There is a huge amount of information on television, in magazines and books and on the internet about sports nutrition. Everyone has an opinion on what to eat and drink, what not to eat and drink, what to consume when training and racing and what to consume to recover. Is any of it correct? Who should be giving this advice? Is it safe? Will it improve your performance? Will it help you achieve all of your goals?
In order to work out what works for you we need to first understand the basics of sports nutrition. We need to know what foods and drinks provide the energy in a form we can use. This will lead to an understanding of what you choose to eat and drink before, during and after training sessions and while racing. It will also allow you to look at what is on the market and you can determine if it is right for you.
Carbohydrate and fat are the most common fuels burned during exercise. We have stores of both of these fuels in our bodies and what we use will depend on the intensity and duration of activity.
The first step is to understand what fuel the body uses at different intensities of exercise.
Romijn JA. et al. (1993) Am J Physiol. 265(3 Pt 1):E380-91.
At lower exercise intensity we predominantly use fat for fuel (plasma FFA and Muscle triglycerides in the diagram above). In order to use fat for fuel we need to plenty of oxygen – so at rest or at low intensity exercise we can do this. Fat provides 37kJ or 9cal energy per gram. It is the most energy dense of all nutrients. Most of us have a fair amount of energy stored in our bodies as fat – even very lean people have some fat stores.
As exercise intensity increases we start to breathe more heavily so we can meet the energy demands but the oxygen we are taking in is not enough to ulilise the amount of fat needed to work at the higher load. So the body looks for another energy source that does not need as much oxygen to be burned. Carbohydrate (muscle glycogen and plasma glucose in the diagram above) fits this bill and becomes more important as exercise intensity increases. It is easily used for fuel with a lower requirement of oxygen. Every gram of carbohydrate provides 17kJ or 4cal of energy.
The higher the intensity of exercise the more carbohydrate is relied upon for fuel. We store much less carbohydrate in our bodies than fat so at moderate to intense levels of exercise it becomes important to continually top up the bodies carbohydrate stores (more on this in following posts).
So it seems logical to want to use more fat for fuel as it gives more energy per gram and it is already in our bodies and it provides two times the amount of energy. The limitation is the intensity of activity is limited with fat for fuel. If you need to increase speed or intensity at any point you will step up into the zones where you run out of oxygen for fat burning – no matter how fit you are.
More to come on:
- Carbohydrate needs during endurance exercise
- Protein needs in endurance exercise
- Nutrition needs for recovery
- Eating before exercise
Rebecca Hay – Accredited Practising Dietitian, Accredited Sports Dietitian & Nutrition Consultant