What are Multiple Transportable Carbohydrates?

In a previous article "Not all Carbs Are Equal" we saw that some carbohydrates are used more rapidly than others, but no carbohydrates are used at rates higher than 60 grams per hour. Why is this? The answer can be found in the way the small intestine absorbs carbohydrates. The capacity of absorption is limited and how much ingested carbohydrate your muscles can use appears limited by how much your intestine can absorb.

To understand what is happening we must look at the absorption process close-up. Absorption is the process of moving a nutrient from the intestinal lumen into the bodies’ circulation. In this process the nutrient has to pass through cells and in particular two cell membranes. These cell membranes are a barrier for unwanted and dangerous substances but also make it more difficult for any nutrient to enter the body. Many nutrients need the help of a transporter. These transporters are proteins that are embedded in the membranes and help the nutrient to move across the barrier. Glucose uses a transporter called sodium-dependent transporter or SGLT1 for absorption. The transport capacity of this transporter is limited as the transporter becomes saturated at a carbohydrate intake around 1 gram per minute (60 grams per hour). This is the main reason that ingesting more carbohydrate than about 60-70 grams per hour will not result in more oxidation of that carbohydrate. The excess carbohydrate is simply not absorbed and will accumulate in the intestine. Imagine we have 100 people in a room and the room has one door. When the meeting in the room is finished everyone will make their way to the coffee machine using that door and the main limiting factor for people leaving the room is the size of that door. The people in this example represent glucose and the door is the transporter. The only way to get people out of the room is to open another door. In our studies we found, that, if you make sure you saturate the SGLT1 transporter by giving 60 grams per hour of glucose and at the same time you use a carbohydrate that uses a different transporter, you can deliver more carbohydrate to the muscle. Fructose is such a carbohydrate. It is transported by a carbohydrate transporter called GLUT5. Because glucose and fructose use different transporters they are often referred to as multiple transportable carbohydrates or MTC. In 2004 we published the first study to show that if you ingested a combination of carbohydrates and observed oxidation rates well above 1 gram per minute. This was more than 25% more than we previously thought was the maximum. At present only two different intestinal carbohydrate transporters have been identified (SGLT1 for glucose and galactose, and GLUT5 for fructose). Studies suggest that carbohydrate oxidation from a sucrose drink is similar to glucose and does not reach the high oxidation rates observed with glucose and fructose (or other multiple transportable carbohydrates).

Optimal Mix and "No Magic" Ratio

We attempted to find the carbohydrate mix that would result in the highest oxidation rates. These studies confirmed that multiple transportable carbohydrates resulted in up to 75% greater oxidation rates than carbohydrates that use the SGLT1 transporter only! The following combinations seemed to produce the most favorable effects:

maltodextrin:fructose
glucose:fructose
glucose:sucrose:fructose

In all cases, the glucose transporter needs to be saturated and this will not happen if less than about 60 grams per hour is ingested. The additional second carbohydrate (fructose) will have to be ingested at sufficient rates to add to the carbohydrate delivery (30 grams per hour or more). If these amounts are ingested it gives you a ratio of 2:1 glucose:fructose and an intake of 90 grams per hour. This is often the recommended ratio and the basis of Neversecond's C-Series fueling products because 90 grams per hour is very achievable by many athletes and higher intakes are often much more challenging in practice. However, it is important to realize that there is no magical ratio. The exact ratio is a function of the type of carbohydrates you ingest and especially the absolute amount.

Performance Effects

In line with the evidence of a dose response relationship between carbohydrate intake and endurance performance, studies have demonstrated that multiple transportable carbohydrates can result in improved performance over and above the performance-enhancing effect of a carbohydrate drink with one single carbohydrate. It has also been demonstrated that multiple transportable carbohydrates may have advantages in fluid delivery and tolerance (gastrointestinal comfort).

Liquids, Gels or Solids?

Important from a practical perspective, such high oxidation rates cannot only be achieved with carbohydrate ingested in a beverage but also in the form of energy gels or low fat, low protein & low fibre energy bars. Therefore, it is possible to deliver the carbohydrate from a range of sources and it is possible to pick-and-mix to achieve the desired carbohydrate intake.

Practical Recommendations

Carbohydrate mixes can be recommended at all durations of exercise but are most effective when the exercise is 2.5 hours or longer. In those conditions, carbohydrate intakes of up to 90 grams per hour are recommended from multiple transportable carbohydrate sources such as C90 High Carb Mix. Glucose or maltodextrin will have to provide around 60 grams per hour, and fructose 30 grams per hour.

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References

Jentjens, R. L., et al. (2004). Oxidation of combined ingestion of glucose and fructose during exercise." J Appl Physiol 96(4): 1277-1284.
Pfeiffer, B., et al. (2010). Oxidation of solid versus liquid CHO sources during exercise." Med Sci Sports Exerc 42(11): 2030-2037.
Pfeiffer, B., et al. (2010). "CHO oxidation from a CHO gel compared with a drink during exercise." Med Sci Sports Exerc 42(11): 2038-2045.
Currell, K. and A. E. Jeukendrup (2008). "Superior endurance performance with ingestion of multiple transportable carbohydrates." Med Sci Sports Exerc 40(2): 275-281.
Jeukendrup, A. E. (2011). "Nutrition for endurance sports: marathon, triathlon, and road cycling." J Sports Sci 29 Suppl 1: S91-99.
Jeukendrup, A. (2014). "A step towards personalized sports nutrition: carbohydrate intake during exercise." Sports Med 44 Suppl 1: 25-33.

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