During prolonged exercise (>2 hours) carbohydrate ingestion:
- Helps to maintain blood glucose concentration
- Helps to maintain high rates of carbohydrate oxidation
- Reduces liver glycogen breakdown
- May reduce glycogen breakdown in the muscle in some conditions (running).
- Delays onset of fatigue
- Improves endurance capacity and performance
- Tolerating high amounts of carbohydrates while running at high intensities takes a lot of training. Our stomachs don’t cope well with high concentrations of sugar. This can result in slower gastric emptying and can cause GI distress.
The gut is very adaptable/trainable:
Intestinal carbohydrate transporters can be upregulated by a high carbohydrate diet and by frequently using carbohydrates during sustained and hard efforts in training. Training the gut may help by improving gastric emptying and the perception of fullness (reduced bloating), improving tolerance to larger volumes and increasing the speed of absorption causing less residual volume and smaller osmotic shifts.
Glucose is absorbed through a protein called SGLT1 that assists the transport across the cell wall of the intestine. When the glucose content of the diet is increased for several days, the number of these transporters increases and the absorption of glucose increases to match this. Fructose is transported by GLUT5 and something similar may happen to GLUT5 transporters.
Absorption of carbohydrates:
Absorption is the process of moving a nutrient from the intestinal lumen into the bodies’ circulation (from left to right in the figure above). The nutrient has to pass through two cell membranes, which protect the body from harmful substances. Many nutrients need the help of a transporter to get past, which are proteins that are fixed in the membranes and help the nutrient to move across this barrier.
If you saturate this transporter (SGLT1) 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 since it is transported by a different one from glucose (GLUT5).
Multiple transportable carbohydrates were discovered to be very helpful in 2004 from the first study by Dr Roy Jentjens at the University of Birmingham. He showed that if you ingested a combination of carbohydrates you could tolerate more (25%) compared to just using one source (glucose).
The studies confirmed that compared to only use one transporter, multiple transportable carbohydrates resulted in up to 75% greater oxidation rates. The following combinations seemed to produce the most favourable effects:
- maltodextrin : fructose
- glucose : fructose
- glucose : sucrose : fructose
The glucose transporter needs to be saturated and this is only possible if about 60g/hour is taken in. The second carbohydrate (fructose) needs to be ingested at around 30g/hour or more. This gives a ratio of 2:1 but if you can tolerate higher intakes, adding more fructose may actually help and you will move towards a 1:1 ratio, but still with taking in 60 g/h of glucose or maltodextrin. It all comes down to personal tolerance and practice training your gut!
Choice of Carbohydrates (CHO):
- 45-75 min – mouth rinse – most forms
- 1-2 hours – up to 30g/hour – most forms of CHO
- 2-3 hours – up to 60g/hour – CHO rapidly oxidized (glucose, Maltodextrin) or Glucose+Fructose
- >2.5 hours – up to 90g/hour – ONLY transportable CHO (Glu + Fru)
Multiple transportable carbohydrates (Gluc+Fruc) ingestion can:
- Increase exogenous CHO oxidation compared with glucose.
- Stave off depletion of glycogen stores and maintain a faster pace for longer without ‘hitting the wall’.
- Enhance fluid availability. Fluid delivery is impaired with high CHO solutions but the negative effects of delivery are reduced when Gluc/Fruc is combined.
- Improve performance compared with glucose only.
- May reduce GI distress.
- Exercising in heat – the body becomes more dependent on carbs but cant deliver as much. One study (2006) showed a 36% improvement in exogenous CHO oxidation in hot conditions.
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.
Examples of foods to try for endurance exercise:
Mix and Match: water, carb drinks, gels, bars, banana, bread + honey (gluc/fruc), low in fat and fibre and protein.
Maurten uses a hydrogel technology to help with tolerance. Normally when a drink high in carbohydrates is consumed, gastric emptying slows down resulting in less fluid and carbohydrate delivery to the body.
Hydrogels are used in the food industry often as thickeners. A hydrogel is a biopolymer and water-based structure with very small pores. It looks and behaves like a kitchen sponge. The hydrogels in the sports drinks are built from two natural ingredients:
- Alginate – extracted from the cell walls of brown algae.
- Pectin – found in apples, lemons carrot, tomatoes and other fruits/vegetables.
Both combined to form a pH-sensitive hydrogel. The drink has high amounts of carbohydrates (glucose + fructose) that instantly converts to hydrogel in the acidity of the stomach, encapsulating the carbohydrates. The hydrogel allows the sports drink to get to the intestine where the water, salt and carbohydrates are absorbed.
Remember to practice these suggestions often during long and intense running efforts in order to best help you come race day!
By Rachel Hannah, RD