We, at High5 Sports Nutrition and Tri Training Harder, understand the importance of getting your nutrition right so that you can maximise your endurance performance and stay fit and healthy before, during and after your session(s). To that end, we have spoken with numerous athletes and coaches to find out some of the most common problems/issues regarding sports nutrition.
We have tried to answer this questions in as generic and simple terms as possible so that you, the athlete, are best equipped to fuel yourself correctly, whatever your event or discipline. The science around sports is the same for every human being; we all burn energy, and that energy needs to be replenished. The confusion begins when we try to apply a ‘solve all’ answer to our individual nutrition needs.
We have tried to answer the question below without referencing specific brands and/or products, so that you can make an informed decision as to your nutritional requirements.
Q: Understanding absorption rates
A: After swallowing your traditional sports drink (that breaks down into glucose, and not fructose); it reaches the stomach before moving down to your intestine. During that journey, the various types of carbohydrate found in the drink are broken down to glucose by your digestive system. Glucose is the main source of energy for the body during exercise. This Glucose must then pass through the intestine wall, by way of Glucose Transporters and into the blood stream to be taken to the working muscles.
However, the Glucose Transporters only allow Glucose to pass through relatively slowly and this results in a bottleneck at the wall of the intestine. It’s thought that this Glucose ‘bottleneck’ is what limits the maximum amount of carbohydrate your body can absorb, from a traditional sports drink, to around 60 gram per hour.
Maltodextrin: This is a carbohydrate used in many traditional sports drinks. It’s a common type of carbohydrate that’s broken down to glucose by digestion and passes through the wall of the intestine at a maximum rate of 60gram per hour.
Fructose (fruit sugar): Is a unique carbohydrate that’s not broken down to Glucose by digestion. Fructose passes through the wall of the intestine using a completely different set of Transporters to Glucose (GLUT5). Fructose does not get caught in the Glucose ‘bottleneck’ and it can provide your working muscles with an additional 30gram per hour of carbohydrate.
A ratio of 2 parts maltodextrin to 1 part fructose has been shown to be the most effective in providing your muscles with carbohydrate. If we consume 60g glucose per hour, then we can provide our working muscles an additional 30g of carbohydrate per hour through fructose. As you can see from the diagram, 90 gram per hour could be absorbed each hour using a 2:1 fructose formulation.
As carbohydrate is the primary fuel for endurance sport, the more carbohydrate you have available, the faster and further you will be able to go. A number of independent research studies, are based on 2:1 fructose drinks, and they have clearly demonstrated a substantial performance and endurance advantage when compared to traditional sports drink formulations.
When considering absorption rates, the aim is to balance liver release and muscle absorption at 1g/min. Despite fuel being used from both the liver and the muscles, hypoglycemia is one of the first reasons athletes fatigue during exercise which takes place when the liver glycogen (fuel) stores are used up. Without carbohydrate ingestion (no sports drinks or food) to suppress liver glucose production, even when only racing or training at between 70-85% VO2Max, these liver glycogen stores will be depleted after around 2 hours.
After swallowing food be it in liquid or solid form, the ability for your body to use the ‘food’ is determined by the following four areas:
Muscle glucose uptake
Oxidation limit carbohydrate use by muscles.
In most studies the stomach has still been fully emptied with doses of carbohydrate between 70-100g/hr.
As described above, the intestinal absorption is balanced at approximately 60g/hr of glucose and a further amount of fructose polymers. This is set to 30g/hr as even a limited amount of fructose (50g/hour) produces gastrointestinal discomfort. This is because there is limited capacity to absorb fructose in the intestine so it then travels to the colon where metabolism by bacteria produces chemicals that can induce colonic discomfort.
Ingested carbohydrate during exercise is burned by the muscles in place of blood glucose derived from the liver, and this rate of use increases up to an intensity of 60%VO2Max. Carbohydrate ingestion during exercise does not however increase the rate of glucose output by the liver during exercise. It simply substitutes all or part of the glucose that would be released by the liver and any excess is stored as liver glycogen stores. Trained athletes may oxidise more ingested carbohydrate than untrained athletes, but only glucose infused straight into the blood stream (i.e injected) allows muscular oxidation rates />to be increased further (up to 150g/hour).