Understanding the Glycemic Index: Carbohydrate Digestion & Absorption By Steve Taylor, R.D.

The glycemic index is one of those things, which at least in name, many of us are familiar with. However, when it comes to the actual understanding of its workings, things can get a bit confusing. What is the glycemic index? What factors affect the digestion and absorption rate of carbohydrates? What are the practical applications?

In today’s article, we are going to briefly run through each of these questions, to help gain a better understanding of this popularly talked about scale, as well as what other factors affect the digestion and absorption rate of carbohydrates, in the “real-world”.

What exactly IS the Glycemic Index?

In short, the glycemic index is essentially a ranking system, a scale, which rates different foods based on how fast the body digests and absorbs them, and consequently how quickly they raise levels of glucose in the blood. However, it should be noted that a food’s G.I. value is based on when that food is eaten in isolation, by itself, after an overnight fast.

The scale goes from 0 to 100, with higher numbers given to foods which are digested/absorbed faster, and raise blood sugar levels more rapidly. Pure glucose (or white-bread in some instances) generally serves as the reference comparison food, to which all other foods are compared, and is given a G.I. value of 100.

Most healthcare organizations rank foods as either high, medium, or low G.I. foods, based on their determined value:

Low G.I. Foods = 0-55

Medium G.I. Foods = 56-69

High G.I. Foods = 70+

When consumed in isolation, those foods with a lower G.I. value (the ones which are digested and absorbed at a slower rate, and cause a more gradual rise in blood-sugar), are said to help control appetite and delay hunger (increase satiation), by providing a sustained release of energy.

Theoretically, if all things are equal, those foods with a lower GI rating should improve satiety and delay time to hunger, by providing a prolonged, gradual, uptake of energy (due to their rate of digestion, etc.). However, because satiety is multifaceted, and includes things like palatability, insulin-mediated responses, and psychological factors alongside the physiological ones, foods with a lower GI rating, do not always result in increased satiety, lower hunger levels or less ad lib intake (as indicated in these studies: 1,2,3,4).

Eating a Food in Isolation Vs. As Part of a Meal

While the above does seem logically sound when consuming a food in isolation, what about when it’s consumed alongside other foods as part of a meal? Does this change things?

Yes, it does.

When different foods are eaten together, the digestion and absorption rate of one, affects the others. In the stomach, everything gets mixed together. The body doesn’t separate foods into different compartments saying, “This space is reserved for that apple you just ate, and this space over here, well that’s for the peanut butter.” It all comes together into a big ball called chyme (this paints a lovely picture, doesn’t it?).

What affects the digestion and absorption rate of carbohydrates?

Many factors affect the digestion and absorption rate of carbohydrates. When carbohydrates are ingested alongside protein, fat and/or viscous fiber, their rate of digestion/absorption is decreased. This results in an attenuated peak in plasma glucose levels (5, 6).

Food structure also appears to be of general importance. Factors such as food-processing or cooking, which disrupt the physical structure of carbohydrate-containing foods, will generally increase their plasma glucose responses (5). For example, over-cooking legumes can increase their rate of digestion and absorption, therefore increasing their G.I. value.

Foods which consist of ingredients containing intact physical structures (example: bread made from intact grains vs. one made from grains which have first been ground into flour), have a high resistant starch content, or are enriched with viscous dietary fiber, will result in reduced blood glucose responses (5).

Generally, the more a carbohydrate-containing food is altered from its “natural” state (via cooking, mechanical separation, milling, etc.), the more it’s natural integrity/botanical or physical structure is altered, the more likely it’s G.I value is going to increase.

Quality AND Quantity Matter

Along with the above factors, which describe the quality (the G.I. value) of a carbohydrate-containing food, the total amount, or total quantity of carbohydrate consumed is also important (this is what leads to the creation of a food’s overall glycemic load).

So even if a food which has a high G.I. value is eaten in isolation, if the total amount of that food is small, say for example, 1 Starburst (and subsequently the total carbohydrate amount is low), then this will still result in an overall small glycemic response.

Thinking logically, it makes a lot of sense. Even if someone were to consume a teaspoon of straight glucose, which has the highest G.I. value, if it’s only 1 teaspoon, that’s still only 4 grams of carbs, even if they are rapidly digested.

Practical Applications

So, what the heck does all of this mean from a practical standpoint?

Although the Glycemic Index can serve as a systematic way of ranking carbohydrate-containing foods, based on how fast the body digests and absorbs them, we can’t forget about all the other factors which also influence a food’s digestion and absorption-rate. As mentioned above, this includes when a food is consumed alongside other foods, the other macronutrients present, and the total amount of each food(s) that were consumed. However, just because the glycemic index is a bit more limiting than commonly perceived, doesn’t mean it’s not still valuable in certain scenarios.

There will be times when we want foods to be digested faster (intra-workout carbs, pre-workout nutrition, etc.), and there will also be times when we want foods/a meal to be digested slower (to potentially help control our hunger levels and appetite when dieting). And because of the many factors we talked about which influence the rate of a food/meal’s digestion and absorption rate, we have options.

When focusing on the outcome we want from a meal (for example, to digest relatively quickly and completely, so that we can have a productive training session and don’t feel bloated), we can look at the characteristics above, and combined with our own physiological responses to various foods, put together a meal (or ingest an isolated food source), to match our desired outcome.

There is virtually an unlimited amount of food combinations for each scenario, and taking the above factors into consideration, allows us to create one which we both enjoy, as well as one which provides us with the desired outcome (appetite control, rapid glycogen replenishment, rapid energy, etc.) we are looking for.

I hope you benefited, and enjoyed reading this article. If there is anything you’d like me to write about in the future, feel free to reach out to me at: stevetaylorRD@gmail.com. You can also find me on:

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Resources

1. Aston LM, Stokes CS, Jebb SA. No effect of a diet with a reduced glycaemic index on satiety, energy intake and body weight in overweight and obese women. Int J Obes (Lond) 2008;32:160–165.

2. Niwano, Y.; Adachi, T.; Kashimura, J.; Sakata, T.; Sasaki, H.; Sekine, K.; Yamamoto, S.; Yonekubo, A.; Kimura, S. Is glycemic index of food a feasible predictor of appetite, hunger, and satiety? J. Nutr. Sci. Vitaminol. 2009, 55, 201–207.

3. Makris AP, Borradaile KE, Oliver TL, et al. The individual and combined effects of glycemic index and protein on glycemic response, hunger, and energy intake. Obesity. 2011;19:2365–2373.

4. Raben A. Should obese patients be counselled to follow a low-glycaemic index diet? No. Obes. Rev. 2002;3:245–256. doi: 10.1046/j.1467-789X.2002.00080.x.

5. Bjorck, Inger, et al. “Food properties affecting the digestion and absorption of carbohydrates1’2.” Am J Clin Nutr 59 (1994): 699S-705S.

6. Spiller GA, Jensen CD, Pattison TS, Chuck CS, Whittam JH, Scala J. Effect of protein dose on serum glucose and insulin response to sugars. Am J Clin Nutr. 1987;46:474–480.