Training And Fat - Part 2

How do we 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. Suggestion of a ultraendurance threshold, maybe.

As was demostrated in Part 1, 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 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 as demonstrated in the graph below. 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 ultra-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 is achieved to the detriment of other fitness aspects.

So based on all the above information I plan on following three general, but simple guidelines in to help enhance the endurance aspects of my performance.

  1. Do a large volume at between 55-75%VO2max (60-80%HRmax)
  2. Maintain a predominantly high carbohydrate diet
  3. Perform occasional training sessions with depleted muscle glycogen

"I may not have gone where I intended to go, but I think I have ended up where I needed to be." Douglas Adams


Popular posts from this blog

New Blog: Running Alive

Race Report: Sandy Point Half Marathon

This Is Forty