
If two monosaccharides are joined together, the result is a disaccharide (two single-sugar molecules – di meaning two).
  • Sucrose, or table sugar (refined sucrose from sugar cane), is a disaccharide. Every molecule of sucrose yields one molecule of fructose and one molecule of glucose. Thus 10 grams of sucrose (or 2 teaspoons) yields 5 grams of fructose and 5 grams of glucose. Lactose (glucose + galactose), the sugar in milk, and maltose (glucose + glucose) are also disaccharides. 
As the number of monosaccharides in the chain increases, the carbohydrate becomes less sweet. Rice syrup for example which is less sweet than table sugar is 45% maltose (glucose + glucose), 3% glucose, and 52% maltotriose (a trisaccharide consisting of glucose + glucose + glucose molecules). So, you can see that it’s still chemically very much a ‘sugar’ but it is described as being ‘sugar-free’ by sugar-free/quit-sugar diet advocates because it does not contain any fructose. Maltodextrins are oligosaccharides (oligo meaning a few). They taste only a little sweet.

What about GI? Table sugar which as I have said is refined sucrose has a GI of between 60 and 65. Remember, sucrose is a disaccharide (double sugar) composed of one glucose molecule coupled to one fructose molecule. So, when we consume sucrose, only half of what we’ve eaten is actually glucose; the other half is fructose. While the blood glucose response to glucose is high (GI 100), it is very modest to fructose (GI 19), because fructose is absorbed and taken directly to the liver where it is immediately used as the source of energy. This explains why the blood glucose response to 50 grams of sucrose is approximately half that of 50 grams of corn syrup or maltodextrins – where the molecules are all glucose.

GI testing by an accredited laboratory
North America

Dr Alexandra Jenkins
Glycemic Index Laboratories
20 Victoria Street, Suite 300
Toronto, Ontario M5C 298 Canada
Phone +1 416 861 0506
Email info@gilabs.com
Web www.gilabs.com

Fiona Atkinson
Research Manager, Sydney University Glycemic Index Research Service (SUGiRS)
Human Nutrition Unit, School of Molecular and Microbial Biosciences
Sydney University
NSW 2006 Australia
Phone + 61 2 9351 6018
Fax: + 61 2 9351 6022
Email sugirs@mmb.usyd.edu.au
Web www.glycemicindex.com