Absorption of carbohydrates: GLUT2 new apical transporter?

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It is undeniable that carbohydrate intake during exercise enhances exercise performance, especially during endurance events, in a dose-response relationship. However, in order to properly plan the carbohydrate intake during the competition or training session it is indispensable to understand the mechanism of absorption of carbohydrates in the intestine.

We sometimes focus too much on the details and forget about the bases, but the impact that details have on scientific knowledge is usually limited compared to the relevance of having a solid knowledge of the bases of the topic in hand. I strongly believe that scientific knowledge builds up by adding details to the bases, the latter being what supports the whole structure.

This is why, on this occasion, I bring you a brief  and basic piece of knowledge, yet necessary to know about in order to properly adjust the carbohydrate intake during competitions or sport events. Today I want to talk about how carbohydrates are absorbed in the gastrointestinal tract, focusing on the different transporters that allow for their absorption as well as its importance in performance nutrition.

First of all, in order to be oxidized by the active muscles, exogenous carbohydrates must be digested, absorbed by the enterocytes of the intestine, and delivered to the blood.

Digestion of carbohydrates involves their breakdown into monosaccharides – single pieces of glucose, galactose and fructose – from their complex forms – disaccharides and polysaccharides. This is a necessary step for their absorption since only monosaccharides will be absorbed by the intestine, and it takes place from the moment we put them into our mouth, until they reach the large intestine.

Once emptied from the stomach and digested into their monosaccharides, the absorption of carbohydrates will take place in the small intestine, specifically in the duodenum and jejunum. Sugar is absorbed by the enterocytes – also called intestinal absorptive cells – that line the inner surface of the small intestine.

After their absorption, carbohydrates are then delivered to the systemic circulation so that the different tissues of the body can use them as energy fuel.

But how are monosaccharides absorbed by the enterocytes and delivered to the systemic circulation? Here is where different carbohydrate transporters come into play.

Let’s dig into that!

Glucose and galactose uptake

Exogenous glucose and galactose uptake into the enterocyte is coupled with sodium transport by a sodium-dependent active transport mechanism. This is carried out by the sodium-dependent glucose transporter 1 (SGLT1), which transports two sodium ions into the enterocyte for each molecule of glucose or galactose transported.

Therefore, glucose and galactose transport into the enterocyte is driven by a sodium gradient, which is maintained by a Na+/K+-ATPase – also known as sodium/potassium pump – located at the basolateral membrane of the enterocyte. Na+/K+-ATPase uses the energy of one molecule of ATP to export three sodium ions and import one potassium ion. Therefore, indirectly, the absorption of glucose and galactose requires energy from ATP.

Different studies have shown that the rate of glucose transport by SGLT1 peaks at 60 gr/h even if larger amounts of carbohydrates are ingested. However, the vast majority of these studies were performed in the absence of prior nutritional training, also known as “training the gut”, a highly promising strategy developed by Asker Jeukendrup (you can learn more about it here). A proper nutritional training would result in an enhanced SGLT1 expression, which would lead to higher rates of glucose transport by SGLT1.

Fructose uptake

Fructose uptake does not use SGLT1 as glucose and galactose. Instead, fructose uses the glucose transporter 5 (GLUT5), which is not sodium dependent and is highly specific to fructose.

The peak rate of fructose transport by GLUT5 has been shown to be around 30 gr/h. Again, further research should be done after nutritional training since GLUT5 protein content at the apical membrane of enterocytes increases following exposure to high levels of fructose, which would result in higher maximal transport rates.

Carbohydrate delivery to systemic circulation

From the enterocyte to the systemic circulation, carbohydrates must cross the basolateral membrane. All three monosaccharides use the bidirectional, sodium independent glucose transporter GLUT2.

Kinetics studies have shown that the capacity of GLUT2 to transport glucose a concentration gradient is very large.

GLUT2: the new apical transporter?

So far, I’ve talked about what is generally accepted by researchers. There is great evidence and consensus to support the model of carbohydrate absorption just described. However, new breakthroughs add details to old models, and this is what is occurring here.

Current evidence suggests that transport of glucose into the enterocyte via SGLT1 promotes the rapid insertion of GLUT2 into the apical membrane – which faces the lumen of the small intestine – by a signal transduction that involves the sweet taste receptors T1R1 and T1R2, calcium, and the activation of PKC. This GLUT2 recruitment to the apical membrane would enhance glucose absorption from the luminal space of the small intestine.

It is proposed that, when luminal glucose concentration is low, apical GLUT2 is also low and SGLT1 accounts for most glucose absorbed. When luminal glucose concentration increases, glucose transport into the enterocyte by SGLT1 induces recruitment of GLUT2 to the apical membrane, providing an additional mechanism of glucose uptake, thus increasing the capacity of carbohydrate absorption.

However, this theory of additional facilitated glucose transport remains controversial and further research must be carried out in order to confirm it, as well as to identify the full molecular mechanism of GLUT2 trafficking.

New insights towards higher carbohydrate recommendations

I love talking about all this technical stuff. However, it is pointless to talk about it if we don’t bring it into practice. The real-life application is what makes knowledge useful. So, why are all these glucose transporters important in the sports nutrition field?

Before we begin with this section, let me remind you that there is plenty of evidence supporting a positive relationship between carbohydrate intake and exercise performance, in a dose-response manner. You can see a blog about the benefits of carbohydrates during exercise here.

Once we know that, let’s see the real-life applications of everything explained above.

First of all, we’ve talked about the rates of both glucose and fructose absorption by SGLT1 and GLUT5, respectively – 60 gr/h for SGLT1 and 30 gr/h for GLUT5. Therefore, if we intake beverages or food with only glucose, our maximal rate of exogenous carbohydrate supply to the working muscles will be 60 gr/h. However, if we want to exceed this limit, adding fructose to a glucose- or maltodextrin-rich drink will enhance the amount of carbohydrate absorbed by engaging the fructose-specific transporter GLUT5 – we will get an extra 30 gr/h. In other words, by using multiple transportable carbohydrates, we can enhance exogenous carbohydrate delivery to the working muscles.

Secondly, the fact that GLUT2 may translocate to the apical membrane and provide an additional carbohydrate diffusion mechanism opens new doors towards higher carbohydrate recommendations that exceed the current 90 gr/h proposed by Asker Jeukendrup, based on the multiple carbohydrate transporters present in the apical membrane of the enterocytes.

And last but not least, the fact that different studies have shown an upregulation of the expression and presence of these glucose transporters after exposure to high-carbohydrate diets – also called “nutritional training” or “train the gut” – means that efforts can be made towards increasing the amount of carbohydrate intake over the 90 gr/h. In fact, many professional athletes, such as Chris Froome, Eluid Kipchoge, etc. are working on this and getting amazingly promising results! It looks like the future of sports nutrition will point to carbohydrate intakes beyond currently inconceivable amounts.

Wrap up

An important step for delivering exogenous carbohydrates to the working muscles for oxidation is their absorption from the intestinal lumen. Different monosaccharides are absorbed by the enterocyte via different transporters. While glucose and galactose are taken up via SGLT1, fructose is taken up via GLUT5. From the enterocyte to the systemic circulation, then, all three monosaccharides are transported via GLUT2.

On a practical level, this help us understand that by using multiple transportable carbohydrates we can maximize the amount of carbohydrates delivered to the active muscles, thus supplying our body with more efficient fuels that help us optimize the athlete’s performance.

Also, GLUT2 has recently been shown to be recruited to the apical membrane of the enterocyte when luminal carbohydrate levels are high, thus providing an additional transport mechanism that opens new doors for increasing the current carbohydrate recommendations of 90 gr/h.

Apart from that, it has been shown an increase in glucose transporters expression and content at the apical membrane after exposure to high-carbohydrate diets, which suggests that a “nutritional training” strategy may be beneficial for increasing the ability of carbohydrate absorption and, therefore, enhance the rate at which exogenous carbohydrates can be delivered to the active muscles, which will ultimately improve exercise performance.

References

  1. Kellett, George L, Edith Brot-laroche, Oliver J Mace, and Armelle Leturque. 2008. “Sugar Absorption in the Intestine: The Role of GLUT2.” Annual Review of Nutrition 28: 35–54.
  2. Jeukendrup, Asker E. 2017. “Training the Gut for Athletes.” Sports Medicine 47 (s1): 101–10.

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