For cyclists, the winter months are often a time when they consciously train less and take a more relaxed approach to their diet. If you are now getting back into (structured) training but have put on a few feel-good kilos over the weeks, you may already be thinking about whether interval fasting would be a viable method of losing weight. Not a bad idea. Even if the studies are still sparse, there are indications that intermittent fasting is not only easy to integrate into everyday life, but also triggers various processes in the body that can have a positive effect on athletic performance, among other things. However, only if the fasting concept is implemented accordingly.
What is Intermittent fasting?
Intermittent fasting, or intermittent fasting, means that phases of abstaining from food and eating alternate - at comparatively short intervals. Unlike religious or therapeutic fasting, for example, it doesn't take several days or weeks before there is something to eat again, but only a few hours, depending on the method. Intermittent fasting also aims to achieve long-term weight loss. However, unlike traditional diets or forms of fasting, it is intended as a so-called permanent diet. This means that intermittent fasting can theoretically be practised for a lifetime as a basic principle of nutrition.
The effects of intermittent fasting
"The longer the fasting phases are, the sooner autophagy kicks in," explains nutrition and sports scientist Dr Katrin Stücher. In simple terms, autophagy describes a renewal process of the body's cells, a kind of endogenous waste disposal. This process always takes place at a basal level (also known as the basal metabolic rate). However, in stressful situations - such as after at least twelve hours without energy and nutrient intake - it ramps up. The body therefore switches into a kind of repair mode, which, according to the current state of science, improves fat metabolism and sensitivity to the sugar-controlling hormone insulin. "Animal and human studies have shown significant improvements, particularly in prediabetes and type 2 diabetes," confirms Katrin Stücher. However, the Helmholtz Association of German Research Centres does not yet have any long-term studies on this in humans. And: endurance sport in itself already has a similar effect on metabolism and insulin sensitivity as intermittent fasting.
More muscle, less power through intermittent fasting?
Nevertheless, intermittent fasting can be interesting for racing cyclists and other endurance athletes. A study published in the journal "Obesity Reviews" in 2011 and a meta-study published in the publication "Nutrients" in 2024, which compared various studies on intermittent fasting, found, for example, that the proportion of fat-free mass in the body is better maintained than with conventional diets despite a reduced calorie intake. In other words, the ratio of fat to muscle mass is more favourable - and the more of the latter there is, the more actively the metabolism works, as muscles burn more calories than fat even at rest.
A small study with 16 cyclists in the U23 and elite categories came to similar conclusions in 2020. In addition, after four weeks of intermittent fasting using the 16:8 method, the group of test subjects had a better peak power to body weight ratio than the control group that did not fast due to the weight loss, and the ratio of neutrophils to lymphocytes had changed; this indicates that there is less inflammation in the body, meaning that its defence against infection works better. However, the fasting test subjects also had lower levels of free testosterone. This hormone plays a role in muscle building and strength development, energy and resilience, susceptibility to injury and the ability to regenerate.
Nutritionist Katrin Stücher, who herself advises professional endurance athletes in various disciplines, also sees risks with interval fasting, particularly on a hormonal level: "If many training sessions are carried out in a fasted state, this can have a negative impact on hormone levels. The body releases more stress hormones and women in particular can experience hormonal imbalances, which can also lead to increased bone loss and a higher risk of osteoporosis in the long term."
Fasting training can stress the body and weaken the immune system, among other things. - Dr Katrin Stücher, nutritionist
Intermittent fasting and cycling
In her experience, endurance sport and interval fasting are difficult to combine, but it is not impossible. Katrin Stücher recommends not getting on your bike at all if possible, but at most once or twice a week on an empty stomach, "ideally for a relaxed ride at around 55 to 65 per cent of your maximum heart rate, lasting no longer than 60 minutes". She advises against intensive units without prior energy intake, firstly "because the desired training effect is lower on an empty stomach" and after 16 hours or more the body could, in the worst case, draw its energy not only from fat reserves but also from the muscles. Secondly, "because fasting sessions stress the body enormously, which can weaken the immune system, among other things".
The aforementioned study with the U23 cyclists was not about weight loss. They consumed 100 per cent of their normal calorie intake within the eight hours of eating and ate a snack either directly before or after the exercise. But, as Katrin Stücher points out, "even those who get into the saddle within the time window for food intake are not necessarily in a position to complete intensive units". Almost inevitably, you will then either train with a full stomach or, due to the lack of time, too few daily calories will end up in your body to optimally process the training stimulus. Ideally, cyclists should practise interval fasting on rest days, especially if they have no experience with fasting training, and take a hypoglycaemic emergency snack with them if they do train (lightly). Incorporating strength training into your training plan at least once a week prevents unwanted muscle loss and increases the production of growth hormones, which have a muscle-building (anabolic) effect.
When does intermittent fasting not help?
Intermittent fasting is not suitable for everyone. Anyone who regularly exercises long and intensively or has a very high calorie requirement may find it difficult to absorb the necessary energy and nutrients in a restricted eating window. People with a high risk of hormonal imbalances or a weakened immune system should also be careful. In the long term, overly rigorous fasting can lead to deficiency symptoms, especially if essential nutrients such as vitamins, minerals and proteins are not sufficiently absorbed. It is therefore important to consciously organise your diet during the eating phases and to favour foods rich in nutrients and carbohydrates, including plenty of wholemeal products and vegetables. To give yourself a little boost in terms of body weight and metabolism as a racing cyclist at the end of the off-season and gentle start to training, sensibly implemented interval fasting can be an option. Nutrition expert Katrin Stücher recommends that anyone who wants to try interval fasting could try it out during this low-training phase, for example using the 5:2 method, i.e. with long fasting phases.
Intermittent fasting: 4 common methods
5:2 method
On five days of the week you eat normally, while on two non-consecutive days your calorie intake is reduced to a maximum of 650 kilocalories.
16:8 method
Here, the time window for eating is limited to eight hours a day, for example from 12 noon to 8 pm. The remaining 16 hours are spent fasting. This variant is considered easy to implement, as the time spent sleeping at night is counted towards the 16 hours of fasting. Other similar forms are the 18/6, 12/12 or 20/4 method, whereby the first number always indicates the fasting phase.
Alternate-Day-Fasting (ADF)
In this method, known as alternating fasting, normal eating without restricting energy intake alternates with fasting on a daily basis, with only around 25 per cent of the energy requirement being consumed on fasting days.
Dinner cancelling
If you practise this variant, you should ideally skip dinner two to three times a week. This should result in a fasting phase of at least 14 hours until breakfast.
Conclusion: Intermittent fasting
Even if hourly fasting is easier to maintain than permanent calorie restriction, intermittent fasting cannot work miracles. For example, a study conducted by the German Cancer Research Centre and the University Hospital in Heidelberg with 150 participants showed that it had no greater effect on metabolism and weight loss than a normal reduction diet. The German Nutrition Society also criticises the fact that there are no or only vague recommendations on food selection and therefore "intermittent fasting does not generally lead to a change in diet towards a nutritionally favourable food selection". In any case, Katrin Stücher is convinced: "Intermittent fasting is not recommended for a healthy, balanced diet that is adapted to sport." At least not as a permanent solution.
