Intermittent Fasting and Other Fasting Methods: An Overview

Fasting has long been more than simply going without food for a period of time. In recent years, numerous different fasting models have become widely known, differing mainly in when and for how long food is consumed. One of the most common approaches is intermittent fasting, in which periods of eating and fasting alternate on a regular basis.

Fasting is by no means a uniform concept. There are substantial differences between a daily eight-hour eating window, individual days with markedly reduced energy intake and several days of complete food abstinence. Accordingly, research findings from one method cannot simply be transferred to other forms of fasting.

The more relevant question is therefore not whether fasting can generally be labeled “healthy” or “unhealthy.” What matters is what happens in the body during a period without food, how different fasting models differ and what can actually be inferred from this for humans.

Table of Contents

1. What happens in the body during fasting?
2. Why metabolism switches between different energy sources
3. Intermittent fasting: What is behind the concept?
4. The main methods of intermittent fasting at a glance
5. 16:8, 14:10 or 18:6: How do the fasting periods differ?
6. What does research say about intermittent fasting?
7. Intermittent fasting and the circadian rhythm
8. What role do AMPK, mTOR and autophagy play?
9. What does fasting mean for longevity research?
10. Is intermittent fasting superior to other dietary approaches?
11. Who might find intermittent fasting useful in everyday life?
12. When is fasting not recommended?
13. What matters in practice when fasting?
14. Common misconceptions about intermittent fasting
15. Limitations of current research
16. Conclusion
17. References

What happens in the body during fasting?

After a meal, the body initially has energy from food available. Glucose from carbohydrates enters the bloodstream and is taken up by cells. Insulin plays a central role here because the hormone signals, among other things, that nutrients are available and can be used or stored.

As the period without food becomes longer, this situation changes. Insulin levels fall while the body increasingly draws on stored energy reserves. At first, glycogen plays a particularly important role. Glycogen is the storage form of glucose and is found especially in the liver and muscles.

As these stores are increasingly used during a longer period without food, the mobilization of fat reserves becomes more important. Adipose tissue releases free fatty acids that can be used by various tissues as an energy source. The liver can also produce so-called ketone bodies from fatty acids, which under certain conditions can serve, among other things, as an alternative energy source for the brain.

Fasting is therefore not a state in which the body suddenly switches to a single energy source. Rather, the relative contribution of different fuels changes gradually. The extent of this shift depends, among other things, on the duration of the fasting period, the previous meal, physical activity and individual metabolic conditions.

Nutrient intake also influences various cellular signaling pathways. When energy and amino acids are available, insulin and the mTOR signaling pathway, among others, promote anabolic processes such as protein synthesis. When nutrient availability is lower, the activity of various signaling pathways and cellular processes shifts.

This adaptability makes fasting interesting for basic research. However, it does not mean that the longest possible period without food is automatically associated with greater health benefits.

Why metabolism switches between different energy sources

The ability to switch between different energy substrates is known as metabolic flexibility. This refers to the metabolism’s ability to adapt its preferred energy source to the situation at hand.

After a meal, a larger proportion of the available glucose is used. During a longer period without food, however, the mobilization of stored energy increases. Both states are part of normal metabolism and regularly alternate in everyday life anyway.

Fasting research is interested in how this switch changes under different conditions. Factors such as metabolic status, diet, physical activity and the duration of the fasting period are taken into account.

It is important, however, to distinguish physiological adaptation from health conclusions. The fact that certain metabolic pathways change during fasting initially means only that the body is responding to a changed nutrient situation. It does not automatically follow that a particular fasting method prevents disease or extends lifespan.

This distinction runs throughout current research. Many of the biological changes can be described relatively well. More difficult is the question of which of them are actually relevant to long-term health outcomes in humans.

Intermittent fasting: What is behind the concept?

Intermittent fasting does not fundamentally prescribe which foods should be eaten. Instead, the focus is on the timing pattern of food intake.

This distinguishes intermittent fasting from dietary approaches that focus, for example, on food composition or a specific calorie intake. In intermittent fasting, daily food intake can, for instance, be limited to a defined time window.

The best-known variant is probably the 16:8 method. In this approach, 16 hours separate the last meal of one day from the first meal of the next. Food is consumed within an eight-hour window.

Other models distribute the restriction across the week. In the 5:2 model, people eat as usual on five days, while energy intake is markedly reduced on two non-consecutive days. With alternate-day fasting, days of strongly reduced energy intake alternate with days of regular eating.

These differences are scientifically relevant. Results from studies of a daily eating window cannot simply be transferred to multi-day fasting. A complete food fast also differs physiologically from an extended overnight fasting period.

A blanket judgment about “fasting” is therefore too simplistic. The key questions are: Which method was studied, for how long and under what conditions?

The main methods of intermittent fasting at a glance

The different fasting models differ primarily in how often and for how long food intake is restricted.

With the daily variants, food intake remains possible every day but is limited to a specific period. With 5:2 or alternate-day fasting, by contrast, the restriction is shifted to individual days.

For practical implementation, how well a method fits into everyday life is also decisive. Working hours, sleep rhythm, family life and shared meals can influence the choice of a particular time window.

Diet quality within the respective eating window also remains an independent factor. A shorter period for meals says nothing by itself about their composition or quality.

16:8, 14:10 or 18:6: How do the fasting periods differ?

A common assumption is that a longer fasting period is fundamentally better. However, current research does not provide a basis for identifying one specific duration as generally superior.

The models first differ in practical terms. With 14:10, a relatively large time window remains available for meals. 16:8 restricts this window more strongly, while 18:6 concentrates daily food intake even further.

Which variant works in everyday life therefore depends on individual circumstances. Someone who, for example, wants to eat breakfast in the morning and have dinner with others in the evening may find a very narrow eating window difficult. For others, such a structure may fit more easily into their daily routine.

Scientific studies also use different time windows. In addition, studies differ in whether participants consciously reduce their energy intake or largely determine their food choices themselves.

For this reason, the number of fasting hours alone should not be the main focus. What matters more is what concrete eating routine results from a particular time window.

What does research say about intermittent fasting?

Research on intermittent fasting has grown considerably in recent years. Changes in body weight and various metabolic parameters have been studied particularly well.

A 2025 systematic review and network meta-analysis published in the BMJ evaluated randomized clinical trials of different forms of intermittent fasting. Among other approaches, time-restricted eating, alternate-day fasting and fasting on individual days were compared with continuous energy restriction or a diet without prescribed energy restriction. The results show that the different strategies vary in their effects, but that the differences compared with continuous energy restriction are limited overall.

A randomized study in the New England Journal of Medicine also examined whether a time-limited eating window provides an additional effect beyond calorie reduction. A total of 139 adults with obesity were assigned for twelve months either to calorie restriction with a daily eating window from 8 a.m. to 4 p.m. or to daily calorie restriction without this additional time window. The study therefore mainly provides an important point for interpretation: A defined eating window does not automatically produce an additional effect compared with a comparable calorie restriction.

This does not make intermittent fasting scientifically uninteresting. Rather, it shows that the timing of food intake and total energy intake must be considered together.

A central question therefore remains open for research: What proportion of the observed changes can be attributed to limiting food intake to a specific time window, and what proportion results from the fact that the time window changes eating behavior and therefore possibly total energy intake?

Intermittent fasting and the circadian rhythm

Not only the length of a fasting period may matter, but also when food is eaten during the day. Human metabolism follows a circadian rhythm that is influenced, among other things, by light, sleep and the timing of food intake.

Many metabolic processes change over the course of the day. These include glucose utilization and insulin sensitivity. It is therefore biologically plausible that the same meal is not processed in exactly the same way at different times of day.

This is where the concept of time-restricted eating comes in. Daily food intake is limited to a specific time window without necessarily prescribing a particular calorie intake. Time windows that shift a larger proportion of food intake into the earlier part of the day have been studied especially intensively.

One randomized study, for example, examined an eight- to ten-hour eating window in adults with metabolic syndrome. The eating window ended at least three hours before bedtime. After three months, changes were observed in body weight and certain cardiometabolic parameters, among other outcomes. However, the study was relatively small and had a short observation period.

Research therefore provides interesting indications of a possible relationship between meal timing and metabolism, but not yet a basis for defining a universally optimal time of day.

In practice, this does not mean that eating late is automatically problematic. Rather, it shows that when fasting is studied, both the duration of the eating window and its position within the day should be taken into account.

What role do AMPK, mTOR and autophagy play?

Fasting changes more than the availability of energy. Different molecular signaling pathways also respond to the altered nutrient situation. AMPK, mTOR and autophagy are studied particularly frequently in this context.

In simplified terms, AMPK can be described as an energy sensor of the cell. When available energy declines, AMPK can become more strongly activated. This supports, among other things, processes that provide energy, while certain energy-consuming anabolic processes are reduced.

mTOR responds, among other things, to nutrients such as amino acids and to signals of energy availability. When nutrients are sufficiently available, this signaling pathway supports cell growth and protein synthesis, among other processes. During a period without food, its activity changes.

Closely related to this is autophagy. This is a natural cellular recycling process. Cellular components that are damaged or no longer needed can be broken down and their constituents reused.

These relationships have contributed substantially to the strong interest in fasting within longevity research. Nevertheless, simple statements should be treated with caution. Autophagy also takes place under normal conditions and is not a process that suddenly begins after a specific number of fasting hours.

In addition, a large proportion of mechanistic research comes from cell cultures and animal models. Molecular changes observed in the laboratory are not automatically equivalent to a demonstrated health benefit in humans.

For precisely this reason, statements such as “autophagy starts after 16 hours” are not sufficiently supported scientifically. Reliable human data are lacking that would allow a particular point in time to be defined as a universal switch.

What does fasting mean for longevity research?

The link between fasting and longevity is one of the most interesting areas of research at the intersection of nutrition and aging biology. In various animal models, calorie restriction and certain forms of food restriction have been associated with changes that may relate to lifespan.

Potential mechanisms under investigation include changes in signaling pathways such as mTOR and AMPK, adaptations in insulin signaling, changes in autophagy and altered responses to cellular stress.

Processes related to mitochondrial function and regulation of inflammation are also being studied in this context. Fasting therefore touches several biological systems that also play a role in research on the Hallmarks of Aging.

The key limitation remains, however: Results from mice, worms or cell cultures cannot be directly transferred to human lifespan.

In humans, long-term randomized studies showing that a particular fasting method actually leads to a longer lifespan are still lacking. Changes in various metabolic parameters are being investigated, but no concrete increase in life expectancy can be inferred from them.

For longevity research, fasting is therefore particularly interesting as a biological research model. It allows researchers to investigate relationships between nutrient availability, metabolism and cellular adaptation.

A proven anti-aging effect in humans cannot currently be derived from this.

Is intermittent fasting superior to other dietary approaches?

This question is especially important for practical interpretation. If intermittent fasting leads to weight loss, this may also be because less energy is consumed overall within the limited time window.

Research suggests that intermittent fasting and continuous calorie restriction produce similar results for many of the outcomes studied. This argues against the idea that a particular fasting window must inherently have a much stronger effect.

However, this does not reduce the practical value of intermittent fasting. For some people, it is easier to limit certain meals or snacks by time than to count calories every day.

Other people find a narrow eating window impractical. Someone who wants to eat breakfast, regularly has dinner with family or friends, or needs flexibility because of work may do better with a different dietary structure.

Practicality in everyday life is therefore an important factor. A dietary strategy must not only work in theory but also be sustainable over time.

Who might find intermittent fasting useful in everyday life?

Intermittent fasting may be of interest to people who want to structure their meals more clearly in time and tolerate longer intervals between meals well.

A limited eating window may, for example, reduce incidental snacking or late-evening eating. Whether this actually results in a lower energy intake, however, depends on individual eating behavior within the time window.

Research on intermittent fasting has so far been conducted particularly often in adults with overweight, obesity or increased metabolic risk. In these groups, there are indications of moderate changes in body weight and certain metabolic parameters.

However, this does not mean that intermittent fasting is necessary or fundamentally better suited for everyone. Other dietary approaches can lead to a comparable energy intake and therefore similar outcomes.

In people with normal body weight, the evidence for long-term preventive effects is much more limited. Current research does not show that intermittent fasting is necessary in order to remain healthy.

The decisive question is therefore whether the chosen dietary structure allows an adequate intake of energy and nutrients and can be sustained in everyday life over the long term.

When is fasting not recommended?

Fasting is not equally suitable for everyone. Especially with certain medical conditions or in particular life situations, a marked restriction of food intake should not be undertaken without professional guidance.

Caution is warranted, for example, in people with diabetes who use glucose-lowering medication. Changes in meal timing can affect the action of certain medications and therefore also the risk of hypoglycemia. Anyone taking such medication should therefore discuss changes in eating patterns with their treating physician.

During pregnancy and breastfeeding, deliberate restriction of energy intake is also not the priority. Energy and nutrient requirements change during these phases of life.

For people with current or previous eating disorders, a highly structured fasting window can also be problematic. Dividing time into permitted and non-permitted eating periods may reinforce restrictive eating behavior in some people.

The same applies to people who are underweight or who have difficulty consuming sufficient energy or protein because of an illness. Additional restriction of food intake can make adequate nutrition more difficult in such situations.

Fasting should therefore not be understood as a universal dietary strategy. Baseline situation, goal and individual tolerance are always decisive.

What matters in practice when fasting?

Anyone who wants to try intermittent fasting does not need to begin with the longest possible fasting window. A moderate start may be easier to integrate into everyday life and can quickly show whether the chosen structure fits the individual daily routine at all.

One option is initially to extend the overnight period without food. Someone who, for example, stops eating after dinner and has breakfast a little later the next morning extends the fasting period without having to reorganize the entire day.

The timing of the last meal can also be considered. An eating window that does not end immediately before bedtime corresponds to concepts studied in research on meal timing. However, no particular clock time can be derived from this as universally optimal.

Within the eating window, the diet should remain nutrient-rich and balanced. Vegetables, fruit, legumes, whole grains, nuts and suitable protein sources can provide an important foundation.

Protein intake deserves particular attention if fasting regularly results in one meal being omitted. Especially at older ages, an adequate protein intake is relevant because maintaining muscle mass and physical function becomes increasingly important.

Fluid intake should also not be overlooked during fasting periods. Water and unsweetened beverages can help meet fluid requirements.

Fasting is not a competition

A longer fasting window is not automatically a better fasting window. Anyone who regularly feels unwell during the fasting period, has difficulty concentrating or experiences excessive hunger afterwards may have chosen a strategy that is not well suited to their everyday life.

The most sensible fasting method is not necessarily the strictest one, but the one that can be maintained over the long term alongside a balanced diet and everyday life.

Common misconceptions about intermittent fasting

“Fat burning automatically starts after 16 hours.”

The body uses different energy sources even after a normal meal. During a period without food, only the relative contribution of these energy sources changes. The use of fat as an energy source increases with longer fasting, depending among other things on the starting situation and physical activity.

There is therefore no fixed point in time at which fat burning suddenly begins.

“The longer I fast, the better.”

There is no robust human evidence for this. Longer fasting periods lead to stronger changes in nutrient availability, but this does not automatically translate into additional health benefits.

“Fasting activates autophagy after 16 hours.”

Autophagy is a continuous cellular process. A period without food can influence various signaling pathways associated with this process. However, there is insufficient evidence for an exact number of hours after which a defined autophagy effect begins in humans.

“Intermittent fasting works regardless of calorie intake.”

An eating window only determines when food is consumed. Total energy intake and meal composition remain separate factors.

“Anyone who skips breakfast is automatically fasting healthily.”

Skipping a meal by itself says little about the quality of the overall diet. Nutrient intake, dietary composition and individual circumstances remain important.

Limitations of current research

The number of studies on intermittent fasting has increased sharply in recent years. Nevertheless, important questions remain unanswered.

Many studies last only a few weeks or months. Such periods are limited when it comes to assessing long-term health developments. Especially in healthy people, studies comparing different fasting models over many years under comparable conditions are lacking.

Another problem is that very different strategies are grouped together under the term intermittent fasting. A daily 14:10 method differs substantially from alternate-day fasting or a multi-day period without food.

The comparison groups are not always identical either. In some studies, a fasting model is compared with a normal diet; in others, with targeted energy restriction. As a result, it is not always possible to determine clearly what proportion of an observed difference is due to the timing of meals.

There are also differences among the people studied. Age, body weight, sex, sleep, physical activity and dietary pattern can all influence the results.

Current research therefore mainly allows conclusions about how different fasting models perform under specific study conditions. It does not provide a basis for a universal recommendation as to which fasting window is optimal for everyone.

The question of long-term effects on aging also remains open. Biological mechanisms from cell and animal models are interesting, but they are not sufficient to infer an effect on human lifespan.

Conclusion

Intermittent fasting is not a single dietary concept, but includes different models in which periods of food intake and fasting alternate. From 14:10 and 16:8 to 5:2 and alternate-day fasting, the methods differ substantially.

Research shows that the timing of food intake can influence metabolism. At the same time, the findings to date cannot be reduced to a general recommendation for a particular fasting duration.

Fasting is particularly interesting for basic research because periods without food alter, among other things, the availability of energy and nutrients as well as various cellular signaling pathways. These include AMPK, mTOR and processes related to autophagy.

This gives rise to interesting questions for longevity research. However, an extension of human lifespan through intermittent fasting has not yet been demonstrated.

In practical nutrition, fasting should therefore not be viewed in isolation. Food quality, adequate energy and nutrient intake, physical activity, sleep and individual circumstances remain important components of a balanced long-term lifestyle.

Intermittent fasting can be one way to structure eating patterns. However, it is neither the only dietary approach nor, based on current knowledge, fundamentally superior to other forms of nutrition.

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