Glycemic Index
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Most people are aware that consuming large amounts of simple sugars is bad, yet few understand why consumption of starches can also be detrimental to health. Many of us in grade school tried the experiment of chewing a saltine cracker for several minutes – it begins to taste sweet as the starches are broken down into sugar. This experiment reinforces the idea that refined starches should be treated as sugars in the diet.
Beyond "simple" and "complex" carbohydrates: glycemic index/load
Glycemic Index (GI) measures how quickly a consumed carbohydrate affects postprandial serum glucose levels in a specified time, (lower=slower=better).
Glycemic Load (GL) accounts not only for how rapidly a food's carbohydrates are converted to glucose but also the relative amounts of carbohydrate the food contains in an average serving. Generally low GI foods also have a low GL. However some high GI foods, like watermelon, may have a relatively low GL. This is because watermelon is mostly water and contains low absolute carbohydrate content per serving.
Examples of GI/GL
Modified from Foster-Powell K, Holt SHA, Brand-Miller JC. International table of glycemic index and glycemic load values: 2002. Am J Clin Nutr. 2002;76:5–56.
Clinical Significance
High GL diets lead to higher risk of developing Type 2 Diabetes Mellitus (1) and low GL diets are associated with better glycemic control (2). Simply put, a low GL diet can both prevent and treat DM2. Perhaps of even more interest to our patients is how eating a low-GI meal can change our eating habits. High GI foods cause a spike in plasma glucose, leading to a spike in insulin, which ultimately causes plasma glucose levels to fall a few hours after eating. This glucose/insulin rollercoaster can lead to altered energy, hunger and poor food decisions!
Compared with an isocaloric low-GI meal, a high-GI meal decreased plasma glucose, increased hunger, and selectively stimulated brain regions associated with reward and craving in the late postprandial period, which is a time with special significance to eating behavior at the next meal (3).
1. Foster-Powell K, Holt SHA, Brand-Miller JC. International table of glycemic index and glycemic load values: 2002. Am J Clin Nutr. 2002;76:5–56.
2. LaHaye SA, Hollett PM, Vyselaar JR, et al. Comparison between a low glycemic load diet and a Canada Food Guide diet in cardiac rehabilitation patients in Ontario. Can J Cardiol. 2005;21:489–494.
3. Lennerz BS, Alsop DC, Holsen LM, Stern E, Rojas R, Ebbeling CB, Goldstein JM,
Ludwig DS. Effects of dietary glycemic index on brain regions related to reward
and craving in men. Am J Clin Nutr. 2013 Sep;98(3):641-7.