A quantitative approach to fuelling performance

Our GPS devices/phones/wearables all give us an indication of how many Kcals we are burning day to day. Let’s assume it is accurate (sometimes a mistake), how can we use this to our advantage of not only knowing our total Kcal expenditure but the breakdown of Kcals into our fuel sources, carbohydrate and fat. This article will help you to understand how to fuel and refuel for your sessions.
Does energy intake matter?
Of course, without energy doing exercise is extremely difficult/impossible. Energy comes in the form of ATP with glycogen (carbohydrates) being the bodies preferred source due to being easily broken down and rapidly absorbed. Unlike fatty acids, our bodies can only store a limited amount of glycogen. This is why many athletes find benefits in being ‘fat adapted’ due to the infinite fat stores we are capable of. Unfortunately fats are difficult to breakdown at higher intensities of exercise (> 70% of maximal heart rate) and the body will solely rely on glycogen stores once aerobic threshold is reached. If glycogen stores are not replaced, intensity of exercise will be compromised by the body in order to avoid itself going to failure. Being fat adapted is favourable when exercising at low intensities, this just means you are able to maintain a faster pace or do greater amount of work at <70% of MHR. This is why low intensity endurance training is important, so you can exercise for longer and build greater endurance adaptations without HR rising enough to favour the glycolytic energy system. Glycogen is costly to burn for the body and results in waste products that must be cleared from the muscles which comes at a further metabolic cost, reducing the bodies aerobic efficiency. However, in order to achieve certain performance targets, exercise intensities will often need to rise above 70% of MHR, limiting glycogen at this point is inefficient to the body for the reasons explained, therefore replacing glycogen stores pre, during and post exercise is favourable.
How do we measure it?
As with the other two macronutrients (Protein and Fat), carbohydrates is measured in grams (g). 1 g of carbohydrates = 4 Kcals and 1 g of fat = 9 Kcals. Further reason why being fat adapted is effective for endurance athletes, you get a lot more bang for your buck when burning fats!
A test of measuring your bodies energy source at different intensity levels is called an RER test (respiratory exchange rate test). When predominantly burning fat, oxygen intake is higher than the amount carbon dioxide expelled, however when predominantly burning glycogen, carbon dioxide expelled is greater than oxygen intake. Therefore the dominant energy source can be known at various intensities. This is a lab based test and although they can be costly they can also be extremely useful. Lactic threshold heart rate (LT) has been shown to coincide with the point at which the body is predominantly using glycogen, we therefore usually use this method to set your training zones and judge your intensity levels during exercise.
Fuel expenditure and regeneration.
When working at 50% of lactate threshold; 45 - 55% of Kcals come from fat, at 75% of LT; 10 - 30% of Kcals come from fat and at or above LT its 0%. We don’t need to worry about fat replenishment from an energy standpoint as we have finite stores. Carbohydrate however needs replenishing. Moderate carbohydrate diets (40 - 50% of total intake) enables the body to store around 1000 Kcals available for lower body exercise. On a high carbohydrate diet (60 - 70% of total intake) these stores can double to 2000 kcal depending on your muscle glycogen storage capacity (usually found to be better in trained individuals). On low intensity days, where you know you have kept HR low and within a fat utilising intensity, you should aim to only replace the Kcals lost through carbohydrate utilisation. Let's look at a relatively low endurance session. If you burn 500 Kcals with 1hr easy running at 70% of LT, an estimated 100 Kcals of that 500 was lost through fat stores which don’t need replacing (20% of 500 = 100). The other 400 Kcals came from carbohydrate stores, meaning you still have roughly 1100 kcal of glycogen stores left. These are estimates, for an accurate breakdown an RER test can be performed at a scientific laboratory. Aiming to replace glycogen stores is the aim of the game, but in this example your body has already taken care of it thanks to the stored glycogen in the lower leg musculature. Continuing to run at 70% of LT for a further hour would total you to 1000 kcal total lost of which 800 Kcals would be from glycogen. Still not leaving you empty, 1500 - 800 = 700 Kcals of glycogen still stored. Continue for a further hour and now you will deplete glycogen stores, this is where fueling your low endurance, longer sessions is beneficial in order to maintain performance and enable you to go further for longer.
Let's look at a higher intensity session. Two hours of exercise at 95% of LT could potentially mean a 2000 kcal hit, roughly 5% of those Kcals come from Fat (100 kcal) and the rest from glycogen (1500 Kcal). Notice the same amount of fat Kcals are used in each session. Hence why for fat loss, low intensity exercise can be more beneficial for people rather than high intensity ’smash fests’. In order to have kept this intensity up, the athlete must find a way to replace lost stores, in this case 400 kcal of glycogen or 100g of carbs. The key to keeping intensity high is to not let the glycogen tank empty, the athlete would not maintain 95% of LT if they let this happen. Timing your intake of carbohydrates is key in a session like this is as 100g of carbohydrate in one hit while exercising will cause GI issues, aiming for 50g per hour is a much better approach to take. Once depletion of glycogen stores has happened intensity will drop and at that point it’s too late. Fueling before energy drop is the key and why a fueling plan is so important.
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By; Tom Walker, Endurance Coach