April 15, 2016
All carbohydrates have a glycemic value; this value determines how quickly the sugar in the carbohydrate is released into your blood stream. It gives you a quick rush of energy, excess sugar that isn’t used gets converted into fat and stored in your fat cells.
We get our energy from carbohydrates as it gets converted into glucose and this glucose is transported to our cells via the blood. Insulin is a hormone from the pancreas that allows the glucose from our blood to be transported to the cells, storing it as glycogen. When you eat carbohydrates such as grains and fruit the body breaks this down into glucose. This then raises your blood sugar level, excess glucose gets sent to the liver where the liver turns it into fat.
Carbohydrates that breakdown quickly and are readily absorbed into the blood stream causing a rise in insulin have a higher GI rating. Carbohydrates that breakdown slowly into the blood stream have a lower GI and cause a slow release of insulin. Foods that have a high glucose level and rapidly digested starches have a high GI value, such as, white bread and rice cakes. Foods that have a low glucose level and have slowly digested starch have a low GI value, such as, pulses and beans. Carbohydrates that are also high in fat and protein have a low GI because stomach emptying slows down the release.
Over the years they have found that just using GI as an indicator didn’t take into account the amount of carbohydrate in the food. The GL is the glycemic load, so a food that has a high GI but has a low amount of sugar ends up having a low GL and therefore is now a food you can eat. For example, watermelon has a high GI but a low amount of carbohydrate therefore what was once a food that wasn’t ok to eat now is. The GL is the amount of available carbohydrate in a food multiplied by the GI of the food/100.
There are other factors that influence the release of sugar from food:
The nature of the starch affects the GI as fibre forms gelatinous gels and delays stomach emptying, lowering the GI. Therefore the more fibre the lower the GI, juicing an apple as compared to a whole apple, the whole apple has more fibre. The more you cook a grain the more the cell wall breaks down and improves the speed of digestion, leaving a rapid release of sugar, where as grains that aren’t cooked the cell wall is intact and have a low GI. If the food is cooled starches are formed which lowers the GI. When eaten with protein and fats it affects the rate of gastric (stomach) emptying, therefore, lowering the GI.
Recent studies show that low GI foods have benefits on risk factors for CVD and diabetes; it is due to the rate of glucose entry into the blood stream as well as the duration of the elevation. Classification of the GI value of foods helps consumers make healthy choices. Knowing the GI of foods influences health on diabetes, cancer, sport performance, appetite control and cognitive performance. Other studies also show that introducing protein and fat reduces the GI.
To find out more about how to get healthy and lose weight, get my book The Body Objective.
Mann and Truswell (2007) Essentials of Human Nutrition, oxford, 3rd edition, p20-22
Bjorck, Liljeberg and Ostman (2000) “Low Glycaemic-index foods”, British Journal of Nutrition, 83, Suppl. 1, S149-S155
Brouns F. et al (2005) “Glycaemic index methodology”, Nutrition Research Reviews, 18: 145-171
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