GU Fuels U: Fructose 101 by by Brent Mann and Bryan Bergman

Fueling is a key component in triathlon and in any endurance event.  There are many different types of nutrition that can be used to fuel your body during long training days and racing.  It can get a little confusing and often time takes trial and error to determine what is going to work the best for you.  The article below can help you in the process of choosing what nutrition is going to work best for  you.  Come on into KONA Multisport to look through your nutritional options and to ask your questions to find out what will be your best source of nutrition for your next race or long workout. 

There are enough types of carbohydrates out there to confuse even the most nutrition-savvy triathlete. Opinions on which is the best fuel vary widely, with fructose getting an especially bad rap. Fructose, however, is an important carbohydrate for endurance nutrition. 

Get to know your carbs

First, let’s take a look at the different types of carbohydrates commonly found in sports nutrition products. 

Glucose, aka: dextrose, is the major form of carbohydrate that circulates in the blood. The liver produces glucose by either degrading stored glycogen or by making it from sources such as lactate, glycerol, and amino acids. When glucose concentration is low, an athlete feels hypoglycemic, which can lead to bonking. Glucose is the major fuel source for high-intensity exercise. Due to its importance in blood glucose regulation, preventing bonking, and delaying glycogen utilization, glucose of some variety is found in most sports drinks on the market.

Maltodextrin is a string of glucose molecules bound together like box cars on a train, with each box car representing a glucose molecule. Maltodextrin allows more calories to be packed into a drink without affecting how well it can be absorbed (osmolarity) compared to the same amount of glucose alone. Also, since the enzymes that chew up maltodextrin can only work on one car at a time, the string of glucose molecules is released at a slower rate than if all the molecules of glucose were consumed as individual molecules.

Fructose is a natural sugar found in fruits. Unlike glucose which has six carbons, fructose only has five. Fructose must be converted into glucose by the liver before it can be used as a fuel by the body. As a result, when fructose is consumed it is a “slower release” carbohydrate. Due to the need to convert fructose to glucose in the liver, studies have reported oxidation rates of fructose lower than those of glucose during exercise when each was consumed individually.

Sucrose is a natural sweetener found in cane sugar, and is commonly known as table sugar. Sucrose is made of a glucose and fructose molecule bound together. The oxidation rate of sucrose is similar to glucose or maltodextrin when consumed in isolation during exercise.

Galactose is another simple sugar which is structurally similar to glucose, and competes with the glucose transporter in the gut for absorption. It is commonly found in dairy products, as a component of the disaccharide lactose, which consists of glucose and galactose combined. Similar to fructose, galactose is also mainly converted to glucose in the liver prior to oxidation by working muscles. The oxidation rate of galactose is approximately 50 perfect as fast as glucose when each is consumed individually during exercise.

Maltose is composed of two glucose molecules bound together. Not surprisingly, it is oxidized at a similar rate as glucose when given alone during exercise.

When you look at the seemingly diverse types of carbohydrates, you are really left with only three basic types of carbohydrates or monosaccharides: glucose, fructose, and galactose. And since galactose is oxidized poorly during exercise, it doesn’t make a lot of sense to include it in a product designed to provide fuel for working muscles. While there are some exceptions (poly-lactate for instance), that leaves glucose and fructose for potential carbohydrate sources during exercise.

Get on the carb-train

The carbs you consume get to where you need them through proteins known as transporters. Glucose and galactose are transported from the gut to the blood by a protein transporter known as the Na+/glucose co-transporter 1 or SGLT1. Like all transport proteins, this transporter can become saturated at high concentrations of glucose, which bottlenecks energy absorption. Basically, there is a limit to how much and how fast glucose can be absorbed and used by working muscles using only one type of carbohydrate transporter. Due to this limitation of transporters, 1g/min used to be the maximal rate of exogenous carbohydrate oxidation which could be achieved during exercise. However, recent research has revealed that there’s a way to increase how much of the carbs we ingest are absorbed and used.

Fructose, a simple sugar, is transported using a totally different transporter (GLUT5). When added to a drink containing maltodextrin (glucose), you’re no longer limited by one carbohydrate transporter. Using more than one transporter allows a greater rate of carbohydrate uptake. It’s like having two exit doors for a croweded room of people to use instead of one. More doors mean more people will be able to exit in the same amount of time. While not a perfect analogy, it is a similar process in the gut, where two types of transporters allow more carbohydrate absorption in the same amount of time.

Recent studies found that the addition of fructose increased total carbohydrate transport and ultimately, oxidation. In one study, peak rates of carbohydrate oxidation using maltodextrin alone were approximately 1.1 g/min, which increased to 1.5 g/min with the addition of fructose. Oxidation rates of exogenous carbohydrate have been found to be as high as 1.7g/min when combining ingestion of glucose, fructose, and sucrose. More carbohydrate oxidation using fructose plus maltodextrin means better sparing of glycogen stores. Additionally, once you have run out of or have very limited muscle glycogen (after 2.5 hours of exercise or so, or less if exercise is performed on back to back days), it also allows an athlete to exercise at a higher percent of VO2max since they are reliant on consumed carbohydrate as a fuel source for intense exercise.

Carbs and performance

We know, oxidation sounds great and all, but how does it affect our race-day performance? In a recent study, subjects exercised at 55 percent of their VO2max for two hours while drinking either water, 1.8g/min glucose, or 1.2g/min glucose plus 0.6 g/min fructose, then completed an exercise time trial performance test. Glucose alone increased time trial power by 10 percent compared to water. However, subjects consuming glucose plus fructose had an eight percent increase in average power during the time trial performance test compared to glucose alone, and a 19 percent improvement compared to water.

Another study had subjects consume 0.6g/min of glucose and three doses of fructose for two hours at 50 percent of VO2max, followed by 10 maximal effort sprints on a stationary bicycle. In this case there were no differences between groups in sprint performance with the addition of fructose to the maltodextrin. However, the authors reported that subjects consuming the additional 0.5 and 0.7g/min of fructose had decreased perceived effort for the same amount of work, and showed more fatigue resistance during the sprints.*

Other benefits to fructose

Gastric emptying (the rate at which the contents of the stomach leave the stomach) appears to be increased after subjects consume equal calories of glucose plus fructose compared to glucose alone or plain water. A similar finding of increased fluid delivery was found in subjects consuming drinks with fructose compared to glucose alone. Both contribute to keeping you better hydrated in your racing and training.

GI distress

A problem for many, gastro-intestinal distress has been attributed to fructose only in fructoseonly beverages. As a result of many studies, it appears that it’s an issue of malabsorption, occuring only when fructose is consumed in excess of glucose. Foods that naturally contain fructose in excess of glucose include honey, and the fruit or fruit juice from apples and pears. 

Individual differences

Your body size doesn’t dictate the maximal rate of glucose you can absorb. What does dictate it is your gut, or, more specifically, the presence of the intestinal glucose transporter (SGLT1). So how can you increase it? Animal studies have shown that a high amount of dietary carbohydrate leads to a greater capacity for glucose transport. Therefore, by eating high carbohydrate diets and simple sugars, you can actually train your body to digest and process more carbohydrate energy. This has obvious advantages for athletes who are trying to consume adequate calories during events–more transporters means lessens the chance of a bad stomach on race day, and can help the body’s ability to absorb and use  more carbohydrates during exercise.

*One of the reasons that this study did not find a benefit of fructose ingestion is that they were not consuming glucose at a dose to saturate the SGLT1 receptors. Only at higher doses of glucose ingestion may the addition of fructose be truly ergogenic. However, the good news is that the addition of fructose did not alter total exogenous carbohydrate oxidation rates – so there was no metabolic downside to the addition of fructose ingestion at low doses.

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