Exercise While Fasting: For whom, How and When

Exercising during a fasted state increases lipolysis in adipose tissue, while stimulating peripheral fat oxidation, resulting in increased fat utilisation and weight loss, if that is what you require at this time.

With athletes, people who exercise regularly or who are close to their ideal weight, the effects may be different and perhaps not as beneficial.

Today we will look at which people can benefit from training in a fasted state, what types of exercise are most suitable and how certain times of the day can increase the beneficial effects of this practice.


People who can benefit from fasting training and how it works

People who are sedentary, overweight, as well as those who want or need to lose stored fat. Also, people who are starting or training 3-5 times a week and must combine it with long hours of office work, are those who can benefit from this practice. Let's see what happens inside the organism.

When you have ingested food less than 4 hours before exercise, your blood glucose will be high, so your body will prioritise the use of glucose for fuel when you exercise.

In a fasted state, on the other hand, blood glucose levels are regulated, so training in a fasted state will force the body to burn fat because it is trying to preserve stable levels of muscle and liver glycogen.

This situation is very favourable for the types of people mentioned above because exercising on an empty stomach effectively helps you burn more fat while you are physically active, as it is the next best source of fuel after glucose (which your body doesn't have at that point). But that does not mean you are losing fat. 

It must be understood that it is the calorie balance that determines whether you lose or gain weight. The fact that you burn more fat for fuel does not mean that you are in a calorie deficit. People who want to lose weight may wonder why then exercise in a fasted state.

Metabolic flexibility

A metabolically flexible state exists when there is a rapid shift between glucose and fatty acids during the transition between the fed and fasted state. This flexibility in fuel choice serves to avoid hyperglycaemia after a meal and, at the same time, ensures that there is an adequate amount of blood glucose for the brain and exclusively glycolytic tissues during fasting.

That said, a regular practice (2-3 times per week) would eventually make you more sensitive to insulin.

Natural Stimulation of Human Growth Hormone (HGH)

According to a study published by the US National Library of Medicine, exercise with brief, well-defined HGH peaks leads to a phosphorylation of differentiated STAT5 (a protein that functions as a signal transducer and activator of gene transcription in the nucleus of cells.  It is involved in processes of proliferation, immunity, apoptosis, and cell differentiation); and subsequent IGF-I gene expression, whereas fasting induces more sporadic MBF bursts and a less differentiated but more persistent activation of MBF signalling.

This explains why the combination of exercise with fasting has more robust effects on the stimulation and duration of growth hormone in the body than either exercise or fasting alone.

Human growth hormone regulates metabolism and electrolyte balance; it increases lipolysis and reduces fat storage in the body.  It increases protein uptake and maintains the body's strength and muscle mass. 

Muscle mass

The truth is that weight training on an empty stomach is not as detrimental as many claim. A strength training session, even when performed during a fasting period, will still allow you to gain muscle.

The truth is that your body has the ability to store a lot of carbohydrates, which can be used to fuel high-intensity exercise, such as high-intensity interval training (HIIT) or endurance training.

Depending on how much muscle you have, your body can store more than 400 grams of glycogen, which is the name for the carbohydrates stored in your muscles and liver. It is not that your glycogen stores are depleted simply by fasting.

To gain muscle, you would need to consume about 250-500 calories in addition to your basic needs. This amount varies depending on your body's ability to build muscle, as well as age, training status, type of training and your genetic heritage, primarily.

Mitochondrial genesis

Restricting carbohydrate intake has been shown to promote mitochondrial production. The reality is that the more mitochondria the body has, the faster it can burn both carbohydrates and fat.


Types of training

Whether you follow a 5/2 protocol (eat for five days, fast for two), or 16/8 (fast for 16 hours, eat for eight), or any other version of intermittent fasting, most people who follow such a dietary protocol end up losing weight.

This is because it is much more difficult to overeat if all the daily calories are concentrated in an eight-hour window than if they are spread over 15 or 16 hours. This is what makes fasting and intermittent fasting such an effective tool for weight loss, because by restricting the time in which you can eat, you also restrict the number of calories you take in.

Now, weightlifting, high-intensity interval training or sprinting depend on good fuel sources such as carbohydrates, so these activities during the fast or at the end of the fast work, but not 100%.

One option is to do your cardio, jogging, or running at 60-70% of your maximum capacity, as well as Yoga or Pilates.  While strength exercises such as weights, high intensity intervals, as well as sprinting or long runs, are more effective after breaking the fast.

Not everyone has this possibility, but my point is that it is better to exercise less intensively for the time you are fasting and the more intensively  after the fast is over, if you are interested in increasing muscle mass or athletic performance.


Training time. Yes, it also has an effect.

According to experiments so far, a person is considered to be in a fasting state when they have not eaten for 4-6 hours.

Training on an empty stomach first thing in the morning is not a problem because, even if you are fasting, your body is rested from the night's sleep.

If you train at the end of the day, it is not as powerful if you have fasted all day. The longer the fasting period is prolonged, the more likely it is that your performance will suffer. If you leave it until the afternoon or evening before training, you won't be able to lift as much weight or do as many reps.

Over time, this decrease in strength means that the growth stimulus provided by a given workout will be weaker than it would be. As a result, muscle will be gained more slowly compared to doing the same training session with a few meals in.

Considering the type of fasting and the time of training, some adaptations can be made.

If you train in the morning, there is no problem with breaking the fast after training.  If you train towards the end of the afternoon, you can do what is called early food restriction whose protocol allows you to consume your calories during the morning and start your fast at about 14h or maximum 16h -17h.

This is a relatively new and interesting approach, because it has proven to be very effective in comparative experiments conducted to date for weight loss, fat loss and cardiovascular health. (article)

The other advantage of training in the afternoon hours is that the body is wide awake and in a way the organism and muscles are naturally "warmed up", which provides a good body readiness for training.


Takeaway Message

Can training on an empty stomach be beneficial?

Yes, it is fine to exercise on an empty stomach and you can do almost anything in a fasted state, and that includes lifting weights.

However, the quality of the workout may be compromised to some extent, depending on:

- The type of exercise you are doing

- The duration of the fast (i.e., alternate day fasting, 5:2 fasting, or time-restricted feeding).

- Whether it is during the fast or after the fast is broken

The important thing to consider when exercising while fasting is whether you need weight loss, fat loss or alternating with long hours of desk work.  In these cases, the combination of fasting and exercise is phenomenal.

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