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the metabolic mix -the fuel for endurance sport

-By: Tomas Swift-Metcalfe

Last modified: April 2, 2014

diet, training for endurance sport

This article targets sports enthusiasts with little knowledge of the mechanisms underpinning training.

The main myth I am going to debunk here is the idea that you burn more fat at relatively lower exercise intensities. This statement is an over simplification that I think doesn’t help anyone wanting to get started training, as it’s leading people to train softly when maybe bumping it up a notch is the better solution, especially for the time constrained.

What is the metabolic mix?

Your body is a remarkable machine capable of burning nearly every type of fat, protein and sugar for fuel.

The metabolic mix is the mixture of energy sources that’s used for a particular intensity of exercise. These substrates (or fuels) are most commonly sugar called ‘glycogen’ in physiology speak and fats. There are more types of ‘fuel’ and metabolic pathways. Metabolic pathways refers to the many ways your body can process and break down energy ‘substrates’.

It’s possible to deduce what mixture of fuels is being burnt in the body through the expiratory exchange ratio: That is the quantity of carbon dioxide and oxygen in the exhaled air. However, a guestimate is accurate enough for our purposes.

At sub-maximal exercise level (that’s sustainable, without burning in the muscles and controlled breathing) it is possible to use the respiratory exchange ratio to know more or less the ratio of carbohydrate to fat being burnt. It’s estimated on the ratio of oxygen consumed and carbon dioxide produced. When this ratio is 1 or higher carbohydrates are the predominant fuel source, as fat doesn’t burn in absence of oxygen only glycogen and very inefficiently.

Diagram of the metabolic mixDiagram showing the various fuel sources at differing exercise intensities.

Fuel sources for exercise

The harder you exercise, the more you burn -of everything- so training at threshold,since energy demands are much higher, the fact your burning proportionately less fat, doesn’t mean your burning less fat on the whole. Maximal rate of fat oxidisation 74% maximum heart rate or 64% VO2max. Above this level the rate of fat oxidisation drops of rapidly. Proportionally more CHO is burnt versus fat as exercise intensity increase. Also the longer exercise last, the more protein you burn, hence (rather ironically) endurance athletes have about twice the protein requirement of whey protein guzzling weight lifters at 1.5 g per kg in times of hard training. P.s. you need not guzzle litres of Whey protein but having a diet rich in beans, fish and milk products is enough!.

Lipid power

Lipid power is one very important factor in endurance sport performance, because although it wont win you races (well, not counting ultra-marathons and the like) it is the engine that does the bulk of the work in any long endurance race. By definition lipid power occurs somewhere around aerobic threshold, a fictitious threshold: A purely aerobic effort doesn’t exist, even when you sit down doing nothing your metabolism is predominantly anaerobic.

Lipid power is easily trainable and some (tough) changes to lifestyle and diet, like favouring proteins and fat over CHO (think in terms of a Mediterranean diet). These changes can bring around an even more succinct change than training alone. Generally, lipid power is trained through ‘long slow distance’, although there are smarter ways to train this. The aim is to teach your body to burn fat in place of CHO at intensities where normally CHO is favoured. The effect of just diet or exercise can be so succinct it even effect metabolic gene expression… It’s quite amazing what the human body can do.

Where it becomes difficult

Keeping with a healthy range for body fat isn’t difficult for normal individuals doing up to 10 h of exercise per week. With elite athlete weight regulation and controlling body fat is not easy, nor is it automatic. It takes a huge amount of control to have weight dip to 5% or bellow in a controlled and healthy manner. The problems arise from the fact the even the smallest slip in diet and regime have an impact. Most of the elite athletes I’ve trained are surprised by just how skinny they need to perform, especially in cycling where every change in gradient, every acceleration is affected by weight.

I’ve kind glossed over this subject here and the truth is it’s very interesting and deserves more attention. So I’ll redress it bit by bit in the future.

References available upon request

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