Fasted Training

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Everyone wants a hack to fat loss and to speed it up by trying to optimize every little thing. One of the most popular hacks is to perform fasted training. This NutriWiki covers this topic and will tell you if you really need to skip breakfast before the gym.

Fasted Training


  • Fasted and fed training generally have similar capabilities for fat loss. For most people, it’s a matter of personal preference.
  • There may be benefits for fasted cardio among very lean individuals looking to diet down further.


To start, let us define fasted and fed training:

  • Fasted training refers to exercise after a period of food restriction, or fasting. This is frequently done in the morning, before the first meal of the day.
  • Fed training refers to training in a fed state. This can be any sort of training that takes place sometime after a meal.

To understand their impacts on fat loss, it is important to remember the energy equation in general: To lose fat, there has to be a greater net energy expenditure than energy intake.

In particular, the key term here is “net.” The body’s fat stores are constantly changing, and never a static amount. You are constantly gaining and losing fat as the day progresses, based on the energy you take in via food, and the energy you expend via activity and bodily processes. It is the sum of those energy exchanges over time that dictate fat loss and gain.

To illustrate, let us assume that a person has a maintenance level of 1800 calories, and that a single bout of activity burns 300 calories, and that they like to eat 3 meals a day, each at about 600 calories apiece. Now let us compare scenarios where that person trains fasted vs. fed.

In a fasted state, that individual has not taken in any calories yet. Therefore, in terms of energy intake and expenditure, their daily balance is at 0. They proceed to exercise, and burn 300 calories. That energy has to be pulled from the stores in their body (Although this will pull from both glycogen and fat stores, for the simplicity of this example, we’ll combine them). Therefore, their balance post-exercise is -300. As the day goes on, they eat their typical 3 meals a day, then they head to bed. In summary:

  • 0 calories to start
  • Exercise (-300)
  • Eat 3 meals (+600, +600, +600)
  • Daily total: 1500
  • Daily total (1500) – maintenance calories (1800) = -300.

This person ended with a 300 calorie deficit for the day, which ends up being pulled from their fat stores. They lost fat.

Now let’s do a fed scenario. Again, this person starts with a daily energy balance of 0. They eat breakfast, then go and exercise. Afterwards, they eat lunch, dinner, and head to bed. Here’s the summary for the fed scenario:

  • 0 calories to start
  • Breakfast (600)
  • Exercise (-300)
  • Other two meals (+600, +600)
  • Daily total: 1500
  • Daily total (1500) – maintenance calories (1800) = -300.

Again, they ended their day in a 300 calorie deficit, and thus lost fat. In the grand scheme of things, both approaches were equivalent for fat loss. Therefore, either approach is equally viable when dieting, and ultimately boils down to personal preference.

Additional Considerations

For the majority of folks, fasted vs. fed cardio is a matter of personal preference. However, there are a couple of situations where fasted training may have some additional benefit. The qualifier is that these scenarios tend to apply to already lean individuals who are looking to diet down even further. Therefore, their applicability is limited to the general population.

First, lean individuals may receive a bit of additional benefit from cardio in a fasted state. As a person leans out, fat mobilization becomes more difficult for a variety of different physiological reasons, including inhibited bloodflow and hormonal changes. Training in a fasted state may help mitigate some of these adaptations1.

Additionally, certain supplements such as yohimbine can assist with fat loss by inhibiting certain receptors in adipose tissue2. However, this compound only works in the relative absence of insulin. Therefore, its benefits are only realized in a fasted state. Since yohimbine is generally used to assist with the last bits of stubborn fat, it is not a supplement recommended for the average dieter.

Further Reading

Yohimbine on


  1. Gjedsted, J. , Gormsen, L. C., Nielsen, S. , Schmitz, O. , Djurhuus, C. B., Keiding, S. , Ørskov, H. , Tønnesen, E. and Møller, N. (2007), Effects of a 3‐day fast on regional lipid and glucose metabolism in human skeletal muscle and adipose tissue. Acta Physiologica, 191: 205-216. doi:10.1111/j.1748-1716.2007.01740.x
  2. Lafontan M et. al. Alpha-2 adrenoceptors in lipolysis: alpha 2 antagonists and lipid-mobilizing strategies.Am J Clin Nutr. 1992 Jan;55(1 Suppl):219S-227S.

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