The GI of a food is affected by many different factors including:
- The type of sugar - eg fructose, glucose, lactose and maltose all have different GI values
- The type of starch - eg. the proportion of amylose to amylopectin and the gelatinisation of the starch. The more amylose a food contains, the less easily the starch is gelatinised (due to the structure of the glucose molecules) and the slower its rate of digestion will be. For example, Basmati rice and legumes have a higher proportion of amylose and therefore have a lower GI. Wheat flour and short grain rice have a higher proportion of amylopectin, which is more easily digested, and therefore have a higher GI.
- The type of fibre - soluble fibres are more slowly digested than insoluble fibres. Foods high in soluble fibre include apples, oats, and legumes.
- How a food is prepared - processing, cooking, pureeing and grinding all increase the GI of a food
- The presence of fat or protein slows down the rate of stomach emptying and therefore slows down starch digestion, as do acids in food (eg, vinegar and lemon juice)
Unfortunately it is difficult to predict the GI of a food due to the variety of factors above – it really must be tested. However we now know the GI of many common foods and new foods are being tested all the time.
Lower GI foods include traditional rolled oats, barley, freekeh, quinoa, cracked wheat, dense wholegrain breads, legumes, corn, pasta (but not most gluten-free varieties), basmati and doongara rice, many fruits including apples, pears, berries, citrus fruit, peaches and nectarines, and dairy foods like milk and yoghurt.
Higher GI foods include white and wholemeal bread, puffed/flaked cereals and crackers, most varieties of rice and potato, and most gluten-free breads and pastas.
Sydney University do most of the GI testing in Australia and maintain a database of foods which have been tested that can be found at http://glycemicindex.com/foodSearch.php. As Asheligh and the GI Foundation have noted above, the GI sympol can help with identifiying low GI foods when shopping. There is also a Low GI Shoppers Guide which you can find on their website http://glycemicindex.com/publicationsList.php
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