The Glycemic Index

The glycemic index is an important number for anyone concerned with the conversion of food to blood sugar (sucrose), for the gylcemic index gives the measured rate of this conversion process. The higher the glycemic index value, the faster the conversion rate to sucrose. There are types of sugar molecules other than sucrose. Glucose, for example, is normally the standard reference for the conversion rate to sucrose, with a value of 100.

Some sample values of the glycemic index for foods are: brown rice, 59; white rice, 88; table sugar, 65; grapefruit, 25; spaghetti, 25 to 45; potato, boiled, 55; potato, baked, 85; and dates, 103. Brown rice has more outer layer intact than white rice, so its lower value is evident. But why would a baked potato have a much higher glycemic index than a boiled potato? And how could the value for dates, or any food, be higher than 100?

Answer

The rate of conversion from one type of molecule to another is a chemical process, with the ratio of surface area to volume for particles in the food being converted being an important factor. Smaller sphere like particles have a higher ratio of surface area (SA) to volume than larger ones. Consequently, since the reactions occur on the surface, material consisting of smaller-diameter spheres convert to sucrose faster than the same material consisting of larger spheres. In fact, for the limiting case of a sphere, the ratio SA/Vol. = 3/R, where R is the radius of the sphere. Physical and chemical processes initiated by the environment occur first at the surface of the particle. In addition, for biological systems, larger particles must be reduced to smaller ones before passing through membranes. So a collection of small particles equal in total mass to one large particle will be reduced to acceptable size faster than the large one because the same amount of chemical solution acts upon a much larger total surface area.

The smaller the particles of ingested food, the faster can be the digestion of the molecules in the intestines because the surface-area-to volume ratio is higher, and the sooner is the uptake into the blood. The higher temperature used for baking the potato makes its particles smaller than for the same potato when boiled at 100°C, so its glycemic index is greater, reflecting its faster uptake into the blood.

Dates contain some maltose, a sugar that is even faster than glucose in its basic conversion to sucrose in the blood, so their glycemic index is above 100.

A popular book The New Glucose Revolution by J. Brand-Miller is available in many libraries and has tables of the glycemic index for numerous foods and some of the recent results from nutrition and diabetes research worldwide. Excerpts from the book and many other resources on the glycemic index and its comparison to the insulin index can be found on the Internet.