Before we dive into carb ratios and what the different sources of carbohydrates are, let’s start on why carbohydrates are so important.
Why are carbs important?
When we exercise, we burn predominantly either fats or carbohydrates. We use mostly fats at lower intensities, while we use carbs at higher intensities. When it comes to fuel stores, we have tens of thousands of calories of fat stored in the body, even in a very lean athlete, however we generally only have enough carbs stored for 90 minutes of exercise. This is why it is so important for us to consume additional carbs when we train and exercise, both to fuel the work that we do, and also replenish our carbohydrate stores for training the next day.
What are carbohydrates with two sources?
When it comes to carbohydrates, there are several different forms, and the main difference between them is how they are processed in the body. The primary ones that we want to be consuming are those that require less breakdown and these are most often found in sports nutrition as glucose and fructose. They are referred to as monosaccharides as they are made up of just one sugar molecule. However, there are uses for polysaccharides (multiple sugar molecules) that we will get into later on.
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Glucose is probably the one we are most familiar with within sports nutrition, as it is one of the most readily absorbed. The way we get glucose into the working muscles is firstly via the intestine. This is where the first limitation of consuming single source carbs occurs. When we consume glucose, it is transported from the intestines to the blood via a protein transporter called Sodium-glucose transport protein 1 (SGLT1) which requires sodium to do this.
This transport mechanism can only transport glucose at a rate of 1g/min, hence why 60g/hour was long seen as the maximum level of glucose that could be ingested during exercise.
Fructose however, cannot leave the intestines via this method, and instead uses a transporter called Glucose Transporter 5 (GLUT5). Both fructose and glucose are then transported into the bloodstream via GLUT2.
Fructose is then synthesized into glucose or lactate (yes, lactate is your friend and a useful fuel!) by hepatocytes (cells) in the liver.
However, during the beginnings of exercise at higher intensities you will be depleting your carb stores, and so topping up with fructose earlier will ensure that you have a more ample supply of usable glucose for when you require it.
What happens when we consume this mix of glucose and fructose, is that we are able to transport the sugars from the intestine at a faster rate, without causing discomfort or bloating.
If we were to consume more than 60g/hr of glucose we would not be able to transport all of that and likely experience discomfort. Likewise just consuming that level of fructose only has been found to result in gastrointestinal distress.
As for why the 1:0.8 ratio? Well, in a study by Rowlands et al., (2015) they tested various ratios of glucose to fructose, and found that the 1:0.8 ratio resulted in the highest exogenous carbohydrate energy and endurance power compared with lower or higher glucose:fructose ratios.
However, consuming lots of glucose and fructose together has the issue of being excessively sweet and not the most palatable. In a study by Wallis et al. 2005, they combined Maltodextrin with Fructose.
Maltodextrin is actually a polysaccharide but is made up of a chain of glucose molecules which are easily broken down. The benefit of using Maltodextrin instead of glucose is that it is absorbed as quickly as glucose, but lacks the excessive sweetness that would be found from a 1:0.8 ratio of glucose:fructose in the quantities required for optimal sports performance. In this study, the carb oxidation rates were found to be 1.5g/min, or 90g per hour. However, this was consumed at 1.8g/min, or 108g/hour. Practically what this means is that for optimal carb oxidation we probably want to consume a bottle of energy drink and a gel during each hour.
So what does dual source carbs do for our performance?
A study by Rowlands et al. (2008) found that fatigue was reduced when consuming a maltodextrin:fructose drink compared to maltodextrin only. And a study by Currell & Jeukendrup, (2008) found that glucose drink improved time trial power output by 9% compared to the placebo drink, but when glucose:fructose was used this was improved by an additional 8%!
There are some big gains to be had from optimizing the amount of carbohydrates we use and by using dual source carbs!
Another benefit of dual source carbohydrates is that as well as being better for fuelling prolonged higher intensity exercise, they also improve muscle glycogen resynthesis (Fuchs et al., 2016; Gonzalez et al., 2017), essentially they assist in recovery so that we can train or race more effectively the next day as well.
To summarise, using dual source carbs allows for greater carb oxidation rates because they can both be transported from the intestine to the blood and then the muscles at the same time, thus increasing overall carb availability. This both improves performance as well as recovery.
References
Wallis, G.A., D.S. Rowlands, C. Shaw, R.L. Jentjens, and A.E. Jeukendrup (2005). Oxidation of combined ingestion of maltodextrins and fructose during exercise. Med. Sci. Sports Exerc. 37:426-432.
Fuchs, C.J., J.T. Gonzalez, M. Beelen, N.M. Cermak, F.E. Smith, P.E. Thelwall, R. Taylor, M.I. Trenell, E.J. Stevenson, and L.J. van Loon (2016). Sucrose ingestion after exhaustive exercise accelerates liver, but not muscle glycogen repletion compared with glucoe ingestion in trained athletes. J. Appl. Physiol. 120:1328-1334.
Gonzalez, J.T., C.J. Fuchs, J.A, Betts, and L.J. van Loon (2017). Glucose plus fructose ingestion for post-exercise recovery - greater than the sum of its parts? Nutrients 9:E344.
Currell, K., and A.E. Jeukendrup (2008). Superior endurance performance with ingestion of multiple transportable carbohydrates. Med. Sci .Sports Exerc. 40: 275-281.
Rowlands, D.S., M. Swift, M. Ros, J.G. Green (2012). Composite versus single transportable carbohydrate solution enhances race and laboratory cycling performance. Appl. Physiol. Nutr. Metab. 37: 425-436.
Rowlands, D. S., Houltham, S., Musa-Veloso, K., Brown, F., Paulionis, L., & Bailey, D. (2015). Fructose–glucose composite carbohydrates and endurance performance: critical review and future perspectives. Sports Medicine, 45, 1561-1576.
