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Metabolic Efficiency

METABOLIC EFFICIENCY

What prompted our interest in a low carb high fat (LCHF) strategy for endurance athletes?

Following the metabolic efficiency measurement of a number of endurance athletes in an overnight fasted state we discovered a mix of responses in terms of carbohydrate (CHO) v FAT utilisation during exercise.

Figure 1 is representative of one athlete'’s response. In the group measured we witnessed that even after an overnight fast and 20 mins at very low intensity exercise there was little stimulation of fat metabolism towards energy during exercise. The group included endurance athletes that exercise in a fasted state on a regular basis. Of course the increasing intensity meant that even greater amounts of CHO were consumed for energy.

This got us thinking along the lines that athletes are unable to utilise fat for fuel as well as we'd thought. When we looked at the amount of CHO required to support energy needs during an endurance event the numbers just didn'’t add up.

What happens when we take on carbohydrate during exercise?

As soon as we ingest CHO it is used as fuel bypassing any storage mechanism that might exist post exercise. Prior to a race or a long intensive training session we top up our muscular and liver glycogen storage levels, however this depletes over the course of exercise. We all know that without replacing this on the fly we run the risk of ‘hitting the wall’ or ‘bonking’. Too great an intake of CHO over the length of an IM distance race risks gastric upset. Currently the CHO ceiling is set at 90 g/hr over a 10hr IM, this is 900 g of CHO (allowing 1hr for the swim), a lot of CHO to consume. For an 80kg front of pack male age-group athlete a 5hr cycle leg would utilise 20000 kJ or ~5000 calories over that time. In one participant this equated to a 50:50 FAT:CHO ratio = 135 g/hr from CHO (Figure 1). Notably a 50:50 ratio at race pace is a great example of fuel efficiency, note also that the caloric expenditure is just the bike portion where you are likely to be taking in as much food as you can. Once you start doing the maths it is not hard to see why we need to have a better fat utilisation in long-term endurance performance.

What other possible performance benefits are there to be gained from LCHF?

Consumption of CHO for energy produces greater ‘fumes’ - this is measured in an exercise test as ventilatory carbon dioxide or VCO2. The higher the CHO usage, the greater the amount of VCO2. Fat burns with a lower cost of emission meaning that the build up of CO2 in the body is lessened. This slower increase in VCO2 appears to reduce the build up of blood lactate, possibly indicating a reduced acidic response within the muscle. From both ventilatory (breathing response) and metabolic (blood lactate) standpoints we see improved economy.

What will eating a LCHF diet do to my glucose, cholesterol and triglyceride levels?

The great thing we are seeing in ‘healthy’ and ‘at risk’ populations is generally a lowering of blood glucose levels and other measures associated with elevated cardiovascular risk levels. Significantly, even endurance athletes are at risk of developing lifestyle related diseases such as Type II and elevated chances of cardiovascular risk despite extensive training. A large part of the diet is simply removing refined CHO from your diet, i.e. sugar and high glycaemic index foods in which sugar is present.

How will a metabolic test (MET) help shape my diet strategies and performance?

Based on your initial MET results you will get an idea of your metabolic profile as in our athlete'’s example above. From there it is recommended that you make an appointment with a nutritionist as this is essential to help you on the track to balancing your dietary intake. A subsequent MET will allow you to lock in your nutrition plan around your latest results. The measurements performed in the lab will transfer to race day. As the majority of IMs are raced (or should be raced) at a steady state, fuel utilisation is reasonably predictable.

Interested in finding out more about your own metabolic efficiency and how to improve it?

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Photo credits: Triathlon New Zealand
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