GLYCEMIC INDEX: Factors and Examples of Foods that Influence GI Values
By Dr. Keith E. Lewis
June 26, 2008
There are multiple factors that influence the GI value of food. Primarily, the physical state of the carbohydrate or the starch in that food is the most important factor influencing GI value. Over the years, our foods have become more and more processed. Foods that have become more and more processed over time have developed higher and higher GI values which directly affect blood glucose levels.
The following are factors that influence the glycemic index value of a food:
1) Viscosity of fiber. Viscous soluble fibers increase the viscosity of the intestinal contents which slows down the interaction between starch and enzymes. Because of this increased viscosity, there is slower absorption of glucose and conversation of glucose from carbohydrate in the blood stream. Examples of foods that are soluble fibers will include any oat product, especially raw oats and rolled oats. Lentils, apples, and beans are all good examples.
2) Particle size. The smaller the particle size, the easier it is for water and enzymes to penetrate. Finely milled flours have a high GI value. Stone-ground flours have larger particles and have a lower glycemic index. So again, the finer and the smaller of the particle, the quicker the rate of absorption, the higher their glycemic value.
3) High amylose and amylopectin ratios. The more amylose a food contains, less water the starch will absorb, and the lower its rate of digestion. Legumes, basmati rice contain amylose which again alters the rate of digestion which results in lower glycemic index.
4) Physical entrapment. The fiber vs coating around beans and seeds and the actual plant cell walls act as a physical barrier which slows down access of enzymes to starch inside. Examples would be grainy breads, legumes, barleys, and pumpernickel. All these, because of the cell structure of the cell wall reducing and slowing the digestive process also lower the GI index.
5) Starch gelatinization. The higher the gelatinization or swollen the starch is, the slower the rate of digestion. This is a good example of spaghetti that is cooked al dente, not overcooked, the result being slower rate of digestion, less glycemic index value.
6) Sugar. The digestion of sugar produces only half as many glucose molecules as the same amount of starch. The other half is fructose. The presence of sugar also restricts gelatinization of the starch by binding water and reducing the amount of available water. We find this in tea biscuits and oatmeal cookies. Some breakfast cereals that are high in sugar have a relatively low GI value for this very reason. Again, it revolves around the rate of digestion.
7) Acidity. Acids in foods slow down stomach emptying, thereby slowing the rate at which the starch can be digested. The result, lower GI index. Vegetables that are pickled, some salad dressings, limejuice, lemon juice, vinegar, are good examples of foods that are acidic, that alter digestion.
8) Fat. Fat slows down the rate of stomach emptying thereby slowing the digestion of starch. A good example of this is why potato chips have a lower glycemic index value than boiled white potatoes. Again, the presence of fat slows down digestion resulting in lower glycemic index.
At this point it may be wise to make a bit of clarification concerning sugar and glucose. Although glucose is the standard by which all foods are measured in terms of glycemic index, table sugar which is sucrose has a GI value of only 60 to 65. That is simply because sucrose or table sugar is a double sugar. It is made of one glucose molecule coupled with one fructose molecule. Fructose in our body is absorbed very quickly and is taken directly to the liver where it immediately is oxidized and burnt for energy. The blood glucose response to pure fructose is very small. Fructose has a GI value of 19. Consequently, when we consume sucrose, only half of what we have eaten is actually glucose, the other half is fructose. This explains why the blood glucose response to 50 g of sucrose is approximately half that of 50 g of corn syrup or maltodextrins where the molecules are all glucose.
We often times in our clinic have patients that find the GI index very confusing, especially when it comes to making better food choices. We have available to you, upon request, hundreds and hundreds of foods that have measured GI values as well as glycemic loads.
I would like to take just a moment and talk about some of the different types of foods and how making better choices can certainly help you control or manage your blood sugar levels better.