As previously discussed carbohydrate (CHO) is more than just a macronutrient but also a training regulator and signal modulator. With this in mind, the use of CHO pre, during and post exercise performance can modulate the outcomes greatly. A high muscle-glycogen concentration will allow you to train at a different intensity and achieve a different training adaptation when compared to a low muscle-glycogen concentration. Both strategies have their pros and cons and that is why it is very important to understand the effects that ingestion of CHO can have on the body. Clearly glycogen is a valuable fuel source for both training and performance and it most certainly isn’t a one-size fits all approach. With this in mind, each different CHO produces a different response in the body, and as such, a key part to every athlete’s tool box is the effect of each foods glycaemic index (GI).
GI of Carbohydrates
The GI is simply the ranking of foods from 0-100 based on their immediate effect of blood sugar levels, a measure of the speed at which you digest food and convert it into glucose. The higher the ranking, the faster the rise in blood glucose. All foods are compared to glucose (the reference food) in equivalent CHO amounts to allow both athlete and nutritionist to make informed decisions about what type of CHO to consume to generate the response you are after. For example, if you wanted to get CHO into your bloodstream and muscle cells rapidly to replenish glycogen stores then consumption of high GI foods would be better than low GI. Foods are classified into high (71-100), moderate (56-70) and low (0-55) GI’s to make it easier to select the appropriate foods pre, during and post exercise.
What effects the GI of foods?
To summarise the factors that influences one food have a high GI and another a low GI is summarised in Table 1 (adapted from Bean, 2009). Consuming low GI foods result in attenuated responses on glucose and insulin with lower periods of perceived feelings of hunger when compared to high GI foods (Ludwig et al., 1999).
The current concept of CHO loading for many athletes comes from the landmark study of Jonas Bergstrom and Eric Hultman (1966) from which they performed repeated muscle biopsies on themselves following ingestion of different diets. They clearly showed that performance increased when consuming a 70% diet when compared to 50% and 10% CHO diets. With this in mind current recommendations regarding acute fuelling strategies for high carbohydrate availability to promote optimal performance in competition has been highlighted in a fantastic manuscript by Louise Burke (Burke et al., 2011) and readers are advised to refer to this.
• for general fuelling preparation for events lasting < 90 min – 7-12 g/kg body mass per 24 h
• carbohydrate loading preparing for events > 90 min of sustained intermittent exercise – 36-48 h of 10-12 g/kg body mass per 24 h
• for speedy refuelling between two high demanding sessions with < 8 h recovery – 1.0-1.2g/kg body mass for first 4 h then resume to daily fuel needs
• pre-event fuelling before exercise > 60 min – 1-4g/kg body mass 1-4 h before exercise
It is important to consider again the context of why you want to arrive loaded at the start of the race or game and if the sport you are participating in needs you to be loaded or not. We store approximately 12-16g/kg body mass wet weight of glycogen in our muscles (300-400g), 80-110g in the liver and ~5g of glucose in the blood plasma. Therefore the days leading up to the event are utilised to saturate our muscles with glycogen and maximise the amount of storage ready for performance where the last few hours (3-5 h) leading up to the start of competition are utilised mainly to replenish the liver glycogen stores that have been used through our overnight sleep.
There are numerous theories by which consuming CHO during exercise may help delay the onset of fatigue. For example, maintaining blood glucose, glycogen sparing, promote glycogen synthesis, improve motor skills and CNS effects. However context comes into play again because it depends on the duration of the event that you are participating in. If your fuelling strategy has been implemented correctly then during an event lasting between 45 minutes and 1 hour (brief exercise) CHO’s are not needed. During sustained high-intensity exercise of between 60-90min small amounts are needed and interestingly even just purely mouth rinsing a CHO drink can play a positive role in enhancing performance by 2-3% by activating regions of the brains that are associated with reward and motor control involving the central nervous system (Carter et al., 2004; Chambers et al., 2009; Jeukendrup & Chambers, 2010). During endurance exercise and stop and start team sports of 1 h – 2.5 h, 30-60g/h should be recommended by specialised sports products from both liquid and solid form. The athlete should really try both forms of CHO in training to see which one best suits them as consuming a liquid or a solid will affect the comfort of digestion and satiety effects for each individual athlete differently. For events lasting more than 2.5-3.0 h then up to 90g/h may need to be consumed through products providing multiple transportable carbohydrates (60g glucose and 30g fructose i.e. 2:1 ratio). However it is more practical and easier on the gut to consume 50-60g/h which can be implemented through two 25-30g feed strategies and therefore only very highly trained ultra endurance athletes may be able to cope with the 90g/h strategy due to GI distress.
Some key considerations to think about with post-exercise CHO consumption are:
• how depleted your glycogen stores are after exercise
• the extent of damage to the muscles
• the amount and timing of CHO you eat
• the athletes training experience and conditioning level
In essence the higher your CHO intake the faster you can refuel your glycogen stores. Most researchers suggest consuming 1g/kg body mass during the 2-hour post-exercise period and the sooner you begin this refuelling process the better. This is because the consumption of CHO stimulates insulin release which in turn increases the amount of glucose taken up by your muscle cells from the blood stream and additionally post-exercise our muscle membranes are more permeable to glucose so they can take up more glucose than normal. With this in mind it’s suggested to consume moderate to high GI foods in the first 2 hours post exercise to increase glycogen replenishment (Burke et al., 2004) and then revert back to low GI foods 24 hrs post exercise.
The use of CHO in sport can have huge effects on the outcome of performances. If correct strategies are planned and implemented well then you will see improvements. However the context of why, how and when to use carbohydrates is very important and three key questions that should always be asked.
Nutrition X Sports Scientist & Widnes Vikings Performance Nutritionist
Twitter – @Jmorehen
ReferencesBean, A. (2009). The complete guide to Sports Nutrition. A & C Black Publishers Ltd, pp. 26.
Bergstrom, J., & Hultman, E., (1966). Muscle glycogen synthesis after exercise: an enhancing factor localized to the muscle cells in man. Nature, 210:5033, 309-310.
Burke, L. M., Kiens, B., & Ivy, J. L. (2004). Carbohydrates and fat for training and recovery, Journal of Sports Sciences, 22:1, 15-30.
Burke, L. M., Hawley, J. A., Wong, S. H. S., & Jeukendrup, A. E. (2011). Carbohydrate for training and competition. Journal of Sports Sciences, 29:1, 17-27.
Carter, J. M., Jeukendrup, A. E., & Jones, D. A. (2004). The effect of carbohydrate mouth rinse on 1-h cycle time trial performance. Medicine and Science in Sports and Exercise, 36, 2107-2111.
Chambers, E. S., Bridge, M. W., & Jones, D. A. (2009). Carbohydrate rinsing in the human mouth: Effects on exercise performance and brain activity. Journal of Physiology, 587, 1779-1794.
Jeukendrup, A. E., & Chambers, E. S. (2010). Oral carbohydrate sensing and exercise performance. Current Opinion in Clinical Nutrition and Metabolic Care, 13, 447-451.
Ludwig, D. S., Majzoub, J. A., Al-Zahrani, A., Dallal, G. E., Blanco, I. & Roberts, S. B. (1999). High glycemic index foods, overeating, and obesity. Pediatrics 103:261-266.