“Glycemic index” is a common indicator of how a food impacts blood sugar, or how quickly blood sugar rises after consumption. High glycemic diets are linked with increased risk of type 2 diabetes, a variety of cancers, and cardiovascular disease. Glycemic index helps classify carbohydrates beyond the categories of simple or complex; simple carbohydrates are sugars or highly refined grains compared to complex carbohydrates, which include unrefined grains, whole grains. This has led to recent trends focused on “low glycemic” foods and sweeteners, like agave syrup or coconut sugar.
The glycemic index (GI) is based from the glycemic impact of glucose, or simple sugar, which has a has variable GI depending on the GI reference list used. GI scores differ across different sources, but always use glucose and white bread as reference points within the index. While this variability in scores has been attributed to methodological differences in the past, it is becoming more clear that GI may be more complicated than researchers once believed.
A recent study at Tufts University found that GI can differ as much as 25% among individuals and 20% within an individual. This study found the same food can have low, moderate, and high glycemic impact, which makes it difficult to create categories that are accurate across individuals. The study used white bread, which is often villainized as a high glycemic food, yet in the study it had moderate to low impact for a number of participants. Additionally, the same foods have variable impact even within the same individual. The results suggest that glycemic index has far less value than was previously believed.
While glycemic index promotes healthy choice in theory, it is not a reliable measure. A senior author of the study suggests that individuals should choose whole foods such as whole grains, fruits, and vegetables, and lean meats. Glycemic index also only observes the impact of carbohydrates in isolation, rather than looking at the effects of a whole meal; individuals eat more than carbohydrates in any given meal, which renders the isolated effects of a single carbohydrate source far less meaningful in real life situations. While advertisements may continue to tout the benefits of low GI food products, research has shown that the benefits are limited.
1: Foster-Powell, K., Holt, S. H., & Brand-Miller, J. C. (2002). International table of glycemic index and glycemic load values: 2002. The American journal of clinical nutrition, 76(1), 5-56.
2: Saris, W. H. M., Astrup, A., Prentice, A. M., Zunft, H. J. F., Formiguera, X., Verboeket-van de Venne, W. P. H. G., … & Vasilaras, T. H. (2000). Randomized controlled trial of changes in dietary carbohydrate/fat ratio and simple vs complex carbohydrates on body weight and blood lipids: the CARMEN study. International journal of obesity, 24(10), 1310.
3: Matthan, N. R., Ausman, L. M., Meng, H., Tighiouart, H., & Lichtenstein, A. H. (2016). Estimating the reliability of glycemic index values and potential sources of methodological and biological variability. The American Journal of Clinical Nutrition, 104(4), 1004-1013.
This post was originally featured here on 03/29/17.