Energy Expenditure and Fueling During Endurance Exercise

Energy Expenditure and Fueling During Endurance Exercise

As covered in our previous blog about the Evolution of Endurance Sports Fueling, recommendations for carbohydrate intake have seen a progressive increase from 60 grams per hour to 90 grams per hour. There is even published evidence that some individuals can consume and burn 120 grams per hour with anecdotes of those able to consume 150 grams (or more!) per hour. 

However, does this high carbohydrate consumption rate apply to everyone? Should an individual who is only weeks or months into their endurance sport journey fuel like professional athletes? As with most recommendations in science and sport, the answer is always… it depends. There are many factors to consider when determining how much an individual should fuel such as training status (fitness), exercise intensity, diet, etc. 

To answer this question, we will walk through three elements in a two part blog series. In this blog we will address:

1) Determining how much energy you expend

2) In what form (carbohydrates and/or fats) is this energy being expended.

In part 2, we will address: How much you would need to consume to maintain performance.

Relevant Components for Understanding Energy Expenditure during Exercise

  • Exercise intensity (Note: think power output on the bike or running speed)
  • Exercise efficiency and economy (Note: this is typically very similar for most people on the bike ranging anywhere between 20-25% efficiency. Running economy is vastly different among individuals, which makes energy expenditure estimates much more challenging for runners compared to cyclists, if you are not making direct measurements.)
    • Training status/fitness
    • Respiratory exchange ratio
    • Grams of carbohydrates and fats used by the body during exercise

    Determining Energy Expenditure During Exercise

    Determining the exact amount of energy you expend during exercise is incredibly difficult and requires precise laboratory equipment. Even then, there are assumptions (such as economy and efficiency) when using these laboratory measurements to determine the total amount of expended energy. So, while the amount of energy and the form which the energy is derived from (fats or carbohydrates) can be reasonably estimated, it is still not 100% accurate. 

    Regardless, the best and most practical way to estimate the amount of carbohydrates and fats expended is through a method called indirect calorimetry. This method measures the amount of oxygen you consume and the amount of carbon dioxide you produce during exercise can be determined. 

    A ratio, called the respiratory exchange ratio (RER), of carbon dioxide to oxygen can provide a close estimate of the ratio of carbohydrates to fats you burn during exercise. This ratio spans between 0.7 to over 1.0. The lower the ratio, the greater the amount of fats that are burned. Conversely, the greater the ratio, the more carbohydrates are burned. At rest, most individuals are around 0.8 and can increase to over 1.2 at very high intensities. The exact amount of fats and carbohydrates (in grams) can be determined using an equation proposed by Asker Jeukendrup

    Now, how can this information be used for athletes? Below is a hypothetical case to better illustrate the concept discussed above. 

    Exercise Intensity

    Let’s say there is a cyclist exercising at 150 watts for one hour. This equates to an energy expenditure of roughly 11 kcal/min. For most people, cycling at 150 watts will require your body to consume roughly 2.1 liters of oxygen per minute and can be measured via indirect calorimetry. There can be some variation between individuals (and even variation day to day within the same individual) due to physiological variability and biomechanical efficiency. The amount of carbon dioxide produced is much more variable, but needs to be measured by indirect calorimetry to establish the RER ratio. 

    Training Status

    The sources of fuel required to exercise at this intensity will vary depending on the individual and is closely tied to training status. The more trained an individual is, the more fats they will utilize as energy and the lower the reliance on carbohydrates. The opposite is true when someone is less trained. In terms of RER, the more trained an individual is, the lower the RER will be (e.g. 0.90) compared to being less trained where RER may be greater (e.g. 0.95); the greater the exercise intensity the more carbohydrates are used and the less fats are used.

    Amount of Fuel Used

    With this information, the amount of carbohydrates and fats utilized per minute of exercise can be determined. Keep in mind one gram of carbohydrate contains ~4 calories and one gram of fat contains ~9 calories. Based on the equation proposed by Jeukendrup, at 150 watts and an RER of 0.90, carbohydrates are oxidized at a rate of ~104 grams per hour, while fats are oxidized at a rate of ~21 grams per hour. At an RER of 0.95, carbohydrates oxidation increases to a rate of 131 grams per hour, while fat oxidation decreases to ~10 grams per hour. As you can see there is a significant amount of carbohydrates being used per hour at 150 watts. 

    Effects of Higher Exercise Intensities

    It comes as no surprise that at higher intensities even more carbohydrates will be oxidized, which can easily exceed the upper limits of reported carbohydrate absorption and oxidation rates by the body. These values are in line with what is reported in the literature by Podlogar et al. and Hearris et al. While this is an estimate of the total carbohydrates used in one hour at 150 watts, it is challenging to know the sources of these carbohydrates (e.g. muscle glycogen, liver glycogen, newly generated glucose molecules, and/or blood glucose). 

    The Carbohydrates Sources

    Since the carbohydrate used during exercise can come from various sources, it is challenging to determine the exact origin without laboratory techniques. Nonetheless, it is well documented that muscle glycogen and blood glucose provide the majority of the body's carbohydrate sources during exercise. Thus, it is critical to consume carbohydrates as there are limited muscle glycogen and blood glucose stores. As most of us know, once these stores become depleted, performance will quickly decline. 

    How Much Should I Fuel?

    Thus, the next logical question is: “How much carbohydrates should be consumed during training and racing to maintain performance?”. As you may have guessed by now, it is nearly impossible to replenish the energy you expend during training or racing, especially at intensities experienced during a 3+ hour race, for example. Regardless, you should fuel according to the demands of your exercise, keeping in mind the duration and intensity. If the intensity is low and duration is short, then fueling is perhaps less necessary. If the intensity is high and/or the duration is long then fueling up to the maximum recommendation may be more favorable for maintaining performance. In Part 2, we will address how this information can be used to help support fueling guidelines.

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