Therapeutic Fasting: Effects & Autophagy — What Happens in the Body

The essentials at a glance

• Therapeutic fasting acts on two clearly distinct levels: fat burning with ketosis on one side, and autophagy on the other.
• The metabolic shift from glucose to fatty acids and ketone bodies begins after approximately 24 to 72 hours (as described in scientific literature).
• Autophagy is the medical term for cellular recycling—or more simply put: it is a deep clean where all recycling processes are activated.
• Mentally, fasting days create a turning point; the need for sleep often decreases, sleep becomes lighter, and a sense of fasting euphoria may occur.
• For certain chronic and metabolic conditions, fasting can be medically beneficial (guidelines and studies are available via the ÄGHE)—always under medical supervision.

What defines the effects of therapeutic fasting

Buchinger therapeutic fasting is the conscious, temporary abstinence from solid food and stimulants. It is not a crash diet or a primary weight-loss method, but a therapeutic approach, a cleansing on many levels, and an encounter with oneself. The question driving this article is different from those regarding procedures or methods: What actually happens in the body when solid food is removed for a few days?

The short answer is that two things happen simultaneously that are often conflated. The body recalibrates its energy supply and reorganizes at the cellular level. The first is fat burning with the transition into ketosis; the second is autophagy. Both are part of fasting, but they describe different mechanisms—and it is precisely this distinction that makes it clear why fasting is more than just weight loss. Those interested in the medically supervised framework can find it in our Body Detox program, embedded in the Therapeutic Fasting and Slim Fir hub.

At Naturhotel Tannerhof, this effect has been part of our practice for over 70 years, medically supervised across three generations of doctors. Every statement in this text is either supported by guidelines and studies on fasting therapy, which can be viewed at the Ärztegesellschaft Heilfasten und Ernährung (ÄGHE), or describes the medically supervised practice at the Tannerhof.

The two levels of action at a glance

Before discussing both levels individually, a comparison is helpful. It shows that fat burning and autophagy are not the same, but rather two processes that run in parallel during fasting.

Both levels occur simultaneously during fasting. Grouping them into a single mechanism would oversimplify their effects. That is why they are presented here one after the other, each in its own right.

First level: Fat burning and the path to ketosis

The first level is energetic. As long as solid food is being consumed, the body derives its energy primarily from carbohydrates, i.e., glucose. During fasting, readily available carbohydrate stores decrease, and the metabolism shifts: away from glucose and toward fatty acids and the ketone bodies derived from them. This is the aspect of fasting where the body increasingly lives off its own reserves.

This shift is well-documented. In their 2019 review article in the New England Journal of Medicine, Rafael de Cabo and Mark P. Mattson classify this exact process as a metabolic switch: the organism transitions from glucose-based energy production to ketone-based production once the rapidly available sugar stores are depleted. The authors describe that this transition typically begins a few hours to days into a fasting phase and is accompanied by a series of adaptive cellular responses. For multi-day therapeutic fasting, this means the shift to fat burning and ketone bodies occurs roughly 24 to 72 hours after fasting begins.

A large observational study by Wilhelmi de Toledo and colleagues, published in PLOS One in 2019, shows how this ketosis behaves over a longer fasting period. The study monitored 1,422 people during fasting periods of four to twenty-one days using the Buchinger method. The transition into ketosis was evidenced by rising ketone body levels, which reached a plateau after about five days—a measurable indication that the body had stably switched to energy production from fat. The resulting weight loss is a side effect of this level, not its actual purpose.

Second level: Autophagy, the cellular recycling process

The second level refers not to fat burning, but to the body's own recycling and cleansing processes at the cellular level. Autophagy—literally the cell's self-digestion—is the established medical term for this: cellular components are broken down, recycled, and reorganized. To put it more elegantly: all recycling processes are activated, and a thorough cleaning takes place everywhere. This does not happen in just one place, but on many levels—cellularly, vascularly, in the connective tissue, in the gut, in the brain, and in the liver and kidneys.

The 2016 Nobel Prize in Physiology or Medicine highlights how fundamental this mechanism is to cellular life. It was awarded to Japanese cell biologist Yoshinori Ohsumi for his discovery of the mechanisms of autophagy. In the 1990s, Ohsumi demonstrated in yeast cells how the cell encloses its own components in small vesicles, transports them to its recycling stations, and breaks them down there to reuse the building blocks. This research turned a long-suspected process into a precisely described one—and it explains why fasting medicine pays so much attention to autophagy.

It remains important to distinguish between the two levels. Fat burning answers the question of where the body gets its energy during a fast. Autophagy describes the order and renewal that occurs within the cells. Both occur together during fasting, but they are not the same process. At Tannerhof, autophagy is the actual goal of fasting—medically proven and positively described as the body's great cleanup.

The mental aspect: A break, sleep, and fasting euphoria

The effects of fasting go beyond metabolism. Mentally, fasting days create a break and an abundance of time with yourself and for yourself. Food is reduced to the essentials, and with it, a good portion of daily routine falls away — a counterpoint to sensory overload and constant acceleration.

Two observations frequently recur, both manifesting differently in each individual:

• Sleep. Sleep requirements often decrease, while sleep becomes lighter. Some people wake up earlier and feel clearer during a fast, without feeling sleep-deprived.
• Fasting euphoria. Some experience an elevated, light mood during the fasting days. This is attributed to changes in serotonin metabolism and does not occur in everyone, nor with the same intensity.

These mental effects are not a promise, but a common experience that the medical team at Tannerhof knows from supporting many fasting weeks. To support the mental side, the hotel also offers psychotherapeutic coaching, nature coaching, business and communication coaching, and art therapy — fasting is, after all, more than just a change in diet for a few days.

When fasting is medically relevant

The two physical levels and the mental aspect explain why fasting can be medically relevant for certain conditions. The guidelines for fasting therapy come from the Medical Association for Therapeutic Fasting and Nutrition (ÄGHE); further studies on fasting therapy can be viewed there. They classify the use of therapeutic fasting under professional supervision in areas including:

• Rheumatic diseases.
• High blood pressure and metabolic syndrome.
• Type II diabetes mellitus.
• Allergies, asthma, autoimmune diseases, and food intolerances.
• Migraines.
• Depression.
• Cancer in certain stages — as an adjuvant, never as a sole treatment.

This classification is always attributed to the ÄGHE and the medical team, never as a guarantee of a cure. The PLOS One study underscores how well longer, supervised fasting is tolerated: among the 1,422 supervised individuals, physical and emotional well-being increased significantly in all groups, and a large proportion of participants with existing health complaints reported improvement. Whether therapeutic fasting is suitable in an individual case is always a matter for medical assessment at Tannerhof — because everyone is different, and the individual is at the center.

How Tannerhof supports this effect

These two levels of effect do not unfold in a vacuum. At the nature hotel Tannerhof, therapeutic fasting is always medically supervised because the hotel's medical experience is an integral part of the fasting process. The doctors at Tannerhof are experts in fasting; they monitor vital signs and circulation as well as the individual progress. The process begins with an initial medical consultation on the first day of fasting, which is holistic in nature, including a medical history, physical examination, bioimpedance analysis to measure body composition, and a medication review. The length of stay is booked before arrival; from the initial consultation onwards, the schedule can be coordinated together and extended if necessary.

The accompanying range of therapies is intentionally broad. Medical applications include bioimpedance analysis, IHHT altitude training depending on the program, and, where medically appropriate, ozone autohemotherapy. In addition, there are wraps and compresses such as the liver-supporting hay flower wrap, aroma wraps, as well as mud and warm packs, baths and showers such as Kneipp contrast showers and magnesium-rich detox baths, massages such as the fasting massage, the detox massage with royal oil or the acidosis massage, lymphatic drainage and foot reflexology, as well as breathing therapy and sauna sessions. Exercise remains part of the fast, dosed in moderation: moderate endurance training and moderate strength training, yoga, Feldenkrais, and Tai Chi. Summit tours and running are possible, only at a pace that suits fasting — the body nourishes itself from within.

Above all lies the location. Bayrischzell, the alpine meadow, the mountain stream, clear and pure mountain air, and the view of the Wendelstein. The environment and medical supervision are equally important: fasting in a moving landscape becomes more intense, and the supervision gives the process direction. My hideaway in the mountains is the quiet setting in which both levels of effect are given space.

Ensuring the effects last in everyday life

Fat burning ends with the rebuilding phase, while autophagy is a recurring process—both can be sustained. In the final consultation with the attending physicians, we evaluate the changes that have occurred and discuss what your diet can look like moving forward. If you are looking for a lasting lifestyle change, you can follow up with the "Schlanke Tanne" Low Carb, our in-house low-carb nutritional program. Intermittent fasting, such as a 16/8 rhythm, is also a recommended way to carry the benefits of compact fasting into your daily life—not as a replacement for Buchinger, but as an extension of it into your everyday routine. For those who want to think about the fasting axis beyond a single stay, our Longevity at Tannerhof program provides the perfect framework.

This is how the impact of a fasting week becomes more than just a short-term effect. It gains continuity—and perhaps that is exactly the old Tannerhof motto in a modern form: Human, become essential.