by Bevan McKinnon, image courtesy of Seven8Digital

Ed. note: Bevan McKinnon, of Fitter Coaching and Fitter Radio, joins us again to discuss training, racing, and nutrition. The early part of the year is always a time when athletes focus on aspects of their approaches to sport that they may have neglected in the previous season. McKinnon is an expert in the field of metabolic efficiency, and he joins us today to demystify some of the principles of that oft-misunderstood term.

When looking at the ‘base’ phase of your training year most athletes are considering building their endurance. For many athletes they only think about the muscular and cardiovascular stress but in reality, successful endurance training is mainly about improving firstly at the metabolic level. It has been recognized that the facilitation of fat metabolism is important for endurance performance and the health-related aspects of exercise. Now before undertaking your base training also remember that each athlete is DIFFERENT! We have DIFFERENT metabolisms and physiology and therefore different parameters and potentially different training zones. It’s important to know the metabolic adaptations that occur whilst exercising at different intensities. In absolute terms, carbohydrate oxidation will increase proportionally with exercise intensity, whereas the rate of fat oxidation will initially increase but will decrease again at high exercise intensities.

Basics of Biogenetics and Muscle Metabolism

The capacity of an athlete to exercise ultimately depends on their ability to transform chemical energy into mechanical energy. Skeletal muscle needs to synthesize Adenosine Triphosphate, or ATP, for muscle contraction. ATP is responsible for the energy processes in human cells. ATP generation is achieved by two mechanisms - anaerobic and aerobic metabolism. Fats and carbohydrates (CHO) are the two substrates primarily used to create ATP. The degree of exercise intensity or metabolic and physiological stress, as well as muscle fibre recruitment patterns, will dictate the level of contribution from each energy system (anaerobic or aerobic) and the degree of fat or CHO that is activated.

A good place to start when wanting to improve your metabolic efficiency (optimised fat burning) is to know that at low exercise intensity the body will “attempt” to oxidise fat preferentially from carbohydrate. However, it’s also important to know that recent metabolic studies have shown a wide variance of fat oxidation from athlete to athlete at low exercise intensities. Factors such as fitness level, sex, exercise duration, age, as well as available liver and intramuscular fat and glycogen content, influences oxidation of metabolic fuels during exercise. We’ve also come to find that both acute and chronic dietary habits play a major role as well. For many athletes approaching a base training phase they may simply choose to follow a few well known rules that will help to promote a fat burning environment.

Want to go super long, like Kanza-long? Improve your metabolic efficiency. Photo courtesy of LiFT Creative Studios

For most athletes this means training at an intensity around Zone 2 (in a 5-7 zone system) heart rate or power. This can be equated to a steady aerobic effort. You should be able to maintain a conversation whilst maintaining an even pace. Very generally this intensity is around 85% of threshold HR and 65% of threshold power. A good rule of thumb for most athletes who want to try improving their ‘fat burning’ is to perform prolonged low intensity exercise with low or no fueling.

Alternatively, you can have your metabolism tested so that you have a more accurate idea of your personal metabolic efficiency and therefore your optimal fat oxidation rates. This can be performed at many Human Performance or Sports Science labs that offer metabolic testing. When looking to target improved fat burning, sports science has defined the exercise intensity at which maximal fat oxidation is observed as FatMax. The FatMax test can show the exact intensity range at which your body is using the maximum amount of fat as fuel. The optimal FatMax intensity for each athlete will differ. Studies have shown that FatMax on average occurs at around 65 percent of maximal heart rate, it can also be as low as 50 percent and as high as 80 percent. This is why it may be important to consider individual testing in order for the accuracy of your training to improve. I prefer to use Inscyd to perform metabolic profiling with my athletes as I can test in both a face to face and remote coaching settings to establish FatMax. If you'd like to read more from Inscyd on the subject, you can do so here.

Workout Execution is King

From a practical perspective we can then focus on workout execution to learn how to optimally ‘train’ an athlete’s FatMax. We must remember that the body draws on all energy systems during exercise, regardless of the type of effort, never closing one off completely. They merely change in the percentage and amount of energy they contribute depending on the duration and intensity of the effort and also on the metabolic profile of the individual.

The anaerobic system is the ‘opponent’ of aerobic metabolism. Yes, it primarily contributes significant energy to short intensive workloads, but it also impacts the metabolic fueling of longer endurance efforts – and therefore also the metabolism of fat. In long distance triathlon the ideal involvement of anaerobic metabolism would suggest that the lower its overall contribution the better.

The anaerobic system can still be at work within the human metabolism even when an athlete is exercising at lower intensities. Therefore, the more your body activates the glycolytic process the more it burns carbohydrates during exercise and this will trigger a higher lactate production. If lactate is present then it inhibits fat burning. simply put, the more lactate, the less we burn fat. For example, let’s say your athlete is performing an intense interval training session. During the high intensity effort, lactate will accumulate. In the recovery phase their intensity may be at the level where lots of fat would usually be combusted (zone 1-2). However, in the recovery phase between or after high intensity intervals, fat combustion is low. Instead, the preferred fuel for muscles in this period is the accumulated lactate. The same is true during training intended for ‘fat burning.’ Any time lactate is accumulating during exercise fat will be pushed out of the aerobic metabolism. Until muscular lactate levels decrease again fat combustion will stay lower. All of this means that it is of utmost importance to avoid short, high intensity bouts which cause high lactate levels during training which is meant to be endurance/FatMax focused.

Be careful with high intensity indoor sessions at this point in the season, as it may complicate what you're trying to accomplish

So, when we’re conducting workouts that are designed specifically for improving the metabolic mechanisms that zone two training elicits, make sure you avoid high intensity efforts that create inappropriate lactate production.

As a coach it’s a common occurrence to look at the execution of workouts using both power and HR data. This is where your training data and specific power files can help you understand how you’re executing your training sessions. There are many occasions where the goal of a FatMax workout can be undermined. I see it happen often in group training environments when the dynamics of the workout become highly variable as pacing is influenced by other athletes or when the terrain forces the athlete into high intensity ranges. Heart rate is slower to respond to intensity changes whereas power can pick up this information straight away. Below are two power files for a prescribed 4-hour endurance FatMax ride. You can see that the second graph (fig. 2) had the athlete spend much more accumulated time around the appropriate intensity range (zones 1, 2, and 3) than the athlete in the first graph (fig. 1—lots of time in zones 4, 5 and 6!) The second graph would have seen the athlete accumulate more FatMax time and therefore improved metabolic efficiency. An easier ride but a better metabolic outcome!

Figure 1

Figure 2

So, when looking to improve your metabolic endurance make sure you understand how intensity affects your fuel use and that you don’t let inappropriate intensity impact the development of your FatMax capabilities. At this point in the year, lower intensity is probably better!