In a previous blog 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 g/h. Why is this? Why can you not use more than 60 g/h? The answer lies in your capacity to absorb carbohydrate. How much ingested carbohydrate your muscles can use appears limited by how much your intestine can absorb.
Absorption of carbohydrates
Lets look at the absorption process close-up. Absorption is the process of moving a nutrient from the intestinal lumen into the bodies’ circulation (from left to right in the figure below). 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 1g/min (or 60g/h). 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.
We found out, however, that if you make sure you saturate this transporter by giving 60 g/h 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.
In 2004 Dr Roy Jentjens at the University of Birmingham published the first study to show that if you ingested a combination of carbohydrates - often referred to as multiple transportable carbohydrate because they use multiple transporters - and observed oxidation rates well above 1g/min (1.26g/ min) (1).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. The 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 favourable 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 g/h is ingested. The additional second carbohydrate (fructose) will have to be ingested at sufficient rates to add to the carbohydrate delivery (30 g/h or more). If these amounts are ingested it gives you a ratio of 2:1 glucose:fructose and an intake of 90 g/h. This is often the recommended ratio. However, I want to make a point that this is NOT a magic ratio. If you can tolerate higher intakes, adding more fructose may actually help and you will move towards a 1:1 ratio, but still with ingesting 60 g/h of glucose or maltodextrin.
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 (4) (see figure above). It has also been demonstrated that multiple transportable carbohydrates may have advantages in fluid delivery and tolerance (gastrointestinal comfort). Recently published recommendations take these findings into account, acknowledging that there may be different carbohydrate needs for different durations of exercise as well as for different levels of athletes (5, 6).
Liquid, gel or solids?
Important from a practical perspective, such high oxidation rates cannot only be achieved with carbohydrate ingested in a beverage but also as a gel (2) or a low fat, low protein, low fibre energy bar (3). 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.
Carb 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 90g/h are recommended from multiple transportable carbohydrate sources. Glucose or maltodextrin will have to provide around 60 g/h. In the next blog we will put together a comprehensive practical guide to carbohydrate intake for different events.
For more in-depth information, the reader is referred to recent reviews on this topic.