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Within the first week of the 2016 Tour de France we have already seen examples of riders who have used up their energy stores by going in early breaks then suffering later in the day.  We have also seen the cumulative effects of long glycogen depleting efforts over several days and observed the efforts teams go to in order to preserve their team leader’s energy stores for the critical points of this 3 week marathon.

Here we look at some of the issues surrounding the fuels used during exercise and how awareness of these concepts may be useful to us in long distance training and racing.

Fuel sources for endurance exercise

Compared to other food major groups fat is both an efficient and significant energy provider and most people carry a significant, if not prolific, energy store, as body fat or adipose tissue.

It is important to recognize that fats are much more than just an energy supply and consist of many different fatty acids, the composition of which in the diet can have profound effects on health and performance.  (another article perhaps but hence why Secret Training produce the omega shots)

It is also recognized that there is increased interest in low carbohydrate ketogenic diets, and other methods to increase ketone levels, that may challenge some of these concepts.  However, it is still useful to understand the importance and limitations of fat as a fuel for endurance performance.

Fat as energy

When you look at fat as a supplier and store of energy, without consideration of the difficulties of accessing that energy effectively, it would be easy to wonder why carbohydrate energy drinks are so effective, or indeed why you would have to bother eating or drinking on a ride at all.

Each gram of fat typically provides 9kcal, some fats such as medium chain triglycerides found in coconut oil contain 7kcal but this is still approximately twice that of carbohydrate at 3.75kcal.  At 15% body fat (slightly less than average) an 80kg cyclist would have an astonishing 108,000kcal of energy stored away as adipose tissue.  That is sufficient energy to fuel more than 10 consecutive queen stages of a major tour without eating anything at all.

Using fat for energy

Unfortunately, when it comes to accessing all that energy stored as fat it is not quite such a rosy picture, since the body has a limited ability to burn fat during exercise. 

At low intensities fat can provide most if not all of the limited energy required to fuel the muscles, but as the intensity increases the energy demands become more significant and the body relies more upon carbohydrate. Absolute fat usage will at first increase more or less linearly with intensity, but then rates of increase will fall as carbohydrate kicks in to top up the energy requirement.  Crucially there then comes an intensity where fat burning tops out (fatmax) and starts to decrease in absolute terms not just as a percentage of the total.

Many people are familiar with the concept of a maximal fat burning zone through the heart rate zone training charts often found in gyms and health clubs. These charts depict that at low intensities fat contributes most of the energy but as the intensity increases the fat contribution falls and the body has to rely more and more on carbohydrate for fuel.  These charts often confuse people into working out at a super low intensity where fat contributes the biggest percentage of the energy, rather than an intensity that burns the most mass of fat per hour. The distinction can be appreciated more easily when individual data is expressed in percentage and absolute terms as in graphs 1 and 2.

The fat burning zone is often misused to justify short easy workouts for weight loss.  If you only have a short amount of time, then a strategy of going as hard as possible in the time available will burn more calories during the workout and increase metabolic rate during recovery.  Interestingly several studies show increased weight loss with short high intensity interval training than longer less intense training sessions.

At exercise intensities above fatmax carbohydrate not only has to provide the energy for the increased work rate, but also to replace those provided from fat at lower intensities.  For short rides where carbohydrate supply is not limiting this is not a problem.  Unfortunately, the body has much less capacity to store energy as carbohydrate than as fat.  Typically, a maximum carbohydrate store would be around 500g composed of muscle glycogen, liver glycogen and some circulating glucose.   It is important to remember that carbohydrate provides much less energy per gram than fat.  For every gram of fat that is not used nearly 21/2 g of carbohydrate are required to make up the difference -on top of those required for the increased work rate. 

Charts showing percentage energy contribution and absolute contribution from an individual cyclist

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