Knowledge Base Nutrition

Fasting training and weight loss: Truths and Myths

Training on an empty stomach is often presented as a miracle cure for losing weight and increasing performance. But exercise on an empty stomach can only be effective if you do it the right way. Above all, you need to know your optimal intensity range. Many people exercise at too high an intensity. This training method is only moderately effective for losing fat deposits.




Colin Glattfelder, sports and exercise scientist and prevention consultant, Medbase Checkup Centre.



Here is what fasting training means

Training on an empty stomach means that the carbohydrate store – one of our main energy reservoirs – is almost or completely empty. This happens when you have not eaten for several hours or start exercising in the morning before breakfast. In this state, the body can only resort to a little rapidly available energy, in the form of sugar. It therefore tries to make more energy available through fat reserves, our second largest energy store. This is precisely the desired effect of fasting training.

Only relatively suitable for losing weight

Although fasting training draws on fat reserves, you do not necessarily lose weight. You need to have a negative energy balance, i.e. consume more calories than you take in. For fat metabolism to be activated (also called fat oxidation, fat flow or lipolysis), moderate or low-intensity exercise is required.

In this zone, there is enough oxygen to prevent an increase in lactate (a by-product of energy production). Thus, you are in the aerobic zone. When the oxygen capacity reaches its limit, you enter the anaerobic zone. Therefore, when you are fasting, you consume less energy in the same amount of time than when training at a higher intensity. Consequently, more time must be invested in burning the same number of calories.

The benefits of training on an empty stomach

It has been shown that if you exercise regularly on an empty stomach, your body can access fat metabolism more quickly and supply energy more efficiently, because more mitochondria (the energy ‘factories’ in cells) are formed. According to studies, this works for 90% of people. For competitions in endurance sports, this starting condition is naturally an advantage. In fact, for professional athletes, fasting training helps improve the ability to use fat as fuel and, indirectly, is a valuable tool for increasing endurance and boosting performance over long distances.

How to find the optimal training intensity

Fat metabolism reaches its optimum at around 65 % of maximum oxygen uptake capacity. This depends on fitness and genetic prerequisites. Performance diagnostics, such as those offered by Medbase in its sports medicine centres, are suitable for discovering one’s personal range. Spiroergometry, for example, measures oxygen uptake and carbon dioxide release. This allows to draw conclusions on the proportions of fats and carbohydrates from which energy is derived during training.

The pulse measurement method is even simpler: if the heart rate is 65 to 75 per cent of maximum frequency, you are usually in a range where a lot of fat is metabolised in proportion. This is the optimal intensity regime, and you should feel like you can hold out for at least an hour in terms of energy and still speak in complete sentences.

When you start increasing the intensity too much

You should not exceed this intensity range or reach the Z3 power zone. Training at too high an intensity necessarily requires sugar for energy. If the body cannot supply the energy it needs via fat metabolism, it enters a state of stress. This can lead to a breakdown of muscle proteins and other protein structures – for example, immune cells. This increases the risk of infections and injuries. When our organism is looking for a quick and effective source of energy, it is forced to take it from the proteins contained in the muscles: the consequence is what is known as muscle catabolism (the muscles burn themselves in order to continue physical activity).

The condition also has psychological effects: people become impatient, tense and irritable. A common phenomenon afterwards is exhaustion and hunger pangs. If you consume more calories than you take in, the hoped-for effect of weight loss is nullified. We recommend fasting training no more than twice a week, as the body is under stress.

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Drinking is allowed

But they should be unsweetened drinks. A little milk in coffee is no problem. Caffeine also stimulates fat metabolism.

Running for 45 minutes, cycling for 1h30 and swimming for 40 minutes

In running, after about 30 minutes most of the energy is provided by the fat metabolism and after 45 minutes it reaches its maximum. Therefore, sessions of at least 45 minutes make sense. If you train for several hours a week, you can do it once or twice without any problems. For only two hours per week, the effect is greater with more intensive training.

Regarding duration in cycling, we recommend not exceeding 1h30/2h at the beginning. As the organism gets used to it, you can go up to 3h (but in this case you will need to eat something after 1h30). For swimming, we recommend not exceeding 40 minutes of training.

Even for people with no great sporting ambitions

If you feel fit in the morning, there is nothing against training on an empty stomach. For the untrained, a brisk walk or light cycling at a slightly elevated heart rate is also helpful. This increases the effectiveness of the hormone insulin, which plays an important role in the metabolism of blood sugar. Studies suggest that this reduces the risk of diabetes and obesity. The degree of tolerance of this method varies from person to person. For some athletes, training on an empty stomach gives them the energy to start the day better. Others are grumpy or hungry in the morning. This can sometimes lead to blood sugar and circulation problems.

Similar effect at noon or in the evening

If you have not eaten for several hours, you get a similar effect to exercising before breakfast. Remember, however, that your glycogen reserves in the morning are almost the same as they were the night before. So if you have consumed a lot of carbohydrates at dinner, you will not have the desired effect the next morning.

Who is Medbase?

Medbase is the largest multidisciplinary sports medicine network in Switzerland and offers specialised sports medicine services for athletes, clubs and sports associations of all activity levels in the areas of sports medicine, sports physiotherapy, performance diagnostics and training counselling.