© Freepik
September 5, 2026
Birgitta Dunckel
Intermittent fasting can offer real health benefits — but is 16 hours enough, or does fasting longer offer more? Here is what the research says about fasting duration and healthy aging.
Find a DoctorSearch the network for suitable specialists and contact them directly
16:8 intermittent fasting, going a full day without eating, or even fasting for several days — is all of this fasting? Few dietary strategies are as frequently linked to longevity as the temporary abstinence from food. And indeed, studies show that fasting can have a beneficial effect on weight, blood sugar, blood pressure, and other metabolic markers. But does the principle of "the longer, the better" apply to fasting? And how many hours actually make sense when the goal is not just weight loss, but healthy aging?
Research does not yet offer a straightforward answer. While the health benefits of certain forms of fasting are now well documented, a harder question remains: Can fasting also influence the biological aging process — and if so, what fasting duration would be optimal for that?
When food is withheld, the metabolism gradually shifts. Insulin and glucose levels change, glycogen stores are drawn upon, and the body increasingly turns to its fat reserves. During prolonged fasting, the production of ketone bodies also rises.
At the same time, the food-free period influences cellular signaling pathways that regulate growth, energy supply, and repair processes, among other things. It is precisely these mechanisms that are of interest to aging researchers.
Caloric restriction — that is, permanently consuming less energy without causing nutritional deficiencies — has been particularly well studied. In various model organisms, it can extend both lifespan and healthspan. Fasting studies in humans also show health benefits, especially for metabolic health.
One of the most intriguing clues comes not from a classic fasting study, but from research on caloric restriction.
In the CALERIE study, 220 healthy adults without obesity were assigned to either a caloric restriction group or a control group. The intervention group was asked to reduce their caloric intake by 25 percent over two years.
Researchers then used various epigenetic methods to determine whether changes in biological aging could be detected. Two so-called epigenetic clocks showed no significant effect. The DunedinPACE method, however, suggested that the rate of biological aging slowed by approximately two to three percent.
The effect was modest, but it is quite noteworthy for longevity research. CALERIE is one of the few randomized controlled human studies to provide any indication that a dietary intervention might influence a marker of aging. Whether this will ultimately translate into a longer healthspan or lifespan cannot be concluded from this data alone.
In everyday life, intermittent fasting is far more popular than permanent caloric restriction. With so-called time-restricted eating, food is consumed only within a set window each day — for example, eight hours in the well-known 16:8 method.
The health benefits of this approach are now well established. A systematic review and network meta-analysis of randomized trials published in 2026 found that time-restricted eating can improve body weight, fat mass, waist circumference, systolic blood pressure, as well as fasting blood glucose, insulin, and triglycerides, among other parameters.
Such effects are particularly relevant for healthy aging. This is because excess weight, high blood pressure, and impaired glucose and lipid metabolism are among the factors that raise the risk of chronic disease in old age.
The widely cited 16-hour threshold, however, cannot be identified from the data as an optimum. Particularly short eating windows did not outperform longer ones across all parameters studied. A scientifically validated ranking along the lines of 18:6 being better than 16:8, and 16:8 being better than 14:10, does not currently exist.
Another form of intermittent fasting is alternate-day fasting, in which fasting days and normal eating days alternate.
A randomized human study published in the journal Cell Metabolism examined this approach in healthy adults without obesity. After four weeks of strict alternate-day fasting, participants had consumed approximately 37 percent fewer calories on average. Body weight decreased by about 4.5 percent, fat mass declined, and various cardiovascular and metabolic markers shifted favorably. LDL cholesterol and the inflammatory marker sICAM-1 decreased, while levels of the ketone body beta-hydroxybutyrate increased.
However, the fasting was accompanied by a substantially lower caloric intake and weight loss. It is therefore not possible to clearly determine what role the length of the fasting periods themselves played in the positive changes.
The body's response becomes even more pronounced during multi-day fasting. A study published in Nature Metabolism in 2024 followed twelve healthy adults who consumed nothing but water for seven days.
The researchers analyzed approximately 3,000 proteins in the blood on a daily basis. Changes in energy metabolism appeared early on, while more extensive systemic changes in the proteome only emerged after about three days. In total, more than 1,000 proteins responded over the course of the fasting period.
This demonstrates how profoundly the body adapts to prolonged food deprivation. Whether these molecular changes confer any long-term health benefit, however, cannot be answered by this small study.
A further study on seven-day fasting also revealed potential drawbacks. In addition to fat, participants lost a significant amount of lean body mass. While maximum muscle strength was preserved, maximum oxygen uptake decreased by approximately 13 percent and performance during intense exercise declined.
This is particularly relevant in the context of healthy aging. Muscle mass and physical performance are important factors in staying fit and independent for as long as possible. Fasting for several days does trigger stronger physical adaptations — but an additional health benefit over shorter fasting periods has not been demonstrated.
Few concepts are as closely linked to fasting and longevity as autophagy. It refers to a natural recycling process carried out by cells: damaged or superfluous cellular components are broken down and their building blocks recycled.
Fasting and nutrient deprivation influence signaling pathways associated with autophagy. However, many of the fundamental insights in this area come from cell and animal experiments. A scientific review on fasting, autophagy, and longevity describes autophagy as one of the mechanisms through which fasting and caloric restriction might influence aging processes. Translating these findings to humans, however, is complex.
"In humans, no fixed number of hours can currently be specified at which autophagy suddenly 'begins.' Statements such as 'autophagy starts after twelve hours' or 'cellular cleanup begins after 16 hours' imply a biological on-off switch that simply does not exist in this form. Autophagy occurs continuously by nature and does not proceed the same way in every tissue.
The more extensive protein changes observed after approximately three days in the seven-day fasting study therefore do not mean that a particularly effective "cellular cleanup" only kicks in after 72 hours.
With daily intermittent fasting, another question is increasingly coming into focus: not just how long, but also when we eat.
In the previously mentioned network meta-analysis published in 2026, early time-restricted eating performed more favorably than late time-restricted eating across several metabolic parameters. An eating window that ends earlier in the day may therefore be metabolically more advantageous than an equally long eating window that extends late into the evening.
This aligns with chronobiology: insulin sensitivity, glucose processing, and numerous other metabolic functions all follow our circadian rhythm.
From a practical standpoint, this is quite significant. Rather than extending the fasting window further and further, it may be more beneficial to shift the eating window earlier in the day and avoid eating very late at night.
Fasting can certainly be good for your health. Human studies show positive effects of intermittent fasting in particular on weight and various metabolic parameters. The question of optimal fasting duration, however, cannot be answered with a single number.
The more compelling longevity question remains open: whether fasting, beyond its general health effects, slows the biological aging process or even extends lifespan has not yet been demonstrated in humans. The CALERIE study on caloric restriction offers an intriguing clue, in that a marker for the rate of biological aging changed to a small degree.
Sixteen hours is therefore not a magic threshold — and 24, 36, or 72 hours are not automatically better. For healthy aging, what likely matters less is how long one goes without eating, and more is finding a form of fasting that can be sustainably combined with a balanced diet and the preservation of muscle mass and physical performance.
Dark circles are often blamed on lack of sleep. But pigmentation, visible blood vessels, genetics, or facial anatomy may also play a role. Dermatologist Dr. Susanne Steinkraus explains why the cause matters when choosing the right treatment – and when caution is advised.
Christine Bürg & Marianne Waldenfels
An interview with
Dr. med. Susanne Steinkraus
"Feel Good, Live Better" – In this podcast episode, Prof. Dr. Carina Riediger explains why liver tumors can be cured more often than you might expect.
Christine Bürg
An interview with
Prof. Dr. med. Carina Riediger, MSc
Medfluencers enjoy a high level of trust and reach millions with health advice. But on social media, reach often counts for more than scientific evidence. Prof. Dominik Pförringer examines the troubling intersection of medicine, influence, and commercial interests.
Prof. Dominik Pförringer
By
Univ.-Prof. Dr. med. Dominik Pförringer
