MarathonIf you are training for a marathon, long distance cycle race or triathlon, you are probably already using some form of carbohydrate during your longer training sessions, and will be aware of its benefits. But are you familiar with latest guidelines for carbohydrate use during exercise?

  • The benefits of carbohydrate ingestion during endurance exercise have been known as a result of sports science research since the 1980s. Taking on carbohydrates during exercise maintains blood glucose concentrations and provides extra fuel, thus sparing your body’s carbohydrate stores (glycogen) 1. This extra carbohydrate might be in the form of glucose, fructose (fruit sugar), sucrose (table sugar) or maltodextrin (a short chain of glucose molecules), and consumed in a sports drink, an energy gel or an energy bar. It is important to take on water too if using gels or bars, in order to accelerate absorption and remain well hydrated.
  • Your muscle uses different types of carbohydrates at different rates: glucose, sucrose and maltodextrins are rapidly utilised, while fructose and some starches are oxidised to energy at a slower rate 2. It used to be thought that the maximum amount of carbohydrate that could be ingested in an hour was 60g. However, more recent research has revealed that as glucose and fructose make use of different transporters from the intestine to the cells (SGLT1 and GLUT5 respectively), they can be used in combination to increase the amount of total carbohydrate that can be effectively utilised in an hour to 90g, without negative side effects such as gut discomfort 3. This can have a significant impact on performance.
  • The amount of carbohydrate that you need will vary with the intensity you are exercising at, the duration of your session, the carbohydrate sources you are using, and your own individual tolerance. For example, some people have problems with consuming fructose during exercise. It is also important to like the taste of the carbohydrate product you are using!  A guideline for recreational endurance athletes is 30g of carbs per hour for low to moderate intensity training, increasing up to 60g per hour for high intensity sessions. However, there is no need, or benefit, to using carbohydrate products during training sessions lasting less than an hour, possibly less than 90 minutes when intensity is low. Your body’s glycogen stores will provide sufficient fuel. Always try a drink, gel or bar in training before using it in a race.
  • If using a sports drink, choose one with 6-8% carbohydrate (6-8g/100ml) as more highly concentrated solutions may impair fluid delivery. So you would need to consume c.500ml per hour while training at a low intensity and c.1000ml of sports drink per hour while racing. Rather than waiting until you start to feel fatigue, take on a moderate amount at the start of the session and then top it up regularly, eg every 15-20 minutes, with smaller amounts in order to meet your target.
  • A number of drinks, gels and bars now contain a source of glucose and/or maltodextrin together with a source of fructose, usually in a 2:1 ratio, giving up to 90g of carbs per hour. These are particularly recommended for use when you are training for more than 2.5 hours in duration, eg marathon training runs or long bike rides, or in races lasting two hours or more. A recent paper looking at the relationship between carbohydrate dose and performance concluded that 78g per hour of a 1:1:1 glucose-fructose-maltodextrin solution provided the optimum performance during a two hour cycling time trial. 4
  • If you are racing, or training at a high intensity, for less than an hour but more than 30 minutes, there are now several pieces of research indicating that you may benefit from swilling a carbohydrate solution in your mouth for around 10 seconds and then spitting it out. 5, 6 The positive effect on performance is thought to come from signals sent from your mouth to your brain, with a subsequent effect on your muscles. The effect also seems to be linked to the length of time since you last ingested carbohydrate, with a greater response during training in a fasted state.
  • Finally, some endurance athletes, particularly those involved with ultra-marathon events, report benefits from following a high fat-low carbohydrate ketogenic diet, to which they become adapted over a period of time and see improvements in performance, using fat as their primary fuel. 7 A lower carbohydrate diet, excluding the use of sports drinks, gels, etc as well as providing a lower proportion of calories from carbohydrate, may also be helpful for those looking to lose weight or experiencing issues with blood sugar and insulin function or gut health. But that needs to be the subject of a whole future blog post!

Jo Scott-Dalgleish BSc (Hons) is a BANT Registered Nutritionist, writing and giving talks about nutrition for endurance sportBased in London, she also works as a Registered Nutritional Therapist, conducting onetoone consultations with triathletes, distance runners and cyclists to help them eat well, be healthy and perform better through the creation of an individual nutritional plan. To learn more about these consultations, please visit www.nutritionforendurancesports.co.uk 

References

1 Coggan AR Coyle EF. Reversal of fatigue during prolonged exercise by carbohydrate infusion or ingestion. J Appl Physiol 1987; 63, 2388-95

2 Jeukendrup, Jentjens & Moseley. Nutritional considerations in triathlon. Sports Med 2005; 35(2), 163-81

3 Currell & Jeukendrup. Superior endurance performance with ingestion of multiple transportable carbohydrates. Med Sci Sports Exerc 2008; 40(2), 275-81

4 Smith et al. Curvilinear dose-response relationship of carbohydrate (0-120g.h(-1)) and performance. Med Sci Sports Exerc. 2013. 45(2), 336-41.

5 Jeukendrup AE. Oral carbohydrate rinse: placebo or beneficial? Curr Sports Med Rep. 2013. 12(4):222-7

6 Sinclair et al. The effect of different durations of carbohydrate mouth rinse on cycling performance. Eur J Sport Sci. 2013 April 11. [Epub ahead of print]

7 Volek J and Phinney S. The Art and Science of Low Carbohydrate Performance. 2012. Beyond Obesity LLC. Available as a Kindle edition.