Therapeutic fasting and side effects: understanding the transition days

The essentials at a glance

• Side effects such as weakness, shakiness, nausea, headaches, or feeling cold are part of the transition days when the metabolism switches to fat burning.
• In the major 2019 PLOS One study of over 1,400 medically supervised fasters, adverse effects occurred in less than one percent of cases; the mild side effects appeared primarily in the first few days and subsided on their own or with simple remedies.
• A more pronounced fasting crisis is the exception and can be identified and managed early under medical supervision.
• Proven remedies include green tea with honey, sufficient fluids, and warmth; the medical team adjusts the pace, duration, and support individually.
• At Tannerhof, therapeutic fasting is always medically supervised; if you are unsure, a preliminary medical consultation by phone is available upon request before your arrival.

What happens during the transition days

Buchinger therapeutic fasting is the conscious, time-limited abstinence from solid food and stimulants. As soon as the readily available carbohydrate stores are depleted, the metabolism switches from glucose to fatty acids and ketone bodies within the first few days. It is precisely in this transition phase, on the second and third day of fasting, that temporary side effects can occur — the body is reorganizing its energy supply, and this is noticeable for a few days.

These side effects are a well-known and well-documented part of fasting. For guests who would like to calmly review a medically supervised fasting program, Body Detox is the appropriate program page; the methodology itself is explained on the Buchinger therapeutic fasting page. The following sections first outline what can occur during the transition days, and then how medical supervision at Tannerhof supports this phase.

Data helps to put the frequency into perspective. The large observational study by Wilhelmi de Toledo and colleagues, published in PLOS One in 2019, monitored more than 1,400 people over fasting periods of four to twenty-one days using the Buchinger method. Under this daily clinical supervision, adverse effects occurred in less than one percent of participants, and there were no serious or lasting incidents. The milder side effects appeared primarily in the first few days of fasting and subsided on their own or with simple remedies. This study and the guidelines on fasting therapy available from the Ärztegesellschaft Heilfasten und Ernährung (ÄGHE) support the following sections.

Overview of possible side effects

During the transition days, symptoms primarily related to short-term fluctuations in blood sugar and metabolic adjustment may occur. In the PLOS One study, mild sleep disturbances were among the most frequently reported observations, followed by headaches, muscle discomfort, and temporarily low blood sugar — all described as minor and temporary. Typical for the transition days are:

• Weakness and fatigue, because the body is adjusting its energy production.
• Shakiness and a feeling of inner restlessness during short-term low blood sugar.
• Nausea, usually mild and temporary.
• Headaches, often in the first one to two days.
• Feeling cold, because the resting metabolism generates less heat.
• Slight changes in sleep patterns, often accompanied by lighter sleep overall.

These phenomena are generally harmless and are a natural part of the transition. In our study, they disappeared spontaneously or with simple, natural remedies, without the need to interrupt the fast. The ÄGHE also notes that initial discomfort is usually followed by a phase of well-being and improved mood—the transition is a process, not a permanent state.

The fasting crisis explained objectively

The term "fasting crisis" refers to a more pronounced cluster of transition symptoms, such as when headaches, weakness, nausea, or circulatory fluctuations occur together and become more noticeable. It typically occurs during the early transition phase and is an expression of the same adjustment process, just more intense. Fluid and mineral balance often play a role, as does starting too quickly without sufficient preparation beforehand.

Under medical supervision, a severe fasting crisis is the exception. The favorable safety profile of the PLOS One study—adverse effects in less than one percent of cases, with no serious or lasting events—is precisely due to the fact that participants were screened beforehand and monitored daily during the fast. This is the true value of supervision: changes are noticed early, and the pace can be adjusted before a minor side effect becomes something more.

How medical supervision supports the transition

At the Tannerhof Nature Hotel, therapeutic fasting is always medically supervised—this is the philosophy of our house, cultivated over three generations of doctors. The doctors at Tannerhof are experts in fasting, and the transition days are precisely when this experience is most valuable. Any medical statement in this text is either supported by the guidelines and studies on fasting therapy available through the ÄGHE or describes the medically supervised practice at Tannerhof.

Many issues can be managed with simple, proven remedies. A green tea with honey can balance a temporary dip in blood sugar without breaking the fast—a spoonful of honey or an espresso is explicitly permitted as a small concession. These include:

• Green tea with honey to bridge a low point.
• Plenty of fluids, around three liters a day, in the form of fasting tea, mineral water, lemon water, or hot ginger water.
• Warmth to combat feeling cold—a bathrobe and hot water bottle are standard in every room at Tannerhof.
• A base supplement if needed, to help with acidic metabolic products and uric acid crystals.
• Moderate exercise and rest in the right balance, tailored to the day.
• Adjustment of pace and duration by the medical team, where appropriate.

Medical appointments provide a calm framework throughout your stay. It begins with an intake interview 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—as well as questions about why you are fasting, what is currently happening in your life, and your previous fasting experience. The length of your stay is booked before arrival; from the intake interview onwards, the schedule and support can be coordinated with the medical team and adjusted as needed. If necessary, a follow-up consultation is scheduled, and the stay concludes with a final review to evaluate the changes. Every person is different, and the individual is at the center of our care.

Why good preparation makes the transition smoother

The transition days can be cushioned before they even begin. A fasting week doesn't just start with the first bowl of soup: before arrival, the doctors at Tannerhof recommend one or more relief days at home, such as a fruit, potato, vegetable, or raw food day. The guest decides how many days to take. This gentle start gives the metabolism time to adjust to the change and takes the edge off the transition.

The environment at the hotel is also designed for this transition. Our supporting treatments are intentionally broad and tailored directly to fasting. These include medical history and examination, follow-up medical consultations and the final review, bioimpedance analysis, and, depending on the program, IHHT altitude training, Kneipp contrast showers, magnesium-rich detox baths, liver-supporting hay flower wraps, aroma wraps, as well as mud and heat packs, fasting massage, detox massage with royal oil or acidosis massage, lymphatic drainage, foot reflexology, breathing therapy, and sauna sessions. Some of these treatments provide warmth and relaxation during the transition days, while others support the metabolism. For the mental aspect, we offer psychotherapeutic coaching, nature coaching, business and communication coaching, and art therapy.

Exercise remains part of the fast, just at a moderate intensity: moderate endurance training and moderate strength training, a hike, or a sauna session at a pace that suits fasting. Summit tours and running are possible, just at a slower pace than in your everyday life—the body is nourishing itself from within. Fasting guests take their meals in the "Fasterstube," a dedicated dining room for those fasting, and fasting expertise is embedded in every department of the house, from reception and the restaurant to the therapists. This ensures that the transition is not a hurdle you face alone, but a phase that is fully supported.

What comes after the transition

After the transition days, your experience often changes significantly. Many people who fast report a growing sense of lightness, a clearer head, and improved well-being. In the PLOS One study, physical and emotional well-being increased across all groups, and over ninety percent of participants experienced no hunger while fasting. Some people experience a "fasting euphoria" due to changes in serotonin metabolism, which varies from person to person. Often, the need for sleep decreases while sleep itself becomes lighter.

This transition is part of the whole experience. Buchinger therapeutic fasting is more than just a diet and is not primarily a weight-loss method—it is a therapeutic approach, a cleansing on many levels, and an encounter with oneself. The body works on two levels simultaneously: it switches to fat burning and initiates autophagy, the body's own recycling and cleansing processes at the cellular level. The transition days are the gateway to these effects, not the price you pay. Above it all is the location: Bayrischzell, the alpine meadow, the mountain stream, clear and pure mountain air, and the view of the Wendelstein. My hideaway in the mountains is the quiet setting where fasting and medical supervision amplify one another.

If you would like to view the specific framework and book online, you can find it in the Body Detox program. If you are unsure before your arrival—for example, due to a chronic illness or ongoing medication—a telephone medical consultation can be arranged upon request. An overview of therapeutic fasting and the "Schlanke Tanne" Low Carb, our in-house low-carb diet, can be found in the Therapeutic Fasting and Schlanke Tanne hub.