Guest article by our nutrition expert Dr. Dorothea Portius
Fasting is one of the oldest therapeutic methods and has a deep-rooted presence in medical, religious, and cultural practices [1]. Already Hippocrates praised fasting as a remedy, and in many cultures – from Ramadan to ritual fasting periods in Christianity – it has been used for thousands of years as a spiritual and health practice. 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 conscious abstention from food or, depending on the fasting protocol, from certain foods or nutrients over a defined period.This abstinence can range from a few hours to several days and includes various types of fasting, such as
- “Natural” Nightly 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 in the body are already taking place (depending on health status and the composition of the evening meal).
- Intermittent Fasting (Interval Fasting): Here, phases of fasting and eating alternate in a defined rhythm, e.g., 16 hours of fasting and 8 hours of eating time (16:8).
- Therapeutic Fasting: Fasting periods that last from 24 hours to several days and are usually conducted under medical supervision, such as z.B. Buchinger therapeutic fasting.
Fasting is by no means a passive state, but an active, highly dynamic process, in which the body deliberately activates mechanisms to adapt to the temporary withdrawal of energy. These processes ensure that the organism functions optimally even with limited nutrient intake. The temporary food withdrawal signals the body to use resources efficiently and to initiate regeneration mechanisms. This "fasting stress," when adequately and appropriately tailored to one's own needs, differs from chronic stress, which can harm the body, as it is time-limited 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 relies on the glycogen reserves stored in the liver (carbohydrate storage) to provide glucose (sugar). After about 12-24 hours, when the glycogen stores are depleted, the body begins to convert fatty acids from the fat deposits 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 cleaning mechanism and regeneration
Fasting induces autophagy, a process in which old and damaged cellular components ("cellular debris") are broken down and recycled.This mechanism protects cells from oxidative stress, reduces inflammation, and supports tissue regeneration, ensuring that cells maintain their normal functions. Even during nightly fasting, these mechanisms and autophagic processes are initiated. Mitochondrial efficiency enhancement Fasting presents a physiological challenge for the mitochondria – the "powerhouses of the cell" – and forces them to optimize their energy production. This leads to increased cellular resilience against oxidative stress and promotes metabolic flexibility. This term describes the organism's ability to adapt to changing energy sources – carbohydrates or fats.A high efficiency of metabolic flexibility is considered an indicator of low cellular aging, while a smooth transition between energy carriers is associated with slowed cellular aging.[6].
Hormonal adjustments
The hormonal adjustments during fasting play a central role in the metabolic changes at the cellular level. The body responds to the energy deficit through a finely tuned hormonal regulation that includes both short-term and long-term adaptation processes.
During fasting, the insulin level decreases, which can improve insulin sensitivity and reduce the risk of insulin resistance and type 2 diabetes. At the same time, the secretion of glucagon, the antagonist of insulin, increases.Glucagon mobilizes stored energy reserves and stimulates glucose production in the liver. Additionally, the increased release of growth hormones such as Human Growth Hormone (HGH) promotes lipolysis (fat burning) and helps 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 increased levels of stress hormones such as adrenaline and noradrenaline, as the energy deficit activates the sympathetic nervous system and puts the organism into an "alarm mode". This leads to a short-term increase in alertness, enhanced lipolysis, and an increased supply of energy for vital organs, especially the brain.This stress reaction is, however, primarily limited to acute phases and serves to adapt to energy deficiency, not for regeneration.
Fasting can bring more stress than health benefits, especially during periods of high physical or psychological strain and for certain groups of people. This includes, among others, 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 heightened activation of the sympathetic nervous system can further burden the organism, potentially diminishing or even reversing the health benefits of fasting.
Anti-inflammatory effects
Fasting exerts its anti-inflammatory effect through several interconnected mechanisms[8]. Temporary food deprivation reduces the production of pro-inflammatory cytokines such as TNF-α and IL-6, while anti-inflammatory mediators like IL-10 are enhanced.
Fasting reduces oxidative stress by decreasing the formation of reactive oxygen species (ROS) and activating antioxidant defense systems such as superoxide dismutase (SOD) and glutathione peroxidase. Through more efficient mitochondrial energy production, as described above, oxidative damage to cell structures is reduced [9]. You can learn more about glutathione in the article on GlyNAC.
In addition, fasting modulates the gut 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 gut barrier but also have a systemic impact on several organ systems. You can read more about butyrate in the article on QBIOTIC.
Diseases where fasting offers benefits
Metabolic diseases
Type 2 diabetes: Fasting improves insulin sensitivity and reduces fasting blood sugar. Studies show that intermittent fasting can significantly lower HbA1c levels [10, 11].
Attention: Patients undergoing therapy with insulin or sulfonylureas should only fast under medical supervision, as there is a risk of hypoglycemia.
Obesity: Fasting can reduce body weight and particularly visceral fat. This has a positive impact on chronic inflammatory processes [5, 12] or inflammaging.
Cardiovascular diseases
Fasting can lower LDL cholesterol and triglycerides, while the HDL level is less affected. However, the ratio of LDL to HDL improves, which can counteract the development of oxidative stress and promote endothelial cell function, crucial for vascular health [13].Blood lipid levels are a very important topic for increased longevity.
Neurodegenerative diseases
Fasting increases the production of the 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 [14].
Cancer prevention and therapy
Tumor cells exhibit a fundamentally altered metabolism compared to healthy cells. They predominantly metabolize glucose at a high rate, even under anaerobic conditions – a phenomenon known as 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 certain tumor types. Studies suggest that fasting in combination with chemotherapy improves the effectiveness of treatment and reduces side effects. [15]
Conditions where caution is advised
Hyperuricemia and gout
Fasting promotes the cellular degradation and renewal process (autophagy), thereby increasing the recycling of cellular components.This releases purines, the breakdown of which leads to the formation of uric acid. As a result, the uric acid level can rise during fasting, which increases the risk of gout attacks in predisposed individuals. You can learn more about autophagy in the article on Spermidin.
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 adequate fluid intake to promote uric acid excretion is essential.
Eating disorders and underweight
Fasting can lead to an additional calorie deficit in patients and individuals with underweight, which exacerbates the already existing deficiency state and further intensifies the catabolic metabolism.Additionally, hormonal adjustment to energy deficiency, such as increased cortisol release, can increase the risk of further health complications such as bone density loss, heart rhythm disorders, and delayed metabolic recovery.
In individuals with eating disorders such as anorexia nervosa, fasting can also reinforce restrictive eating patterns and further promote pathological relationships with food.
Recommendation: Individuals with underweight, eating disorders, or a history of such conditions should generally avoid fasting and ensure a balanced, regular food intake, ideally under medical or nutritional guidance.
Pregnancy and Breastfeeding
Fasting is not recommended during pregnancy and breastfeeding, as a continuous supply of energy and nutrients is crucial for the health of both mother and child during these phases. The increased calorie and nutrient requirements support the growth and development of the fetus as well as milk production. A calorie deficit from fasting could lead to deficiencies that may increase the risk of complications during pregnancy and inadequate nutrient supply for the infant. Additionally, fasting can negatively affect maternal metabolism and hormone balance, which could impact the physical and mental health of the mother as well as the development of the child.
Recommendation: Instead of fasting, attention should be paid to a balanced and nutrient-rich diet to meet the special needs during these life phases. In case of uncertainties, consulting 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:
Sufficient adjustment time
The body needs time to get used to fasting – especially when it comes to longer fasting phases or new fasting protocols. Abrupt or extreme fasting can overwhelm the body, which may manifest in symptoms such as headaches, weakness, or concentration problems.
Individual Adjustment
Fasting is not a "one-size-fits-all" approach. Factors such as age, gender, metabolism, hormonal status, and existing diseases influence how well the body responds to fasting. Women in the premenopausal phase or individuals with metabolic disorders often require individually tailored fasting periods to avoid negative effects such as hormonal imbalances.
Sufficient Nutrient Supply During Eating Phases
Between fasting periods, it is essential to provide the body with sufficient nutrients.A lack of essential vitamins, minerals, or calories can reverse the positive effects of fasting and lead to fatigue, muscle loss, or a weakened immune function.
Appropriate 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.
- Longer fasting (24–72 hours): Can initiate deeper repair processes but requires careful planning and medical supervision to avoid overload or nutrient deficiency.
- Chronic fasting or excessive calorie restriction: can put the body into a state of chronic stress, disrupt hormone regulation, and increase the risk of health problems, e.g., through the breakdown of muscle mass or disturbances in the menstrual cycle.
Conclusion: The art of proper fasting
Fasting is a natural and scientifically based method for promoting health. It can activate regenerative processes, optimize metabolism, and unfold anti-inflammatory as well as cell-protective effects. It is important to note: Even short fasting phases, such as overnight fasting, provide positive stimuli, and extending the fasting interval (primarily with an earlier intake of the evening meal) can enhance health benefits.
However, fasting is not a universally suitable concept, but requires individual adjustment. In certain life phases, such as during pregnancy, breastfeeding, or adolescence, as well as in certain diseases like gout or eating disorders, fasting can potentially cause more harm than good. Too frequent or too intense 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 increasingly also as a complementary therapy option. A mindful approach that considers personal conditions is crucial to optimally utilize health benefits and minimize potential risks.
