An 8–10 hour window or alternate-day fasting; choose what you can sustain. Adherence matters more than the exact method.
Landmark review: intermittent fasting triggers a 'metabolic switch' toward the use of ketone bodies that may improve metabolic health and, according to the evidence, favorably influence aging and several diseases. General framework; the specific effects depend on the protocol and the person.
View the study (study) →12-month randomized trial in adults with overweight or obesity: fasting 3 non-consecutive days per week (4:3) led to an average of 2.9 kg more weight loss than daily calorie reduction, both within an intensive weight-loss program.
View the study →⏱ 3 min read
Fasting is one of the oldest metabolic interventions in human history and, paradoxically, one of the most misunderstood in the contemporary world: it is not simply skipping meals or a passing weight-loss fad, but a practice that activates a specific set of physiological responses that the body has spent millions of years of evolution refining in order to survive periods of scarcity, and which, when applied intentionally, can trigger processes of cellular repair and regeneration with potential impact on aging and the prevention of chronic diseases.
From a biochemical perspective, fasting operates through what is known as the metabolic switch: under regular feeding conditions, metabolism is oriented preferentially toward the oxidation of glucose as the main energy source; when fasting extends between 12 and 36 hours, depending on the individual's glycogen stores and level of physical activity, the body makes a transition toward the beta-oxidation of fatty acids and the production of ketone bodies, mainly beta-hydroxybutyrate, which acts not only as an alternative fuel but also as a signaling molecule with its own anti-inflammatory and neuroprotective effects.
Time-restricted eating, popularly known as the 16:8 protocol, is the most studied modality and probably the most accessible: it consists of concentrating all caloric intake in an eight-hour window, typically between noon and eight in the evening, and fasting for the remaining sixteen hours; this pattern aligns with the circadian rhythms of liver metabolism, since the liver has a greater capacity to process nutrients during the first half of the day than at night, which explains why concentrating meals in the morning produces more pronounced metabolic improvements than concentrating them in the afternoon or evening.
Alternate-day fasting and prolonged fasting of two to five days are more intense modalities that activate autophagy mechanisms and stem cell production in some tissues more deeply; the fasting-mimicking diet, developed by Valter Longo and his team, replicates the metabolic effects of prolonged fasting through a very reduced caloric intake, between 700 and 1,100 kilocalories daily, for five days a month, with a specific macronutrient composition that keeps IGF-1 and mTOR signaling low while allowing substantially better adherence than complete fasting.
Autophagy, literally "eating oneself," is the process by which the cell degrades and recycles its own damaged or obsolete components, including misfolded proteins, dysfunctional organelles, and superfluous cytosolic material; this mechanism, which earned Yoshinori Ohsumi the Nobel Prize in Medicine in 2016, is activated particularly strongly during fasting because the drop in nutrients reduces mTOR signaling, which is the main inhibitor of autophagy, and increases the activity of AMPK, which is the cellular energy sensor that stimulates it.
The relevance of autophagy for health and aging is hard to overstate: deficient autophagy is associated with the accumulation of protein aggregates that are the hallmark of diseases such as Alzheimer's, Parkinson's, and ALS; with greater susceptibility to cancer, because damaged organelles generate mutagenic oxidative stress; and with a diminished immune response, because macrophages need functional autophagy to eliminate intracellular pathogens efficiently.
One especially intriguing study examined the effect of fasting in patients who had recently suffered a myocardial infarction: participants assigned to a time-restricted eating protocol showed significant improvements in markers of cardiac function, inflammation, and oxidative stress compared with controls who ate on their usual schedules, which opens a promising line of research on the use of fasting not only as a preventive strategy but also as a component of the treatment of established cardiovascular disease.
Warnings are necessary: fasting is not appropriate for everyone, and contraindications include pregnancy and breastfeeding, people with type 1 diabetes, those taking hypoglycemic medications, individuals with a history of eating disorders, and any medical condition that requires regular food intake for symptom or medication management; in all other cases, it is advisable to start gradually and to have follow-up by a health professional familiar with the clinical applications of fasting.
Information, not medical advice. Scientific evidence reflects the quality of the studies.