// Prevent the bonk - eating for endurance performance

by Tanya Lewis

Your client is training for a 100km charity bike ride and mentions that the last hour of their weekend ride is really difficult, saying ‘I can get home ok, but just can’t maintain the sort of speed I start out with, after a few hours, my legs feel empty. How can I prevent this?’


It’s only 25 minutes into a one-hour training session and your client is unable to maintain intensity of their third interval on the bike, saying ‘I’m stuffed, my legs feel drained’. You ask them what they have eaten today, and get the reply ‘two pieces of toast for breakfast and a salad for lunch.’


You have a good breakfast, then run with your first two clients, do a weights session in the middle of the day, are on the floor all afternoon and then struggle through a running interval set with a client you are usually able to easily out-run – but on this day you are ‘spent’ or as some in the cycling fraternity say when fatigue sets in, you’ve ‘bonked’.

These are all examples of carbohydrate depletion, a limiting factor to endurance exercise performance. While the research behind these principals has largely been developed for athletes competing at an elite level, the application for personal training clients wanting to get the most out of their training sessions is highly relevant.


1. Carbohydrate is the key fuel source for high intensity activities and the higher the intensity of activity, the more the body relies on carbohydrate to maintain output.

At rest, fat provides the majority of energy needs but as activity intensifies, energy needs increase and the rate of carbohydrate utilisation rises to become the primary fuel source. So, when carbohydrate is not available, there is inadequate delivery of fuel to the muscles to maintain work at high intensity.

2. The body can store carbohydrate as a fuel reserve. After digestion and absorption, glucose is converted to glycogen and stored in the muscles and liver. For an average 70kg male, about 1,100kJ worth of carbohydrate is stored in the liver and 5,900kJ of carbohydrate can be stored in the muscles. This will fuel ~60 minutes of hard running or ~3 hours of walking.

3. Factors affecting how long muscle glycogen stores last

• Inadequate daily carbohydrate intake – low carbohydrate diets mean the muscle glycogen stores are never full and will run out quickly. Equate it to only ever filling your car with a quarter of a tank of petrol; you would quickly and frequently run out of fuel.

• Time since last meal – skipping lunch will certainly impact on an evening training session.

• Training background – physiological adaptations allow well trained athletes to increase glycogen storage in muscles specific to workload (e.g. cyclists will have lots of glycogen in leg muscles, swimmers will store more glycogen in their arms as well)

• Elapsed time since last training session – short recovery (particularly < 12hours) means that glycogen stores do not have enough time to completely refuel.

4. Usually protein provides less than 5 per cent of fuel needs, but when muscle and liver glycogen stores run low, the body starts to break down protein to convert some amino acids to glucose. This is one reason why it is not ideal to train with insuffi  cient carbohydrate.

5. Daily carbohydrate requirements depend on activity levels and size, specifically lean body mass. Sports dietitians prescribe carbohydrate requirements relative to body weight i.e., grams per kilogram of bodyweight per day. These equations must consider total daily physical activity levels (PAL) and percentage bodyweight. The equations will not be accurate for overweight clients.

•  a generally active person who completes 3 to 4 hard training sessions each week but who spends the majority of time in an offi  ce job would need 3 to 4g of carbohydrate per kilogram of bodyweight every day.

• a committed recreational athlete training at a moderate to hard pace for around 8 to 10 hours/week require 5 to 7g carbohydrate per kilogram of bodyweight every day.

• a serious professional cyclist training over 18 hours a week should aim for at least 10g carbohydrate per kilogram of bodyweight every day!


• Ensure sufficient carbohydrate has been consumed over the previous 24 hours, not just in the meal prior to exercise.
• For early morning training sessions, it is not necessary to eat beforehand, providing adequate carbohydrate has been consumed the day before and the session lasts for less than 90 minutes.
• Good examples of pre-exercise meals include: toast, banana, yoghurt.

•  Carbohydrate replenishment during exercise is necessary when muscle glycogen stores are at risk of depletion, i.e., normally only significant for exercise >60 – 90 minutes, depending on the intensity of activity, pre-exercise intake and starting glycogen stores.
•  500ml of Gatorade, 12 jelly beans or a large cereal bar all provide ~30g carbohydrate.

•  It is important to eat carbohydrate soon after exercise because this is when the muscle is most eff ective at glucose uptake and glycogen resynthesis.
•  Immediate recovery is particularly important if the person will be exercising again within 12 hours.
• Good recovery foods contain protein as well as carbohydrate, for example, ‘nutritionally complete’ liquid meals, tuna/ham sandwiches or sushi rolls.


Bear in mind that carbohydrate is not the only reason behind fatigue and increasing carbohydrate is not going to help all your clients. Be careful not to confuse walking three times a week for an hour with regular high intensity training. For clients serious about their performance or confused about carbohydrate requirements it is best to seek individual assessment from an accredited sports dietitian – for more information, fact sheets or to find a sports dietitian in your area, visit www.sportsdietitians.com By taking these principles onboard you can help your clients to improve their endurance exercise performance – and avoid the ‘bonk’! 


Tanya Lewis
Tanya is an accredited practising dietitian, sports dietitian, personal trainer and a director of Life Personal Trainers (www.lifept.com.au) in Adelaide. She works mainly in the areas of sports nutrition, eating disorders, paediatric nutrition, weight management and corporate health. Tanya promotes the value of life-long good nutrition and exercise habits, rather than quick-fi x diet cycles, for children and adults of all ages.

NETWORK • SUMMER 2008 • PP55-56