Well-being | 1.10.2026
Whether intermittent fasting or a week-long fast, does it really cleanse the body, mind and soul? Can calorie restriction really boost the immune system? Professor David Fäh, a nutritionist and specialist in preventive medicine, answers these questions and more.
Fasting periodically comes back into fashion, particularly in the first half of the year. But it is far from a modern invention. For centuries, people have abstained from eating and drinking until after sunset during the Islamic fasting month of Ramadan. Christian fasting, which is generally less strictly observed, spans the seven weeks from Ash Wednesday until Easter. “Nowadays, fasting is less about the spiritual experience and more focused on the idea of detoxing and cleansing,” says Professor David Fäh, a nutritionist and specialist in preventive medicine.
“Nowadays, fasting is less about the spiritual experience and more focused on the idea of detoxing and cleansing.”
In specialist clinics, patients can fast for two to four weeks under medical guidance. People fasting independently might spend a week consuming nothing but tea, clear soup or highly diluted juices, and no solid food. “In principle, anybody who is healthy and not underweight can fast,” says David Fäh. “However, you should seek medical advice if you have diabetes or are on medication.” Moreover, weight loss should never be the main focus when fasting – according to Dr Fäh, any weight lost is likely to return swiftly when you stop fasting if you resume your previous eating habits.
What impact does calorie restriction have on health? Dr Fäh explains that you shouldn't expect miracles. However, “Recent studies show that cells undergo a kind of spring clean if they stop receiving nutrients. They break down cellular components that are no longer needed. This process, known as autophagy, can extend the lifespan of cells.”
However, David Fäh says there is no proof that this process prevents disease in humans, boosts the immune system or prolongs life. “There’s a lack of long-term studies. Unfortunately, at present in the field of nutrition, there’s a lot of fragmented knowledge and very little hard evidence.”
And what’s the story behind terms like cleansing and detoxing? David Fäh: “There is no such thing as toxins accumulating in the body in the way many people imagine. A healthy body detoxifies itself naturally by converting or breaking down unwanted substances in the liver and eliminating them via bile, urine and the intestines. An example of such a substance is alcohol.” However, he says, cutting calories can in fact reduce fat content in liver cells, which has a positive impact on inflammation in the body and insulin effectiveness.
Therapeutic fasting is certainly an interesting concept, according to our expert. “You start to question your own eating habits. If you don’t eat for a week, your senses become sharper – simply because the body isn’t busy consuming, digesting and storing food. Even the taste buds become more sensitive,” explains David Fäh.
He recommends combining this type of fasting with a break from everyday life in a different environment, and enjoying some meditation or walks in nature. “Ideally use this time to increase your awareness of stress factors and reflect on whether your eating habits are being used to compensate for other needs.”
“If you don’t eat for a week, your senses become sharper – simply because the body isn’t busy consuming, digesting and storing food. Even the taste buds become more sensitive.”
Dr Fäh suggests that those who want to give their body a break and lose a little weight along the way could also try intermittent fasting. This approach involves resting the digestive tract for 16 hours, for example, and only eating during an 8-hour window each day, by skipping breakfast or dinner.
Following a varied, balanced diet with limited processed foods is a far better and more sustainable way to look after your health than any type of fasting, says the doctor. “And the importance of making physical activity and regular exercise part of your daily routine should not be underestimated – as numerous studies show.”
Prof. David Fäh is a lecturer at Bern University of Applied Sciences. As a nutritionist and specialist in preventive medicine, he advises companies and individuals and is the author of numerous publications.
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