Should you be training on empty?

We often hear that it is important to eat carbohydrates before a workout in order to have enough energy. But we also hear that training fasted can have benefits from improving performance to helping with weight loss. So which camp should we believe?

What is fasted training?

Fasted training is exercising with low stores of carbohydrate in the body. This is achieved by excluding carbohydrate intake before training and is typically associated with not eating for several hours before exercising, typically in the morning when your last meal was dinner.

Carbohydrate in the food that we eat in the days and hours before exercise is converted to glycogen and stored in our muscles and liver. This is used as energy during short and high intensity exercise. For workouts of a longer duration, our fat stores provide most of the energy. Consuming food directly before exercise will not be stored as glycogen but will influence the fuel mixture your muscles burn during the workout.

Can fasted training improve sports performance?

Research into the effect of fasting on performance remains unclear, some studies suggested no effect while others report a decrease in performance (1). Further research suggests the type and duration of exercise is important. Intake of carbohydrate before a workout can improve performance for longer, but not shorter duration exercise (2,3). For low intensity exercise such as yoga, walking or slow running a pre-workout meal is less important as the overall calorie burn is lower than, for example, a high intensity workout. The same applies to weight training (high intensity but intermittent activity) as this places a smaller demand on glycogen stores and the energy expenditure is lower than continuous exercise.

However, for longer and higher intensity workouts, a pre-workout meal is likely to be important and fasting beforehand should be avoided (1). Consuming a high carbohydrate meal or snack before a high intensity workout, longer than 60-90 minutes, for example a hard cycle or run, can help to increase endurance. The more intense the exercise, the more the body uses carbohydrates as energy. This is because at a high intensity, fat is less effective as a fuel source than carbohydrates. The intake of carbohydrates will also help to raise blood glucose, helping to delay fatigue.

Eating pre-workout can additionally help to keep hunger levels down throughout the session.

A 2018 systematic review highlighted that pre‐exercise feeding strengthens prolonged aerobic performance (3). Evidence of improved performance includes an increase in VO² max, how much oxygen the body can absorb and use during exercise. This was observed during a 6 week study where individuals who ate carbohydrate before training had a significantly greater increase in VO² max compared to those that fasted.(5). Similarly, consumption of a carbohydrate meal before a treadmill run at 70% of VO² max improved endurance running capacity (4). Combining this with a carbohydrate-electrolyte solution during the workout improved endurance capacity further.

Fasted training and weight loss

Fasted training has recently became more popular as a weight loss strategy with claims being made that it helps to ‘burn more calories’. The theory behind this is that fasted training encourages the muscles to burn more fat and less carbohydrate for fuel.

Indeed, one study found that in a sample of 12 male volunteers, individuals who exercised on an empty stomach burned up to 20% more fat than those that had breakfast during a moderate-intensity run (6). Another found that exercising in a fasted state burned more fat and increased the capacity of the muscles to burn fat in preference to carbohydrate compared to individuals that consumed carbohydrates before and during workouts. This was in a sample of 20 physically active males over a period for 6 weeks. (7). It should be noted that these studies are limited to male participants and only show effects in a small sample of individuals.

This doesn’t necessarily mean overall weight loss will be achieved or that it will be achieved quicker. Another study of 20 female subjects found no significant differences in weight loss between a group which trained fasted and a group which trained after consumption of a meal (8). This was following 1 hour of steady-state aerobic exercise performed 3 times per week across 4 weeks. Another review suggests there is little evidence indicating enhanced fat burning capacity following long-term fasted-state training (2).

To lose weight we have to be in a calorie deficit, energy intake or calorie intake needs to be less than energy used or calorie intake. This also needs to be maintained over a period of time for example days or weeks. Furthermore, weight loss is extraordinarily complex. Factors including sleep, stress, genetics, lifestyle and socioeconomic status all come into play, to name only a few. As an example, in the above study were weight loss was observed (8), participants were provided with nutritional counselling and supervised throughout the study. There was more than just fasted training to the effects.

Ultimately changing body composition is determined by overall energy balance. What we eat, or don’t eat before training only determines what we are using to fuel a session.

Individual variability

It is important to point out that there is a lot of individual variability when it comes to training fasted. For individuals that train on an evening, fasted training will be tricky to implement. Additionally, those that are really hungry on a morning may find restricting food until after a workout difficult. On the contrary for those that workout early in the morning and have no appetite, fasted training may be convenient.

If training fasted is something you wish to do, especially early in the morning, make sure to have an adequate meal containing plenty of carbohydrates, protein and healthy fats the night before. Try to stick to low or moderate intensity workouts for fasted sessions too. Also make sure you’re doing it because you feel it benefits you, works for your routine and makes you feel good.

If training fasted makes you feel lightheaded or hungry during your workout, you absolutely don’t need to do this, consider having a snack or small meal beforehand. If you chose to eat and feel nauseous or uncomfortable, think about changing or reducing the size of the meal before training fasted.

Let’s summarise

Fasting before a workout may be a convenient option for lower intensity or intermittent workouts. However consuming carbohydrate before a high intensity endurance workout is likely to be advantageous. Exercising fasted is not essential to lose weight or burn calories. You will get the same effects from moving your body whether you have eaten beforehand or not. It doesn’t make your training any more superior. The difference lies with what we are using to fuel the session if we are doing it fasted or fed.