Training And Fat - Part 1

Continuing on from It's All About Fat And Sugar, I thought it was time to address how training affects the use of carbohydrates (CHO) and fats during exercise. In order to understand the intent of training a few simplified concepts need to be understood. Exercise intensity influences the contribution that CHO and fat have in fueling exercise. The cross-over concept outlines that there is a certain exercise intensity at which fat and CHO both contribute 50% of the energy required. Below this point the majority of energy is from fat, and above this intensity the majority of energy is from CHO as demonstrated in the graph below.

This cross-over point occurs somewhere between 40-55%VO2max depending on a number of factors, this is often the intensity recommended as the so-called fat burning zone. Unfortunately it is a case of a little knowledge can be a dangerous thing. It needs to be remembered that the total amount of energy from all sources increases as exercise intensity increases. While the relative percentage of the total amount of energy from fats is highest below the cross-over point, the maximal rate of fat oxidation typically occurs at between 60-65%VO2max. Up to about 75%VO2max, it is the absolute rate of CHO oxidation that influences the relative contribution between the two substrates.

If we look at the absolute rate of fat oxidation versus exercise intensity (see graph below), we see that between 40-55% of VO2max fat oxidation remains relatively stable. Above this level it shows a marked increase, peaking at between 60-65%VO2max before then declining again. In fact once the intensity reaches 70%VO2max or above, there is a very obvious rapid decline in fat oxidation. This is a very important point that has implications to both training and racing.

If we are interested in racing and training to optimise fat oxidation (and we are), then this suggests we to need to train in the insensity range somewhere of 55-75% VO2max, or roughly 60-80% HRmax.

In reference to fat oxidation the goals of training could be:

  1. Increase the intensity at which the cross-over point occurs
  2. Increase the intensity at which the maximal rate of fat oxidation occurs
  3. Increase the total amount of fat oxidation across intensity zones

Now the question is are these three points possible, and if so how can they be achieved? To answer this we need to delve into what limits fat oxidation during exercise. Fats (lipids) are stored in the body in the form of something called triacylglycerols in adipose tissue, muscles, liver and in lipoproteins in the blood. One problem is that muscles cannot oxidise triaclyglycerols directly. Instead, the triacyglycerol needs to broken down into its two components, free fatty acid (FFA) and glycerol. This breakdown is called lipolysis. The FFA then travels through the blood supply and eventually is taken up into the muscle. Once inside the muscle energy is derived from the FFA through a complicated process requiring a significant amount of oxygen in the so-called powerhouse of the cell, the mitochondria. There are a number of enzymes and hormones that either speed up or slow down all of the above processes.

In general endurance training increases the number and size of mitochondria in the muscle. This means there are more and better powerhouses working to generate energy from fat. Capillary density is increased and combined with an increase in blood volume, there is an increase in supply of blood to the muscle. This provides more oxygen, FFA's and removal of waste to and from the muscle. Increased relative cross-sectional area of Type I muscle fibres increases the strength capacity of the muscles, meaning more work can potentially be achieved for a lower cost. Increased intramuscular content of triacylglycerol along with an increased capacity to use intramuscular lipid as an energy source during submaximal intensities of exercise. The improved oxygen and altered hormone responses from endurance training work to decrease the rate of utilisation of muscle glycogen, blood glucose and decreases the rate of lactate accumulation.

What is the best way to train to enhance fat oxidation?

I will attempt to answer this in Part 2.

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