Guest article by our nutrition expert Dr. Dorothea Portius
Fasting is one of the oldest therapeutic methods and is deeply rooted in medical, religious and cultural practices [1]. Even Hippocrates praised fasting as a remedy, and in many cultures - from Ramadan to ritual fasting periods in Christianity - it has been used as a spiritual and health practice for thousands of years. But how does fasting affect the body, and what biochemical processes make it a potent tool for health and regeneration
What is fasting
Fasting describes the deliberate abstinence from food or, depending on the fasting protocol, from certain foods or nutrients over a defined period of time. This renunciation can range from a few hours to several days and includes various types of fasting, such as
- "Natural" nocturnal fasting: The period between the last meal of the day and breakfast the next morning. During this natural fasting phase, which typically lasts 8-12 hours, regenerative processes are already taking place in the body (depending on the state of health and composition of the evening meal).
- Intermittent fasting (intermittent fasting): Here, phases of fasting and eating alternate in a defined rhythm, e.g. 16 hours of fasting and 8 hours of eating (16:8).
- Medical fasting: Fasting periods that last from 24 hours to several days and are usually carried out under medical supervision, such as z.BBuchinger therapeutic fasting.
Fasting is by no means a passive state, but an active, highly dynamic processin which the body specifically activates mechanisms to adapt to the temporary withdrawal of energy. These processes ensure that the organism functions optimally even when the supply of nutrients is restricted. The temporary food deprivation signals the body to use resources efficiently and initiate regeneration mechanisms. However, this "fasting stress", if carried out adequately and appropriately to one's own needs, differs from chronic stress, which can damage the body as it is temporary and promotes adaptive processes [2, 3].
These adaptive processes are promoted by fasting
Energy metabolism and metabolic switching
In the first 6-8 hours, the body draws on the glycogen reserves (carbohydrate stores) stored in the liver to provide glucose (sugar). After approx. 12-24 hours, when the glycogen stores are used up, the body begins to convert fatty acids from the fat stores in the liver into ketone bodies. Ketone bodies serve as an alternative energy source, especially for the brain. Better blood sugar levels are also important in the fight against insulin resistance.
Autophagy - cellular cleansing mechanism and regeneration
Fasting induces autophagy, a process in which old and damaged cell components ("cellular debris") are broken down and recycled. Dthis mechanism protects cells from oxidative stress, reduces inflammation and supports tissue regeneration and ensures that the cells maintain their normal functions. These mechanisms and autophagic processes are already triggered during overnight fasting [4, 5].
Mitochondrial efficiency increase
Fasting represents a physiological challenge for the mitochondria - the "power plants of the cell" - and forces them to optimize their energy production. This leads to increased cellular resistance to oxidative stress and promotes metabolic flexibility. This term describes the organism's ability to adapt to changing energy sources - carbohydrates or fats. High efficiency of metabolic flexibility is considered an indicator of low cellular aging, while smooth switching between energy sources is associated with slowed cell aging [6].
Hormonelle Anpassungen
Hormonal adaptations during fasting play a central role in metabolic changes at the cellular level. The body responds to the lack of energy through a finely tuned hormonal regulation that includes both short-term and long-term adaptive processes.
During fasting insulin levels fall, which can improve insulin sensitivity and reduce the risk of insulin resistance and type 2 diabetes. At the same time, the release of glucagon, the antagonist of insulin, increases. Glucagon mobilizes stored energy reserves and stimulates glucose production in the liver. In addition, the increased release of growth hormones such as Human Growth Hormone (HGH) promotes lipolysis (fat burning) and helps to maintain muscle mass by limiting catabolic processes [7]. You can find more sugar tips in our article about Jessie Inchauspé - also known as Glucose Goddess.
However, the body also releases more stress hormones such as adrenaline and noradrenaline, as the lack of energy activates the sympathetic nervous system and puts the body into "alarm mode". This leads to a short-term increase in alertness, increased lipolysis and an increased supply of energy for vital organs, especially the brain. However, this stress response is primarily limited to acute phases and serves to adapt to the lack of energy, not to regenerate.
Fasting can cause more stress than health benefits , especially in phases of high physical or mental stressand in certain groups of people. These include women in the luteal phase, competitive athletes, pregnant women, adolescents and people with acute infections. In individuals who are already under increased physiological or psychological stress, the increased activation of the sympathetic nervous system can put additional strain on the organism, so that potential health benefits of fasting are weakened or even reversed.
Entzündungshemmende Effekte
Fasting exerts its anti-inflammatory effect through several interlinked mechanisms[8]. Temporary food deprivation reduces the production of pro-inflammatory cytokines such as TNF-α and IL-6, while anti-inflammatory mediators such as IL-10 are increased.
Fasting lowers oxidative stress by reducing the formation of reactive oxygen species (ROS) and activating antioxidant protection systems such as superoxide dismutase (SOD) and glutathione peroxidase. More efficient mitochondrial energy production, as described above, reduces oxidative damage to cell structures [9]. You can find out more about glutathione in the article on GlyNAC.
Fasting also modulates the intestinal microbiome and thus promotes the production of anti-inflammatory short-chain fatty acids such as butyrate [5]. These short-chain fatty acids not only strengthen the intestinal barrier but also have a systemic effect on several organ systems. You can read more about butyrate in the article on QBIOTIC .
Diseases for which fasting offers benefits
Stoffwechselerkrankungen
Type 2 diabetes: Fasting improves insulin sensitivity and reduces fasting blood glucose. Studies show that intermittent fasting can significantly lower HbA1c levels [10, 11].
Caution: Patients on insulin or sulfonylurea therapy should only fast under medical supervision due to the risk of hypoglycemia.
Obesity: Fasting can reduce body weight and visceral fat in particular. This has a particularly positive effect on chronic inflammatory processes [5, 12] or inflammaging.
Herz-Kreislauf-Erkrankungen
Fasting can lower LDL cholesterol and triglycerides, with HDL levels being less affected. However, the ratio of LDL to HDL is improved, which can counteract the development of oxidative stress and promote the function of endothelial cells, which is crucial for vascular health [13]. Keeping blood lipid levels in check is a very important issue for greater longevity.
Neurodegenerative Erkrankungen
Fasting increases the production of neurotrophic factor (BDNF), which promotes the growth of new nerve cells and protects the brain from degeneration. There is evidence that fasting, especially intermittent fasting, can reduce the risk of Alzheimer's and Parkinson's disease [14].
Krebsprävention und -therapie
Tumor cells have a fundamentally different metabolism compared to healthy cells. They predominantly metabolize glucose at a high rate, even under anaerobic conditions - a phenomenon known as the Warburg effect This increased glycolysis makes them particularly dependent on a constant supply of glucose and insulin-mediated growth signals.
Fasting reduces the secretion of insulin and the activation of growth-promoting signaling pathways such as mTOR, which can inhibit tumor-specific metabolic processes and slow the growth of some tumor types. Studies suggest that fasting in combination with chemotherapy improves the effectiveness of treatment and reduces side effects. [15]
Diseases for which caution is advised
Hyperuricemia and gout
Fasting promotes the cellular degradation and renewal process (autophagy), whereby the cell's own components are increasingly recycled. Among other things, purines are released, the breakdown of which leads to the formation of uric acid. As a result, uric acid levels can rise during fasting, which increases the risk of gout attacks in predisposed individuals. You can find out more about autophagy in the article on spermidine.
Recommendation: Gout patients should only fast in consultation with a doctor or nutritionist. A cautious start with shorter fasting windows (e.g. 12-14 hours) and sufficient fluid intake to promote uric acid excretion are essential.
Essstörungen und Untergewicht
Fasting can lead to an additional calorie deficit in patients and people who are underweight, which increases the already existing deficiency state and further exacerbates the catabolic metabolism. In addition, hormonal adaptation to the lack of energy, such as increased cortisol secretion, can increase the risk of further health complications such as bone density loss, cardiac arrhythmias and delayed metabolic recovery.
In individuals with eating disorders such as anorexia nervosa, fasting may also reinforce restrictive eating patterns and further promote pathological food use.
Recommendation: People who are underweight, have eating disorders or a history of such illnesses should avoid fasting as a matter of principle and ensure a balanced, regular food intake, ideally under medical or nutritional supervision.
Schwangerschaft und Stillzeit
Fasting is not recommended during pregnancy and breastfeeding, as a continuous supply of energy and nutrients is crucial for the health of mother and child during these phases. The increased calorie and nutrient requirements support the growth and development of the fetus as well as milk productionA calorie deficit due to fasting could cause deficiencies that may increase the risk of complications during pregnancy as well as inadequate nutrient supply to the infant. In addition, fasting can have a negative impact on the mother's metabolism and hormone balance, which could affect the physical and mental health of the mother as well as the development of the child.
Recommendation: Instead of fasting, a balanced and nutritious diet should be followed to meet the special needs of these phases of life. If you are unsure, advice from a doctor or nutritionist can be helpful.
When does fasting become a positive signal?
For fasting to act as a health-promoting stimulus, certain conditions must be met:
Ausreichende Anpassungszeit
The body needs time to get used to fasting - especially when it comes to longer periods of fasting or new fasting protocols. Fasting that is too abrupt or extreme can overtax the body, which can manifest itself in symptoms such as headaches, weakness or concentration problems.
Individuelle Anpassung
Fasting is not a "one-size-fits-all" approach. Factors such as age, gender, metabolism, hormonal status and existing illnesses influence how well the body responds to fasting. Women in the premenopausal phase or people with metabolic diseases often need individually adapted fasting times to avoid negative effects such as hormonal imbalances.
Adequate nutrient supply during eating phases
It is essential to provide the body with sufficient nutrients between fasting periods. A lack of essential vitamins, minerals or calories can reverse the positive effects of fasting and lead to fatigue, muscle loss or weakened immune function.
Adequate duration and intensity
- Short fasting (12-16 hours): Promotes regeneration and activates processes such as ketosis and autophagy without putting too much strain on the body.
- Prolonged fasting (24-72 hours): Can initiate deeper repair processes, but requires careful planning and medical monitoring to avoid overloading or nutrient deficiencies.
- Chronic fasting or excessive calorie restriction: Can put the body in a state of chronic stress, disrupt hormone regulation and increase the risk of health problems, such as loss of muscle mass or disruption of the menstrual cycle.
Conclusion: The art of fasting properly
Fasting is a natural and scientifically proven method of promoting health. It can activate regenerative processes, optimize the metabolism and have anti-inflammatory and cell-protecting effects. The following applies: even short periods of fasting, such as fasting at night, provide positive stimuli, while extending the fasting interval (primarily with an earlier evening meal) can increase the health benefits.
However, fasting is not a universally suitable concept, but requires individual adaptation. In certain phases of life, such as during pregnancy, breastfeeding or adolescence, as well as with certain illnesses such as gout or eating disorders, fasting can potentially do more harm than good. Too frequent or too intensive food restriction can also reverse the positive stimulus into stressful "metabolic" stress.
With growing scientific evidence, fasting is establishing itself not only as a preventive measure, but also increasingly as a complementary therapy option. The key is a mindful approach that takes personal circumstances into account in order to make the most of the health benefits and minimize potential risks.
