Put as simply as possible, the glycemic index is a measure of how a food influences the body’s blood sugar concentration after that food is consumed. The glycemic index was first developed in the 1980s in order to attempt to help patients with Type 2 diabetes achieve better glycemic control. However, since then, it’s been a controversial topic in nutrition — mainly because there has been a poor understanding of it, but also because of a variety of methodology issues. In order to calculate the glycemic index of a food, a subject is given a portion of the food, usually containing 50 grams of carbohydrate, and the blood glucose response is then compared to a reference carbohydrate (usually a glucose solution or white wheat bread).
Then there’s glycemic load, another concept related yet distinct from glycemic index. It was introduced after the glycemic index in the late 1990s and is used to quantify a food’s total glycemic effect compared to its carbohydrate content. Therefore, glycemic load is comprised of the glycemic index (i.e., the way the food affects blood sugar) as well as the total carbohydrate content of that food.
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What Puts GI Into Question
There are multiple problems with the glycemic index, including the ways in which it has been measured in research studies and also how it behaves in the world outside of the lab. The first problem is that glycemic index is measured for a single, specific food in the lab, yet most humans consume foods in combination with other foods. For example, pasta’s glycemic index could be measured in the lab, but you are most likely not going to simply consume that pasta alone for a meal. You’ll probably add a sauce, a protein source, a salad and maybe even breadsticks.
Even though there is a decently reliable correlation between the glycemic index of a food and the blood glucose concentration after eating that food, the GI of a mixed-food meal appears to be 20 to 50 percent lower than the value the glycemic tables predict for a food on its own. The fact is that foods eaten together can change the glycemic index of the meal in multiple ways. For instance, the interactions of the food in the gastrointestinal tract of a human can slow the rate of the glucose absorption, thus reducing the overall glycemic index.
Another example of the significance of a mixed meal is that when proteins are consumed along with the carbohydrate, the protein’s properties can also impact the post-meal blood sugar. Fiber is another potential confounding factor in GI research because high glycemic index products tend to also be low in fiber, while low glycemic index products tend to be higher in fiber. This could mean that the introduction of a higher-fiber diet, rather than a lower-glycemic diet, is responsible for the health benefits and tendencies seen in the research.
Consider that even if the food is “the same food” as the glycemic index value measured, foods can actually vary widely from each other, making it impossible to know the actual GI of the exact item you ate — unless you take that identical food to the lab and have it measured right then and there. This puts into question the clinical and general use of the glycemic index and creates an obstacle for getting unbiased data.
Another issue is that in order to measure GI you have to see how it interacts with blood inside the body of a test subject, which is hardly an analytical tool that is easy to control for outside variables that could be influencing the values. The body’s complexity tampers with GI measurements, so it’s difficult to isolate variables for clear research conclusions. Additionally, the amount of a food sample tested in the lab to determine its glycemic index may not be the amount of the food that is typically eaten at a meal by a consumer, which complicates the ability to deduce whether the body reacts to that amount the same as it would to the amount usually consumed.
As if there is not enough complicating the usability of the index, finding reliable values for the food you’re consuming is not straightforward. The measurements in the literature of the glycemic index are not nearly as precise as once thought, and researchers have interchanged terms in the literature, creating murky conclusions. Still, despite the controversy and problems associated with glycemic index, if it is measured correctly, it is possible to distinguish between high glycemic index and low glycemic index foods with 95 percent certainty.
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When You Should Pay Attention to the Glycemic Index
Keep in mind that most of the randomized controlled trials that have studied the benefits of eating foods with lower GIs and glycemic loads have been short-term studies. That limits the validity that this type of diet is similarly beneficial in the long-term. At the 2015 International Scientific Consensus Summit from the International Carbohydrate Quality Consortium, it was concluded that there are still questions about how applicable GI is for general use. It seems that individuals with diabetes and metabolic syndrome markers (such as higher systolic blood pressure) tend to have the most perks from using a low glycemic index/load diet for weight reduction, control of diabetes and minimizing coronary heart disease risk.
As mentioned, there is mixed evidence about whether or not a low glycemic diet lowers the risk for Type 2 diabetes or other conditions like cardiovascular disease. Plus, there is conflicting evidence about whether or not this type of diet is helpful for weight loss. If you are not diagnosed with Type 2 diabetes, it may be most helpful to focus on recommendations like choosing more fruits and vegetables, increasing soluble fiber, balancing lower glycemic index foods with higher glycemic index foods and choosing fewer processed foods. If you are concerned about your blood sugar control throughout the day, be sure to check in with your registered dietitian nutritionist and doctor for an individualized recommendation for your specific health needs and goals.
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- AND, Evidence Analysis Library: Prediabetes
- AND, Evidence Analysis Library: Adult Weight Management — Low Glycemic Diets
- Nutrition, Metabolism and Cardiovascular Diseases: Glycemic Index, Glycemic Load and Glycemic Response: An International Scientific Consensus Summit from the International Carbohydrate Quality Consortium
- Progress in Retinal and Eye Research: Dietary Hyperglycemia, Glycemic Index and Metabolic Retinal Diseases
- Nutrition Reviews: Informing Food Choices and Health Outcomes by Use of the Dietary Glycemic Index
- IUBMB Life: The Application of the Glycemic Index and Glycemic Load in Weight Loss: A Review of the Clinical Evidence
- Nutrition: A Glance at… Glycemic Index
- Clinical Diabetes: The 3 R’s of Glycemic Index: Recommendations, Research, and the Real World
- Nutrition, Metabolism and Cardiovascular Diseases: Long-term Effects of Low Glycemic Index/Load vs. High Glycemic Index/Load Diets on Parameters of Obesity and Obesity-Associated Risks: A Systematic Review and Meta-Analysis
- Current Diabetes Reports: Use of the Glycemic Index for Weight Loss and Glycemic Control: A Review of Recent Evidence