By Cat Hine

Metabolic testing has been used in nutritional physiology for a number of years to reveal how an individual athlete uses calories. Metabolic testing helps in creating training, racing, and nutrition plans specific for that individual athlete so they can train and fuel in the way that they can best improve.

The athletes most visibly doing metabolic testing in aid of their training? The Norwegian Olympic team.

What happens in metabolic testing?

The athlete will wear a face mask, complete with a breath filter and sensors in order to collect data. Working up to approximately an 85% effort either on a treadmill or turbo trainer, inhaled and exhaled gases are measured in order to identify the fuel sources being used at a given power output or speed. Using the ratio of carbon dioxide produced to oxygen consumed, testing equipment can establish the number of calories used and the percentage of these which come from burning fat and carbohydrates. This means that at any given intensity of heart rate, power, speed or perceived exertion an athlete can calculate the number of calories that they have burned or will burn over a set period of time.

The testing protocol follows the pattern of a standard ramp test.  Intervals are however three minutes long, giving the body time to settle and adapt to each increase in intensity.  The incremental increases in power or speed will be relatively small (25 watts on the bike, or 15s/KM on a treadmill) in order to gain the most accurate data possible.  The test will often be completed in around 20 minutes.

Finding thresholds

Information gained through exercise metabolic testing establishes aerobic thresholds: the point at which the body reaches maximum fat burning capacity, and signifies maximum endurance efficiency.  By working at or below the body’s maximum aerobic efficiency, an athlete can train for longer without reaching fatigue. This is because they eliminate the risk of ‘bonking’ by not tapping into their limited carbohydrate stores (approximately 1500-2000 calories of carb stores available at any one time within the human body).

This aerobic threshold is often reached at about 50-60% of maximum intensity. Without testing the average data and significant margin for error can significantly impact training zones.  This can result in under- or over-training and has significant detrimental impact on efficiency, fitness, and/or strength gains.

Anaerobic threshold can also be identified within the testing protocols.  At anaerobic threshold, the body produces increased levels of carbon dioxide as the body attempts to metabolise a build up of blood lactate.  The body will also consume large amounts of oxygen, which results in rapid carbohydrate utilisation and fatigue. Thus, understanding anaerobic threshold or lactate threshold can be of vital significance for any athlete that wants to pace their race effectively, and avoid ‘hitting the wall’ or ‘bonking’.  Testing also allows you to find how much time is needed to recover and go from anaerobic back to aerobic threshold.

How to use metabolic testing data

Once these data points have been collected, a specific nutrition, training, and racing plan can be created for an athlete.

We can set training zones as a percentage of a maximal number; for instance, Zone 1 is usually 50-70% of maximum heart rate, 65%-84% of lactate threshold, or less than 55% of functional training power. However, training zones for various sports will often be different. For example, individuals can often hold a higher heart rate when running in comparison to cycling. It has also been noted that a tendency to fuel the bike to support the run can result in a more efficient fat utilisation in the run phase of the event. Thus, testing should occur across disciplines and a specific nutritional strategy and training plan put in place for each discipline.

During long endurance rides and runs the athlete can work within their own ‘Zone 1’ or ‘Zone 2’ in order to ensure that they are using fat sources efficiently and not risking premature fatigue. Any increased efforts above Zone 2 will start to burn carbohydrates and will therefore limit endurance.

Interval training efforts can be set to anaerobic threshold limits, in order to help train the body to process lactate as efficiently as possible.  The rest period between intervals (so that the body switches from carbohydrate burning back into fat burning) can also be identified to ensure that recovery is incorporated even within the session.  The specific number of calories required to refuel after a session or during a race can also be established.  This will, again, support recovery and maximise performance.

By altering fuelling strategies prior to different sorts of training sessions, the body will start to physiologically adapt.  This can result in an extension of Zone 2 for endurance athletes, meaning that an athlete can go longer and harder while still only burning fat sources.  Top-end speed and power developments can also be supported through effective and carefully timed carb intake.

A focus on nutrition and training zones as a result of metabolic testing can prime the body for adaptations that will support future goals and enable you to wring out the best performance possible.

Cat Hine is an MX Endurance Race Team ambassador.