Dr Alan Barclay
Not all refined carbs are high GI.
Despite frequently being used interchangeably in the media and scientific reports ‘refined carbohydrate’ and ‘high GI’ are not the same thing. In fact using them as if they were may have adverse effects on some people’s health as some refined carb foods (like pasta) have a low GI and some ‘whole’ and unprocessed foods like most potatoes have a high one. Simply recommending we eat ‘less-refined foods’ is not enough to help people prevent and manage weight gain or type 2 diabetes.
While refining and/or processing may modify the overall GI of a food, it doesn’t necessarily mean the final product will have a higher GI – it all depends on the GI values of the sugars and starches in the actual foods and how the foods have been refined and/or processed. The following examples illustrate how.
Sugary foods The GI of sugars ranges from 15 for fructose to 105 for maltose – a seven-fold difference! Most fruit-based products (dried and canned fruit and fruit juices) and dairy foods have a low GI, because the predominant carbohydrate in these products (e.g., fructose and lactose/GI = 46) are low GI. On the other hand, most ‘foods/beverages’ like confectionery and soft drinks made primarily from table sugar (or sucrose/GI = 65) will have a medium GI – not a high GI as is often assumed. Of course, unlike fruit and dairy foods, soft drinks and confectionery are occasional TREATS (not everyday foods) even if the balance of sugars were changed to lower their GI, they would still remain treat foods.
Starchy foods Unlike sugary foods, the type of starch contained in a food is not as strong a predictor of its GI for a variety of reasons. There are two main types of starch – amylose and amylopectin, with amylose having a lower GI than amylopectin. So a food with more amylose may have a lower GI than one higher in amylopectin, but this is not always the case. This is partly because the starches in unrefined grains like hulled barley, brown rice or wheat berries are encapsulated by the germ and bran, which when left intact can make the starch - regardless of type - very hard to digest. Of course, this is why we process them to provide us with more digestible forms (e.g. pearl barley, white rice and bulgur) or into flour. We have found that the milling method (e.g., stone grinding versus modern steel roller milling) generally has a more significant effect on the ultimate GI of grain foods. Traditionally stone ground flours retain much more of the germ and bran and have more coarsely ground endosperm (where the starch is found), so the starch is still much harder to digest than modern roller-milled grains. You can see an example of 'traditional' stone milling to make tortillas in this 10-minute movie Maiz made in around 1960 by Roger Sandall in Mexico.
Photo by Sheridan Rogers: www.sheridanrogers.com.au
Breads do not always have a high GI although the main ingredient (flour) is usually a highly refined starch. This is because other aspects of the bread making process affect the GI. Gluten matrices form in flour after it is mixed with water and kneaded into a dough – gluten encapsulates the starch molecules making them harder to digest. In addition, the organic acids that form when the dough rises can lower the pH of the final product – organic acids slow digestion and absorption of carbohydrates, further lowering their GI. This is why authentic sourdough breads made the traditional way (in which lactic acid and propionic acid are produced by the natural fermentation of the starches and sugars by the bacterial starter culture) have a much lower GI (54–58 ) than most sandwich breads (average 71) on supermarket shelves. Both use refined flour, but the method of processing has a significant effect on their GI.
What do we recommend? Reduce your intake of carb-rich foods with ‘added refined sugars’ and ‘added refined starches’, particularly those with a high GI and join us in pushing for a dietary guideline for breads and cereals that looks something like this: ‘Eat plenty of cereals, breads and pastas — preferably wholegrain with a lower glycemic index’.