Training and Fat
In order to appreciate the intent of training a few simplified concepts need to be understood. Exercise intensity influences the contribution that carbohydrate (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.
|From Go Hard|
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.
|From Go Hard|
In reference to fat oxidation the goals of training could be:
- Increase the intensity at which the cross-over point occurs
- Increase the intensity at which the maximal rate of fat oxidation occurs
- 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?
The simple answer seems to be to do a lot of training in the intensity zone of 55-75%VO2max, based on where the levels of fat oxidation are highest. Many athletes and coaches have advocated training at this level. Gordo Bryn and Mark Allen both have their guidelines that essentially achieve this. The problem I see is that this is a very large range of intensity to train at, and begs the question of are there important differences within this zone? To put it another way, does training at 55%VO2max give a different result to training up near 75%VO2max?
The answer depends a lot on your current fitness level and fat oxidation abilities. Less fit means lower fat oxidation. A perfect example of developing the ability to utilise fat as a fuel throughout a career is Mark Allen. Over a fifteen year career, he has documented the speed at which he runs what he terms is maximal aerobic speed. Over the years he increased this speed from 4:05/km to 3:19/km.
As was demostrated above, there is a point at which there is a sharp decline in the oxidation of fat. A reason often given for this, is that the rate of energy required exceeds the body's ability to utilise it mainly from fat stores. The theory sounds reasonable at first glance, and it has long been accepted as the main reason. The problem is that it would be in the body's best interest to keep burning fat while topping up the supply from the various carbohydrate stores. This doesn't seem to happen. There must be another reason to explain the significant reduction in fat oxidation at about 70-80% VO2max. What else occurs around this intensity and is affected by fitness level?
The answer is an increase in intramuscular and blood lactic acid levels. It is often termed the anaerobic or lactate threshold, however, it should be noted that most measures of this threshold give an intensity higher than when fat oxidation reduces on an individual basis. It appears that increased levels of lactic acid inhibit fat oxidation (down regulates oxidative phosphorylation). This is important for a couple of different reasons. Firstly it helps provide a guideline and objective measure to training status and the upper intensity limit when aiming to train for fat oxidation. Secondly it helps to provide another reason for sensible pacing in long course racing. If increased lactic acid levels inhibit fat oxidation, then it suggests that those short, but intense bursts throughout the race may be detrimental in the long term. An important element in choosing race pace strategies should be to take into account aiming to minimise lactate accumulation.
How do we decide on which intensity to train at?
The theorectical answer is to train at the intensity right at or just below where there is a large decline in fat oxidation. That is, train at the speed that limits lactate accumulation, or below the lactate threshold. The theory is idealistic and is difficult to be precise in practice. The problem is there are a number of other factors that affect lactate levels and other components of fat oxidation that come into play.
One important factor to take into consideration is that fat oxidation varies over time. For example if an athlete was to ride at exactly 70% of their VO2max, the contribution from fat oxidation versus carbohydrate sources will vary as the exercise session progresses. Generally speaking, as time progresses, fat oxidation increases.
How does diet affect fat metabolism?
Is it possible to increase the body's ability to utilise fat during exercise by eating a fat adaption diet? The short answer is yes. Eating a relatively high fat, low carbohydrate diet over a few days does lead to an increase in fat utilisation and a reduction in the rate of muscle glycogen usage. This suggests that with the increased fat metabolism, there is a sparing of carbohydrate stores, which in theory should translate to improved performance in ultra-distance racing. The research has not been able to show any performance benefit. In fact, a fat adaption diet may actually lead to decreased performance. There appears to be number of reasons for this, one of the obvious is that athletes report extreme difficulty in maintaining their normal training volume and intensity when on a high fat, low carbohydrate diet. Another important reason, is that what was initially presumed to be a sparing of muscle glycogen , may actually be a reduced ability to use these carbohydrate stores.
It has been proposed by a number of people that starting long sessions with depleted carbohydrate stores may improve the body's ability to utilise fat. This is still controversial as the majority of scientific evidence supports that a high carbohydrate diet leads to a superior training response during high training loads. However, recent research has demonstrated that commencing exercise with depleted muscle glycogen leads to enhancement of the transcription of a number of genes involved in training adaptations. It is thought that several transcription factors have glycogen-binding domains and when muscle glycogen is low these factors are released, becoming free to associate with certain proteins. The suggestion is that a cycling of muscle glycogen stores may lead to improved endurance response. At this stage there is no real evidence of performance benefit in trained athletes with this approach.
Should enhancing fat adaptation be the primary goal?
The simple is no. Many people seem to make the incorrect assumptiom that endurance events only involve submaximal performance. This leads to the interest in increasing fat utilisation and conserving carbohydrate stores. It needs to be remembered that this is only one aspect to performance. Racing and training will involve at times much higher intensities and different energy pathways. Taking steps to purely enhance fat utilisation may actually harm race day performance if achieved to the detriment of other fitness aspects.
Based on all the above information I have incorporated on following three general, but simple guidelines in to help enhance the endurance aspects of my performance.
- Do a large volume at between 55-75%VO2max (60-80%HRmax)
- Maintain a predominantly high carbohydrate diet
- Perform occasional training sessions with depleted muscle glycogen