Understanding the Glycemic Index

A term commonly used in nutrition discussions about diet is glycemic index. Glycemic index is formally defined as the amount that a food consumed by itself by someone in a fasting state will raise that person’s blood glucose level over a two-hour period of time relative to a reference carbohydrate consumed under the same conditions.  Higher glycemic index foods are simple sugars and highly refined grains because they are quick to digest, pass thru the stomach, and be absorbed into the blood stream from the intestines.

It is generally considered advantageous to health to avoid eating large amounts of simple sugars and refined carbohydrates.   An important reason for this is that, except during vigorous physical exertion, the sudden spike in blood glucose levels that results from high glycemic index foods can lead to an exaggerated rise in insulin levels.  This in turn raises serum triglyceride levels and low-density lipoproteins (the so-called bad cholesterols).  Both of these lipids are associated with increased disease risks.  Additionally, the spike in serum glucose has been associated with increased vascular and organ inflammatory mediators which are thought to contribute to development of atherosclerosis and multiple different organ diseases.

Nearly all studies looking at our dietary patterns over the last several decades have reached the same conclusion: Americans obtain too many of their calories from processed and refined carbohydrate sources, the so-called “bad carbohydrates.”  Restricting these makes sense in any diet for weight loss or health promotion.  When other calories are not consumed in the place of these eliminated food sources, then, by definition, the individuals who are restricting these carbohydrate sources will be achieving a negative energy balance and will lose weight.  This is important to understand:  the stored energy (fat mass) loss that comes from a carbohydrate restriction diet comes from a reduction in total energy intake so the only way to lose fat mass is to consume fewer calories than you burn.  It just so happens that excess high glycemic index foods are a convenient way to do this because they are almost always over-consumed by overweight and obese individuals.

A common observation from patients when they implement a carbohydrate restriction diet is that they experience rapid weight loss.  This is indeed often the case since simple sugars are most often the main source of excess calories in the American diet.  Unfortunately, the rapid loss of large amounts of weight, out of proportion to the actual caloric restriction, that patients often report is not caused by fat loss (what one wants) but by excretion of large amounts of water that ordinarily is bound to stored glycogen.  This water weight loss will re-equilibrate over time, and, in point of fact, carbohydrate-restriction diets have never been shown to be superior to other calorie restricting diets for long-term weight loss.  Therefore, I tell patients that trying to avoid all carbohydrates so they can lose weight is not a productive long-term strategy and misses the most important point that there is a significant difference among the types and sources of carbohydrates.  They should restrict high glycemic index carbohydrates, but fruits, vegetables, and whole grains are not intrinsically concerning, and should, in reality, continue as important energy sources.  Individuals should consume health-promoting whole food carbohydrate sources and avoid the “bad carbohydrates” such as those high in added simple sugars, added high fructose corn syrup, and highly refined grains which all have a high glycemic index.

By Dr. Eric Zacharias

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