Fructose: What is it, Uses, Intake

“Fructose is the sugar from fruit”

No matter who you ask, this is what they are going to tell you. However, is it really that simple?

What is Fructose?

It is a sugar, particularly a monosaccharide, just like glucose or galactose.

These 3 monosaccharides share their molecular structure (C6H12O6) and weight.

However, they have mild structural differences: fructose is the “most distinct” one, which is why its pharmacodynamics is significantly more different from the other two molecules.

For example, it has different transmembrane transports, which is why it has particular features regarding its absorption.

Do you want to find out more about fructose? Then, keep reading!

Food sources of fructose

In nature, it is available in fruit and vegetables.

Although the last ones have very low amounts.

There are fruits with a higher fructose content, mostly dehydrated fruit. For example, figs, apricots and raisins. So, once again, the amount of fructose available in natural products, even fruit, is actually quite low.

Perhaps the best source of glucose from nature is honey. In fact, it contains a 41% of fructose, which is an impressive amount.

However, the main source of fructose in developed countries is:

What is it used for?

Fructose performs a clear function: recharging the hepatic glycogen concentrations.

The body absorbs it with different transporters from the ones used for glucose and galactose.

To put it simply, glucose is absorbed in the apical membrane of the enterocytes SGLT1 pathway while fructose uses the GLUT5 pathway.

This is the reason why combining a ratio 2:1 of glucose and fructose improves the amount of sugar we can absorb in a 51%. Why? Because we are able to take advantage of both transport systems.

This is specially useful for athletes who do exercises that use the glycolytic system. Usually, this requires the intake of carbohydrate and electrolyte drinks in order to improve the sport performance.

According to the ACSM (2016), this is necessary for efforts that last more than 45 minutes.

Figure I. Recommended carbohydrate intake during the workout by the ACSM.

Fructose as Sweetener

It is a sugar that sweetens between a 50 and 80% more than table top sugar.

That is why you will need around half less the amount of sucrose you would have to use in order to sweeten a drink.

Why take Fructose

It is a great carbohydrate, since many athletes have higher levels of carbohydrates in the liver than sedentary people.

Fructose or Glucose, which one is better?

It is not possible to give a completely correct answer to this question.

But it is important to explain what is the role of physical exercise when it comes to assessing the dose of fructose.

Figure II. Most frequent glucose transporter subtypes and their organic location.

The previous image shows the main glucose transporter subtypes and the tissues where they are mostly located.

What about fructose?

Its absorption is limited by the GLUT transporters in the bowel. But we can increase the expression of these transporters by taking more fructose every day. Consequently, we will be able to absorb more and more (Taskinen et al., 2019).

The fructose that is absorbed reaches the blood flow and goes to the liver. This is due to the fact that it reaches the cells through GLUT2,  a monosaccharide transporter without expression in the muscle cells. That is why fructose cannot be absorbed in the muscle cells and ends up being trapped by the liver.

Figure III. Simplified metabolism of glucose and fructose in the liver.

Fructose is the main source to recharge the hepatic glycogen that is lost during the workout. That is the reason why it is a great source of sugar for athletes.

Fructose Intolerance, how can we know?

Fructose intolerance is the inability of our body to absorb this sugar.

Currently, we do not know exactly why it happens. All the theories have failed up until now. The only thing we know is that our body gets used to it as we grow old, unlike what happens with lactose.

Those who suffer this intolerance also experience leaky gut syndrome, gases and diarrhea.

Bibliography

1. Ebert, K., & Witt, H. (2016). Fructose malabsorption. Molecular and Cellular Pediatrics, 3(1), 10.
2. Ferraris, R. P., Choe, J.-Y., & Patel, C. R. (2018). Intestinal Absorption of Fructose. Annual Review of Nutrition, 38, 41–67.
3. Johnson, J. M., & Conforti, F. D. (2003). FRUCTOSE (B. B. T.-E. of F. S. and N. (Second E. Caballero, ed.).
4. Litwack, G. (2018). Chapter 6 – Insulin and Sugars (G. B. T.-H. B. Litwack, ed.).
5. Rao, S. S. C., Attaluri, A., Anderson, L., & Stumbo, P. (2007). Ability of the normal human small intestine to absorb fructose: evaluation by breath testing. Clinical Gastroenterology and Hepatology: The Official Clinical Practice Journal of the American Gastroenterological Association, 5(8), 959–963.
6. Saris, W. H., van Erp-Baart, M. A., Brouns, F., Westerterp, K. R., & ten Hoor, F. (1989). Study on food intake and energy expenditure during extreme sustained exercise: the Tour de France. International Journal of Sports Medicine, 10 Suppl 1, S26-31.
7. Thomas, D. T., Erdman, K. A., & Burke, L. M. (2016). American College of Sports Medicine Joint Position Statement. Nutrition and Athletic Performance. Medicine and Science in Sports and Exercise, 48(3), 543–568.
8. Wilder-Smith, C. H., Li, X., Ho, S. S., Leong, S. M., Wong, R. K., Koay, E. S., & Ferraris, R. P. (2014). Fructose transporters GLUT5 and GLUT2 expression in adult patients with fructose intolerance. United European Gastroenterology Journal, 2(1), 14–21.
9. Zugasti Murillo, A. (2009). Intolerancia alimentaria. Endocrinologia y Nutricion, 56(5), 241–250.

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