Fast sugars, slow sugars: a wrong concept

Starchy foods or white sugar: same speed!

For a long time, it was believed that complex carbohydrates, a priori more difficult to "break" into elementary bricks, penetrated more slowly into the blood than simple carbohydrates. Hence their classification into 2 categories well known in the sports world: slow sugars and fast sugars. However, according to studies conducted in the 1980s, this concept is false: all carbohydrates have the same rate of intestinal absorption. The glycemic index is now the reference used to compare the hyperglycemic power of foods. However, this notion of 2-speed sugars is still mentioned today in many sports dietetics articles, including those advocating the glycemic index. The point in this file.

Fast sugars, slow sugars a wrong concept

From the concept of 2-speed sugars to that glycemic index

Carbohydrates: an efficient but limited fuel

The athlete uses 2 main sources of fuel:

Carbohydrates (formerly called hydrocarbonates) brought by the blood or stored as localized reserves in the muscles (glycogen) and liver. These reserves are limited (2000 to 4000 kcal at most).
The fats stored in fat cells in the form of triglycerides. These reserves are almost inexhaustible. They are mainly used in low to medium intensity efforts.

Glycogen stores, easier to transform into energy that can be used by muscles than fat, are used systematically during a sporting effort. In intense efforts (competition, fast jogging, climbing a mountain pass by bike), they constitute more than 90% of the energy provided. Before embarking on an effort, it is therefore important to replenish them well by eating carbohydrates. But which carbohydrates should be promoted?

"Slow" sugars and "fast" sugars: it was all about structure

Carbohydrates exist in different forms:

Simple carbohydrates (monosaccharides and disaccharides) such as glucose, fructose, sucrose contained in fruits, honey, sugar, most sweet foods. They are composed of one or two glucose molecules and are sweet in flavor.
Complex carbohydrates (polysaccharides) such as starch or cellulose contained in starchy foods (cereals, legumes, vegetables, pasta, rice, etc.). They are composed of hundreds of glucose molecules. They do not have a sweet flavor.

A distinction must also be made between digestible and non-digestible carbohydrates. During digestion, digestible carbohydrates are converted into simple carbohydrates (fructose, glucose and galactose) to be able to pass into the bloodstream. This is the phenomenon of hydrolysis. Indigestible carbohydrates, such as fiber, ferment in the colon. They do not enter the bloodstream but promote intestinal transit and bacterial activity in the colon.

It has long been thought that the size of carbohydrates (simple or complex) consumed during a meal has an influence on the more or less rapid increase in blood sugar, i.e. The level of glucose in the blood. It seemed logical that a complex carbohydrate, a priori longer to reduce into simpler molecules, takes longer to enter the blood than a simple carbohydrate. Carbohydrates were then classified into 2 categories: slow sugars and fast sugars.

This concept was used in particular in the field of sport to indicate the sugars to be preferred according to the desired objective: sugars with a rapid effect during exercise or sugars with a delayed effect for longer outings and for the replenishment of reserves. Who has never heard of the famous "pasta party" in the days before a competition?

But this concept of slow and fast sugars is false. We'll see why below. We must no longer compare carbohydrates, but the foods that contain them and the comparison should no longer be made on the basis of the rate of absorption of carbohydrates but on their ability to increase blood sugar. It is the glycemic index that is used today to make this comparison.

Storage mechanisms: fats or glycogen, make the right choice!

Let's not call a sugar a sugar

For some, eating carbohydrates is synonymous with weight gain. For others, such as the athlete, it is essential! However, there are carbohydrates and carbohydrates. Not all of them have the same effect on our figure or on our sports performance. This is why it is interesting to understand the mechanisms that operate in our body when we consume them, in particular the principle of regulation of blood sugar.

Blood sugar

Blood sugar is the concentration of glucose in the blood. Its normal fasting level is between 0.65 and 1.10 g per liter of blood in a healthy individual. Our body, in normal times, consumes 10 to 15 g of glucose per hour, including 4 g just for the brain. This glucose is converted into energy in the cells. It is important that blood sugar remains constant so that the vital functions of our body are ensured. Hypoglycemia is when blood sugar is too low and hyperglycemia when it is too high.

Insulin and blood sugar: storage mechanisms

Insulin is a hormone secreted by the pancreas in order to keep blood sugar levels constant. After a meal, blood sugar levels rise. Insulin is then released to bring excess carbohydrates into the cells and thus bring blood sugar back to a normal level.

Excess sugars are either used immediately (in case of physical exertion) or stored. Storage is in the form of triglycerides (fats) in fat cells or as glycogen in the liver and muscles. The type of storage depends on the intensity of the peak of insulin secreted:

If blood sugar rises sharply, the pancreas releases a significant or even excessive amount of insulin. Carbohydrates are then stored mainly in the form of triglycerides (fats) in fat cells. Excess insulin causes blood sugar to return to below normal and so-called "reactive" hypoglycemia.
If blood sugar rises slowly, insulin is secreted in moderate amounts. Carbohydrates remain available longer in the blood and are then essentially stored as glycogen in the muscles and liver (glycogenogenesis), at least as long as the reserves are not full.

The type of carbohydrates ingested therefore influences the way they will be stored: in the form of fats (in the case of sugars that strongly modify blood sugar) or muscle and liver reserves (in the case of sugars that do not alter blood sugar). A difference that deserves a close look!

Carbohydrates and sport go hand in hand: fuel first and foremost

When doing sports after consuming carbohydrates, excess sugars in the blood are not stored. They are oxidized in the muscles to produce energy. They thus help muscle reserves.

The effects of insulin are indeed counterbalanced by the secretion of antagonistic hormones (adrenaline, norepinephrine, glucagon) produced to Destock the glycogen reserves necessary for exercise. To save his precious reserves, the athlete therefore has an interest in consuming, during the effort, any sugar that greatly increases blood sugar (gels, exercise drinks, etc.)

Outside of periods of exertion, in a trained athlete, excess carbohydrates in the blood will be more likely to be stored in the form of glycogen reserves while in the sedentary, they will be more easily stored as fat in the adipose tissue. It is therefore easier to allow yourself small deviations when you are a sportsman!

Slow sugars, fast sugars: an attractive but false concept

Simple sugars and complex sugars: same rate of absorption!

For years, it was commonly accepted that complex sugars (polysaccharides) such as starch contained in cereals, tubers, legumes, consisting of hundreds of glucose molecules, were more slowly assimilated than simple sugars (monosaccharides, disaccharides), such as sucrose, fructose or honey consisting of one or two glucose molecules. It seemed logical that a longer time would be necessary for digestive enzymes to break down large carbohydrate molecules into elementary units that could be assimilated by the intestine.

A distinction was made between so-called "slow" sugars and so-called "fast" sugars:

Slow sugars: longer to "break", they would slowly and gradually diffuse glucose in the blood, so would be at the origin of a moderate peak of insulin promoting storage in the form of glycogen in the muscles.
Fast sugars: immediately assimilated, they would quickly increase blood sugar levels, an advantage for the athlete in full effort looking for a supplement of energy. At rest, these sugars would create a spike in insulin promoting storage in the form of fats.

According to this principle, the athlete had to favor "slow" sugars before his exit, in order to fill up with energy, and "fast" sugars during the activity, when energy needs are important, and possibly in the recovery phase just after the effort. In the person looking to lose weight, it was of course necessary to avoid "fast" sugars promoting fat gain.

However, studies conducted as early as the 1970s (R. Spaethe et al., 1972, Crapo et al., 1976) and concretized in the early 1980s (Jentkins, 1981) have shown that this concept of 2-speed sugars is false (1, 2). On the one hand, experience shows that the increase in blood sugar is not directly related to the complexity of sugar. For example, with equal amounts of carbohydrates, fruits, which contain fructose and sucrose, sugars that are quickly digested, produce a lower blood sugar spike than white bread, a complex carbohydrate.

On the other hand, the time elapsed between the ingestion of a food on an empty stomach and the occurrence of the peak of blood glucose (when absorption is maximum) is almost the same regardless of the nature of the carbohydrate it contains (simple or complex): the peak of blood glucose is observed after about 30 minutes and the return to normal blood sugar after about 2 hours (3, 4). In reality, most nutrients, especially carbohydrates, are hydrolyzed in the first meter of intestine. That is, the absorption of food and the passage into the blood does not take hours as was thought.

On the other hand, the variation in blood sugar (peak height, surface of the blood glucose curve), for the same amount of absorbed carbohydrates, differs according to the food and depends on various factors that we will see later. Thus, the variation in blood sugar does not depend just on the type of carbohydrate consumed, but on the food that contains it and various complex parameters (preparation, cooking, amount consumed, etc.). It is this difference that must therefore be considered to categorize sugars, or rather foods, between them.

Finally, the rate of intestinal absorption should not be confused with the overall digestion time: gastric emptying can be slowed down by various factors, including the type of carbohydrates consumed, and thus influence the time of passage of carbohydrates in the blood. We will discuss this point below.

(1) Jenkins D.J.A., WoleverR T.M.S., Taylor R.H., Barker H.M., Fielden H., Baldwin J.M., et al., 1981. Glycemic index of foods: a physiological basis for carbohydrate exchange. American Journal of Clinical Nutrition 34, 362-366.

(2) Jenkins et al. Simple and complex carbohydrate: lack of glycemic difference between glucose and glucose polymers. Journal Clinical Nutrition Gastroenterol. 1987, 2 : 113-116

(3) Wahlqvist ML, Wilmshurst EG, Murton CR * Richardson EN. The effect of chain length on glucose absorption and the related metabolic response. American Journal of Clinical Nutrition 1978; 31:1998-2001.

(4) Schenk S, Davidson CJ, Zderic TW, Byerley LO, Coyle EF, [2003], Different glycemic indexes of breakfast cereals are not due to glucose entry into blood but to glucose removal by tissue, American Journal of Clinical Nutrition 78(4):742-748

A new concept: the hyperglycemic power of food

From this observation, we can note the following 2 essential points:

There are no slow or fast absorption carbohydrates

Since all carbohydrates have more or less the same rate of absorption, we must rather talk about the ability of a food to increase blood sugar levels. "Fast" sugars would correspond more to sugars capable of sharply increasing blood sugar and thus providing for the urgent needs of the athlete in full effort. "Slow" sugars would correspond to sugars producing a low peak in blood sugar, therefore insulin and which are therefore preferred to reinflate its muscle reserves. But the notion of speed should no longer be considered, at least when talking about intestinal absorption.

The hyperglycemic power of a food is not necessarily related to the size of the carbohydrates it contains

The categorization "simple sugar versus complex sugar" that had been done until then to define the ability of a sugar to increase blood sugar more or less quickly is false. The complexity of a carbohydrate alone is not enough to indicate its hyperglycemic power. We must also consider the food and not just the carbohydrate it contains.

This observation leads to a complete review of well-established habits such as the pasta dish taken the day before a competition! White pasta a little overcooked or mashed potatoes have a hyperglycemic power equivalent to white sugar! These dishes can thus cause reactive hypoglycemia! Conversely, a fructose drink does not present these risks.

The storage principle inherent in the insulin peak remains correct, however. It is the categorization of carbohydrates that is not and the fact that it was thought that the diffusion of carbohydrates in the blood differed from one carbohydrate to another.

Professor David Jenkins proposes a new index to classify foods according to their hyperglycemic power: the Glycemic Index (see below).

Glycemic index and glycemic load

As we have seen, the absorption of carbohydrates in the intestine is the same regardless of the type of carbohydrate ingested. It is the evolution of blood sugar that differs according to the food consumed, in particular the proportion of sugars absorbed in the intestine. Glycemic index and load are recent concepts for classifying foods according to their hyperglycemic power. They replace the erroneous concept of slow and fast sugars.

What is the glycemic index?

"Glycemic index and absorption time of a carbohydrate are not related. Carbohydrates, simple or complex, take the same time to enter the bloodstream (about 30 minutes). »

The glycemic index or glycemic index (GI) is an indicator for comparing foods containing carbohydrates. It indicates the hyperglycemic power of a food, i.e. its ability to raise blood sugar levels compared to a reference food (generally glucose in Europe and white bread in the United States). The higher the index, the greater the glycemic response.

The value of the index makes it possible to anticipate how carbohydrates will be stored (fats or glycogen). This is the reason why it is used in weight loss methods. However, it should be used with caution. The GI is not a 100% reliable value because its measurement is influenced by many parameters and differs from one individual to another. Moreover, it alone is not sufficient to determine whether a particular food is good or bad for the intended purpose. The digestion of carbohydrates is a complex phenomenon involving other parameters.

What is glycemic load?

Glycemic load (GC) is a quantitative index that takes into account the amount of carbohydrates present in a normal serving of a food (e.g. a portion of melon). It is the product of the glycemic index by the weight of carbohydrates present in the food.

If we take for example the case of watermelon, this fruit has a relatively high GI, 75, more than that of white sugar. However, a serving of 150 grams provides only 8 grams of sugars (about 5% of the weight) or 32 kilocalories (a little more than a piece of sugar). Not enough to trigger the famous insulin spike and store fat! The glycemic load thus makes it possible to compare standard portions with each other (a slice of melon, a portion of rice, etc.) and also, by adding the load of each portion, to have an idea of the overall glycemic impact of a meal.

The glycemic load is useful to anticipate the effects consequent on the consumption of a food or a meal and limit the risk of weight gain.

Factors slowing down the rise in blood sugar

If the absorption of digestible carbohydrates in the intestine is independent of the type of carbohydrates consumed, it can be slowed down further upstream in the digestion process.

Gastric emptying may delay carbohydrate absorption

Gastric emptying is an important factor in the process of carbohydrate absorption. It may explain why some carbohydrates take more or less time to reach the intestine, therefore in the blood, and thus influence the glycemic index.

For example, our stomach can only evacuate 750 ml to 1 liter of drink per hour. If you drink too much at once, you slow down your abilities. Similarly, the sweeter a drink is, the longer it will take for the stomach to evacuate it. A highly concentrated solution can stay up to 45 minutes in the stomach while pure water is drained in about twenty minutes!

Temperature also plays an important role: a hot drink will take longer to digest than a drink at room temperature.

Other parameters influence the speed of gastric emptying such as the nature of the food (solids, liquids), the outside temperature, the physical exertion at the time of digestion, the state of form. Lipids slow down gastric emptying and delay the appearance of carbohydrates in the intestine.

Carbohydrates partially absorbed by the intestine

The lower increase in blood sugar for foods with a low glycemic index is partly explained by the fact that some of the carbohydrates ingested do not enter the small intestine. Thus, part of the calories ingested is not stored!

Starch, for example, is a complex carbohydrate and the main constituent of our diet. 5 to 10% of the starch consumed through the diet would escape digestion (Cummings, 1983). This residue is referred to as resistant starch. This starch ferments in the colon.

Fiber, by swelling, prevents digestive juices from reducing starch, limiting their penetration into the blood. Foods rich in fiber thus provide fewer calories to the body than they contain.

Fiber itself is an indigestible carbohydrate. They are not absorbed by the intestinal wall and ferment in the colon.

Certain elements, called anti-nutrients, such as tannins, phytates, lectins or saponins, slow down and reduce the digestion of carbohydrates (Trustwell, 1992).

Fructose is one of the exceptions when it comes to the rate of intestinal absorption. This simple carbohydrate is indeed less quickly assimilated by the intestinal mucosa than other carbohydrates (Crapo, Henry). In addition, the absorption capacity of fructose by the intestines is limited to about 50 g per hour (Takuji Fujisawa). The excess ferments in the intestines, causing gas or even diarrhea.

Good to know

High GI does not mean fast sugar!

If the concept of fast and slow sugars is now abandoned in favor of the glycemic index, many authors, including health professionals, still talk about the speed of carbohydrate absorption. Their discourse is to say that there are slow sugars and fast sugars but that it is the glycemic index, and no longer the complexity of sugar, that explains this distinction. The classic mistake is to say that a food of high GI behaves like a fast sugar, that is to say that its action is fast compared to a food of low glycemic index which gradually diffuses glucose. No! As we explained above, the rate of absorption is the same regardless of the type of carbohydrate. In addition, we do not classify a carbohydrate but the food that contains it.

The correct interpretation is to say that with the same amount of carbohydrates, foods of high GI (100 for example) lead to a greater increase in blood sugar than foods of low GI, therefore a greater peak of insulin and thus a more easily carried out storage in the form of fats. Conversely, foods with a low GI promote storage in the form of glycogen. They are to be preferred by the athlete in preparation for a competition or long outings (bike, marathon, etc.).

Pasta, rice, bread, puree: "slow" sugars that are not

Beware of excess sugars!

Eating high glycemic index carbohydrates regularly poses a risk for diabetes

According to a study at Harvard University, drinking more than one soda a day increases the risk of type 2 diabetes.

First, we must forget the notion of "slow" sugars and "fast" sugars: as studies show, all carbohydrates have the same rate of absorption. It should be remembered that foods containing carbohydrates have a more or less hyperglycemic power: they contribute to a more or less strong increase in blood sugar.

Secondly, the fact that a carbohydrate is simple or complex is not enough to indicate its hyperglycemic power.

Thus, white bread or mashed potatoes are complex carbohydrates. Yet, with the same amount of carbohydrates ingested, they have an effect on blood sugar equivalent to that of white sugar!! Conversely, fructose, a simple sugar, has a glycemic index of 20, which is less than that of rice. We will therefore be more likely to make a reactive hypoglycemia and gain fat mass by eating a dish of puree than by absorbing the equivalent of a fructose-sweetened drink!

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Eating sugars can cause... hypoglycemia!

If you consume, in significant quantities, foods with high hyperglycemic power, such as sucrose or even a dish of white pasta or mashed potatoes, blood sugar rises sharply. Our body, in response, then produces a strong surge of insulin. Nothing abnormal so far. But now, the amount of insulin secreted tends to exceed the amount that would have been sufficient to bring blood sugar back to its normal level causing very quickly a depletion of carbohydrates: this is the reactive hypoglycemia.

This phenomenon explains why one falls asleep after a meal loaded with carbohydrates. In the athlete, it is necessary to avoid consuming sugars with a high glycemic index just before departure and favor a drink of the expectation low sweet.

What carbohydrates in the athlete?

The athlete must make sure to recharge his glycogen reserves before his exit and just after. He must also pay particular attention to his diet during the effort to prolong his muscle reserves as much as possible and maintain his optimal performance. The diet must include carbohydrates whose glycemic index will be chosen according to the current stage (preparation, before departure, race, recovery).