Teaching your body to burn fat more effectively can go a long way to helping you to avoid hitting the dreaded wall during a marathon, or at least reduce its impact.
So what precisely is the marathon wall, what processes give rise to it and most importantly what can we do about it?
Glycogen vs Fat
When we exercise, we use two main energy stores; glycogen (carbohydrate) and fat. At any stage during exercise, we need to use a combination of these sources.
Glycogen is stored in your muscles and liver, and is rapidly and readily converted into glucose to fuel respiration, the chemical reaction that releases energy for life and which is summarised below.
The respiration equation
However, the liver and muscles can only store a limited amount of glycogen, so in an event like the marathon, it is likely to run out unless replenished. This typically occurs around the 18 to 20 mile mark and leads to the effects that we associate with hitting the wall (ie. weakness, disorientation, blurred vision and generally feeling pretty dreadful). The photo on the right shows the famous but distressing sight of the British athlete Jim Peters, then the world marathon record holder, trying in vain to reach the finish line in the 1954 Commonwealth Games in Vancouver. He was 17 minutes ahead of the field and ten minutes ahead of his own world record schedule with less than 400 metres to go, but his exhaustion was so great that he just could not make it to the finish unaided. Peters retired from racing that day and recognised that he could have died, a point that we should all consider if we hit the wall this badly (or become in any way seriously distressed during the race). Remember there are always other races on other days.
Fat is stored in a variety of locations and provides a longer lasting supply of energy, though it is released more slowly. However, the metabolic pathways involved mean that fat cannot be efficiently burnt without some glycogen, so when the glycogen runs out you can’t effectively access the fat stores. This then leads the body, in its desperation, to utilise a third, emergency energy source – protein.
This is bad news for the athlete because the skeletal muscle cells that you are hoping to power you to the line are being sacrificed as an (inefficient) energy source. So it’s no surprise that things start to go awry at this stage. A key part of the solution to this problem is to find a way of teaching your body to burn a higher proportion of fat during the earlier phases of the race, and therefore better preserve the limited glycogen supplies.
So how do you train your body to burn more fat?
The ratio of fat to carbohydrate (from glycogen) burning is related to the speed or intensity of the exercise and it is no surprise to find that the slower burning, longer lasting energy store (ie. fat) is utilised more during long slow runs. Therefore, one of the key training benefits of completing long runs at a comfortable pace is linked to the development of fat burning efficiency. Slow and steady really does win the race!
The graph below illustrates how the proportion of fat burnt increases (and carbohydrate use reduces) as the intensity of exercise reduces.
What else can you do to delay the onset of the wall?
Nutrition, throughout your training programme, in the last couple of weeks before the race and during the race itself are clearly key factors in helping the athlete to reduce the impact or even avoid the wall altogether.
Questions to ask might include:
Do I need to do anything special other than maintain a normal balanced diet, as perhaps typified by the “Eatwell plate” on the right?
Is there an ideal ratio of carbohydrate to protein in my diet during the main training period?
Should I alter my diet in the days leading up to the race (eg. carb loading)?
Should I use energy gels or other fast absorbing food sources, such as ripe bananas, during the race?