Energy – How we get it – Where we get it from

During exercise, our muscles mainly use fat and carbohydrates (‘carbs’) for energy. Fat from our diet is stored throughout the body in fat cells known as adipocytes. Carbohydrates are converted to glycogen (a special type of glucose) which is then  stored in the muscles and the liver. At low exercise intensities fat is the preferred energy source - although some glycogen is always used. At higher exercise intensities (around 50-60% of max), fat can’t keep up with the increased energy demands, so our muscles start to use more glycogen. The reason this happens is because glycogen can provide energy much faster than fat. So, while fat can produce energy in much higher amounts, glycogen can produce energy at much higher speeds. This is why carbs are essential during competition or training, when high intensity effort is needed. Only carbs (via muscle glycogen) can provide the energy fast enough!

The other thing to know is that, even at low exercise intensities, carbs are always used for energy production. The ‘conventional wisdom’ that fat is the sole fuel used during low intensity exercise is not supported by research 1. At ‘race pace’ intensity, athletes are ‘burning’ carbs at ~2-3 g/min. However, even during low intensity exercise, although the majority of energy requirements are being supplied by fat, most individuals are still burning carbs at ~1-2 g/minute. Carbs are therefore required during all levels of competition and for both high and low intensity training2. For exercise lasting longer than 1.5 - 2 hours, proper carbohydrate intake (and glycogen content) is crucial.

The important take home message

Be aware that glycogen is consistently used at all levels of effort. Pay close attention to your glycogen levels and be better able to optimize them for maximizing performance and recovery. The ability to monitor glycogen stores ‘live’ via the MuscleSound technology is an essential tool that removes guesswork from the equation.

  1. van Loon, LJC et al. The effects of increasing exercise intensity on muscle fuel utilisation in humans. Journal of Physiology, 536.1, pp.295–304, 2001
  2. San Millán, I. The Importance of Carbohydrates and Glycogen for Athletes. Training Peaks:, 2013

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