Forget About the Glycemic Index

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The glycemic index is a measure of the expected rise in blood sugar levels after consuming a specific food.

It is measured by comparing it to a reference value, which is usually pure glucose, or, much less commonly, white bread. The glycemic index (GI) is expressed on a scale starting from 0, with 100 representing the increase in blood glucose levels over a 2-hour period after consuming 50 grams of glucose. It is considered low if it ranges between 1–55, moderate between 56–69, and high if it is above 70.

Although it was developed to help diabetics regulate blood glucose levels, the glycemic index has been linked to various health effects in healthy individuals, such as increased risk of obesity, cardiovascular diseases, and diabetes.

Unjustifiably so.

The glycemic index “hides” the primary characteristics of foods, their nutritional value, such as fiber content. In other words, it’s not the glycemic index that positively affects health, but the nutritional quality of the food itself.

Even if the health effects were due to differences in the body’s glycemic response, in the following text I will explain why the glycemic index is neither a realistic nor an applicable measure, and why we should not base our dietary decisions on it – whether we are healthy or living with diabetes.

A critique of the glycemic index

Its functionality is limited to strictly controlled laboratory conditions, as several real-life factors significantly affect its accuracy and usefulness:

  1. First of all, it’s not the same whether we consume 1 or 100 grams of carbohydrates. Or 50 vs. 100 grams. As a reminder, GI is measured exclusively after ingesting 50 g of carbohydrates from a single food. The body’s glycemic response will differ significantly depending on the quantity of carbohydrates consumed. Let’s not forget: The dose makes the poison. Or the medicine.
  2. Secondly, a major issue with GI is its poor reproducibility. If we measure the GI of several apples, we may get different results depending on whether the apples are of the same variety, grown in the same climate, same soil, or even from the same tree. These compositional differences will affect their glycemic index (1).
  3. Moreover, the glycemic response to the same food can vary between individuals by up to 30% (2), and GI also shows significant variability within the same individual—even under strictly controlled laboratory conditions (3).
  4. How often do you eat a single food on its own? Namely, the addition of other foods and their nutrients significantly alters your glycemic response. Adding protein, fat, and fiber to a meal will significantly lower its GI. The method of preparation can also drastically change the glycemic response. In other words, the GI tells us nothing about how an entire meal affects glycemic response.
  5. The GI is calculated after a 10–14 hour overnight fast (4). This means it is only applicable once a day—in the morning.

Without much doubt, we can conclude that the glycemic index is an imprecise measure that is unusable outside of laboratory conditions.

There is a measure derived from the GI: glycemic load (GL). Glycemic load is more realistic and applicable than the gylcemic index. Unlike the GI, it is not fixed and takes into account the amount of carbohydrates consumed. However, it only solves one of the five major problems with the gylcemic index, which still leaves it impractical.
Regardless, the GI is still far more widely used—both among laymen and scientists.

The glycemic index as a measure of food quality

As I mentioned in the introduction, the glycemic index is often referenced in discussions about food quality, and even the quality of the overall diet.

(Unfortunately), a considerable number of studies suggest the GI has value as a measure of food and dietary quality. But as you may know (do you?), we often cannot draw causal conclusions from many scientific studies. And this applies here as well.

Looking at epidemiological and observational studies, we can see a positive correlation between GI and the development of certain diseases (5). However, such studies—and their interpretations—often lack depth and critical analysis (6). Furthermore, these findings are in contrast with results from randomized controlled trials, which conclude that it is highly unlikely that the GI of individual foods or an overall diet is directly linked to disease risk (7–9).

There is no evidence that a low-GI diet has positive effects on blood lipids, blood pressure, or insulin resistance (10,11). Similarly, research does not support the claim that a low-GI diet reduces feelings of hunger (8). Nor will choosing low-GI foods help you lose extra weight (12).

Many scientists agree that most of the benefits attributed to the GI or GL are actually due to the fiber content of the food (10).

Generally speaking, foods with a lower glycemic index are those with a higher content of fat, fiber, and/or protein. Each of these nutrients slows down the body’s glycemic response, thereby lowering the GI.

For example, olive oil has a GI of 0. But it’s oil; it contains no carbohydrates. Trans fats also have a GI of 0. Does that mean we can consume them without concern in exuberant amounts?

Ice cream has a low GI. Does that mean it’s a “healthy” food that we should eat as much as possible?

Carrots have a GI of 70, the same as watermelon. Does that mean we should avoid them?

Potato chips have a lower GI than boiled potatoes, due to their oil content. Does that make them the better choice?

(All of these questions are rhetorical and imply a negative answer.)

Foods with a high glycemic index are not necessarily low-quality foods, nor are low-GI foods inherently high-quality. A high GI can “hide” nutritionally valuable foods, while a low GI can mislead us into increasing consumption of low-nutrient foods.
The glycemic index is not a suitable measure of food quality, nor should it be used as the foundation for our dietary choices.

Life After the Glycemic Index

Focusing on the GI of individual foods is not only misdirected, but it can actually have a negative impact on diet quality. Given the limited focus capacity of Homo sapiens, choosing foods based solely on their lower glycemic index could lead us to neglect the overall quality of food and nutrition, which do affect health.

So what should replace the glycemic index? Nothing. There is no single objective measure that can accurately signal the quality of a specific food. A healthy diet is the result of combining diverse nutrients from a variety of foods, each contributing in its own way to the overall quality of what we eat.

Still unsure how to move forward without the glycemic index?
Send a message to our nutritionists. You’ll get a response in no time.

References

  1. Foster-Powell K, Holt SHA, Brand-Miller JC. International table of gylcemic index and glycemic load values: 2002. Am J Clin Nutr. 2002;76(1):5-56. doi:10.1093/ajcn/76.1.5
  2. Vega-López S, Ausman LM, Griffith JL, Lichtenstein AH. Interindividual variability and intra-individual reproducibility of glycemic index values for commercial white bread. Diabetes Care. 2007;30(6):1412-1417. doi:10.2337/dc06-1598
  3. Whelan WJ, Hollar D, Agatston A, Dodson HJ, Tahal DS. The glycemic response is a personal attribute. IUBMB Life. 2010;62(8):637-641. doi:10.1002/iub.365
  4. Brouns F, Bjorck I, Frayn KN, et al. Glycaemic index methodology. Nutr Res Rev. 2005;18(01):145. doi:10.1079/NRR2005100
  5. Barclay AW, Petocz P, McMillan-Price J, et al. Glycemic index, glycemic load, and chronic disease risk – A metaanalysis of observational studies. Am J Clin Nutr. 2008;87(3):627-637. doi:10.1093/ajcn/87.3.627
  6. Tuomainen TP, Mursu J, Voutilainen S. Bold conclusions from inadequate evidence. Am J Clin Nutr. 2008;88(2):477-478. doi:10.1093/ajcn/88.2.477
  7. Vega-López S, Mayol-Kreiser SN. Use of the glycemic index for weight loss and glycemic control: A review of recent evidence. Curr Diab Rep. 2009;9(5):379-388. doi:10.1007/s11892-009-0059-9
  8. Vega-López S, Venn B, Slavin J. Relevance of the Glycemic Index and Glycemic Load for Body Weight, Diabetes, and Cardiovascular Disease. Nutrients. 2018;10(10):1361. doi:10.3390/nu10101361
  9. Kristo AS, Matthan NR, Lichtenstein AH. Effect of diets differing in glycemic index and glycemic load on cardiovascular risk factors: Review of randomized controlled-feeding trials. Nutrients. 2013;5(4):1071-1080. doi:10.3390/nu5041071
  10. Sacks FM, Carey VJ, Anderson CAM, et al. Effects of high vs low glycemic index of dietary carbohydrate on cardiovascular disease risk factors and insulin sensitivity: The OmniCarb randomized clinical trial. JAMA – J Am Med Assoc. 2014;312(23):2531-2541. doi:10.1001/jama.2014.16658
  11. Clar C, Al-Khudairy L, Loveman E, et al. Low glycaemic index diets for the prevention of cardiovascular disease. Cochrane Database Syst Rev. 2017;2017(7). doi:10.1002/14651858.CD004467.pub3
  12. Das SK, Gilhooly CH, Golden JK, et al. Long-term effects of 2 energy-restricted diets differing in glycemic load on dietary adherence, body composition, and metabolism in CALERIE: A 1-y randomized controlled trial. Am J Clin Nutr. 2007;85(4):1023-1030. doi:10.1093/ajcn/85.4.1023