Whole-day fasting (5:2 and alternate day): who is it worth it for?
Whole fasting days can have a stronger effect on insulin and fat metabolism than a shorter eating window. But they also ask more of you: in daily life, in your head and in your hormonal system. Here you can read what studies show, how the demands compare with 16:8 and how to tell whether the step to 5:2 may suit you.
More restriction does not automatically mean more health. The right form of fasting is the one your system can carry, without you having to win every fasting day against yourself.
Maybe you're at exactly this point right now.
16:8 has been running for months. You no longer miss breakfast, and your afternoons have become calmer. And yet the number on the scale has almost stopped moving.
Then you read about people who eat almost nothing on two days a week. Or every other day. More autophagy, they write. Better insulin levels. Part of you thinks: I can do that.
Another part of you thinks of dinner with friends on Thursday. Of the birthday party on Saturday. And of the mood you're in when you're properly hungry.
If you recognise yourself here, the question is not whether you are disciplined enough. It is whether this form of fasting suits your body and your life.
I'm writing this as a physician who values fasting and takes its limits seriously. Three questions interest me. What do whole fasting days offer compared with 16:8 and with a daily calorie reduction? What do they demand of you? And who might the step be worth it for?
You'll find the basics of a shortened eating window in Intermittent fasting 16:8. How to ease gently into fasting is covered in Fasting for beginners. This article is about the next step: whole days with very little or no food at all.
What to expect in this article
- What exactly 5:2 and alternate day fasting mean
- What studies show on weight and insulin
- What lies behind autophagy and ketones
- How demanding fasting days are for hunger, daily life and hormones
- A suitability check for switching from 16:8
- When whole-day fasting is not a good fit
- Three levers for a careful trial
- Answers to the most common questions
How I mark the evidence
Clinical trial Randomised controlled trials and meta-analyses in humans.
Human observation Studies without a control group, cross-sectional studies, case series and reviews. They show associations and mechanisms, not necessarily cause and effect.
What is whole-day fasting? 5:2, alternate day and the difference from 16:8
In short: With whole-day fasting, fasting days and normal eating days alternate. On the 5:2 diet there are two fasting days per week, with alternate day fasting it is every other day. In many studies a fasting day does not mean zero, but about a quarter of your energy needs, usually around 400 to 650 kilocalories. With 16:8, on the other hand, you eat every day, just within a shorter window.
The terms sound technical. Behind them lies a simple idea. Instead of eating a little less every day, you cut back sharply on certain days.
Imagine your energy balance as a bank account. 16:8 mainly shifts when you make withdrawals. A daily calorie reduction saves a small amount every day. Whole-day fasting saves a lot on certain days and lets you spend more or less normally on the others.
Review A team at the University of Illinois Chicago led by Varady summarised the evidence on the three best-known forms of fasting. It describes alternate day fasting as fasting days with 0 to 500 kilocalories alternating with eating days with no set target. The 5:2 diet consists of two fasting days and five normal days per week. With time-restricted eating, you eat every day within a set time window.
For you, this means: not all fasting days are the same. Whether you eat zero or 500 kilocalories makes a big difference to your daily life.
Varady KA et al. Nat Rev Endocrinol. 2022. DOI: 10.1038/s41574-022-00638-x16:8
Time-restricted eating- Eating every day, within a window of about eight hours
- Fasting phase of around 16 hours, mostly overnight
- No calorie target
- Usually fits easily into daily life
5:2
Two fasting days per week- Two non-consecutive fasting days
- In studies, mostly about 400 to 650 kilocalories on these days
- Five days with no set target
- Fasting days can be moved within the week
Alternate Day
Every other day- Three to four fasting days per week
- 0 to 500 kilocalories on a fasting day
- With no food on the fasting day, around 36 hours in a row
- The strictest of the three forms
Filled dots stand for fasting days in a sample week. An in-between form is 4:3, with three fasting days per week, which was tested in a large study in 2025.
The most important difference lies in the duration. With 16:8 you are in a fasted state for a few hours every day. If you eat nothing at all on a fasting day, around 36 hours pass between dinner the evening before and breakfast the morning after. That was the case in a study of zero-calorie fasting days. With a small meal on the fasting day the break gets shorter, but the calorie gap stays large.
Whole-day fasting is not a tougher version of 16:8. 16:8 organises your day. 5:2 and alternate day fasting change your week. That sounds like a small difference, but in everyday life it feels like a different tool.
And now you know why the jump from 16:8 to 5:2 is not another step on the same staircase, but a move to a different staircase.
Does more restriction bring more? What studies show on weight and insulin
In short: When it comes to weight, whole fasting days are mostly on a par with a daily calorie reduction. In a large network meta-analysis in the BMJ, people on alternate day fasting lost on average 1.3 kilograms more than with daily calorie reduction, mainly in shorter studies. A Cochrane review found hardly any difference compared with standard dietary advice. For insulin, there are signs of an added benefit, especially when insulin resistance is already present.
Maybe you're hoping that more restriction automatically brings more results. That's a very human thought. The studies paint a calmer picture.
Weight: a small lead that shrinks over time
Network meta-analysis A team from Toronto and Harvard led by Semnani-Azad analysed 99 randomised trials with 6,582 adults. Alternate day fasting was the only form of fasting that showed slightly more weight loss than daily calorie reduction, on average 1.29 kilograms. Compared with time-restricted eating the difference was 1.69 kilograms, and compared with 5:2-style whole-day fasting around 1 kilogram. In the 17 trials lasting 24 weeks or more, an advantage only showed up compared with eating without guidelines, and no longer between the diet types.
For you, this means: fasting every other day can give you a small lead in the short term. In the long run, what counts most is what you can stick with.
Semnani-Azad Z et al. BMJ. 2025. DOI: 10.1136/bmj-2024-082007A second network meta-analysis with 24 trials and 1,768 participants arrived at a similar ranking: alternate day fasting at the top, followed by daily calorie reduction and time-restricted eating. Overall, however, people on intermittent fasting lost about as much weight there as with classic calorie reduction. The paper was produced largely at the research institute of a food company, and that belongs in the picture too.
Cochrane review A team from Buenos Aires analysed 22 trials with 1,995 adults with overweight or obesity for Cochrane. Compared with standard dietary advice, intermittent fasting made little to no difference to weight within one year, on average 0.33 percentage points. The authors recommend weighing readiness, everyday practicality and sustainability individually for each person.
For you, this means: intermittent fasting is one tool among several. Whether it suits you depends partly on your daily life.
Garegnani LI et al. Cochrane Database Syst Rev. 2026. DOI: 10.1002/14651858.CD015610.pub2RCT, n=100 At the University of Illinois Chicago, 100 adults with obesity spent a year either eating only a quarter of their energy needs every other day or three quarters of it every day. A third group had no guidelines. After twelve months, both diet groups had lost a similar amount of weight, 6.0 and 5.3 percent compared with the control group. In the fasting group 38 percent dropped out, in the daily reduction group 29 percent. On fasting days the participants ate more than intended, on eating days less.
For you, this means: even in a well-supported study, many people found the fasting day hard. The plan on paper was stricter than what life allowed.
Trepanowski JF et al. JAMA Intern Med. 2017. DOI: 10.1001/jamainternmed.2017.0936RCT, n=165 In Colorado, a team led by Catenacci tested a variant with three fasting days per week, on which participants cut their energy intake by 80 percent. The comparison group ate 34 percent less every day, so the weekly deficit was the same. Both groups received an intensive support programme with group meetings and an exercise recommendation. After twelve months, the fasting group had lost on average 2.89 kilograms more.
For you, this means: with good support, going without food for whole days can bring a moderate advantage. Whether it shows up in a similar way in daily life without this structure is an open question.
Catenacci VA et al. Ann Intern Med. 2025. DOI: 10.7326/ANNALS-24-01631Insulin: this is where whole fasting days might have a lever of their own
With insulin, things get more interesting. Insulin is the hormone that tells your cells: store, don't burn. If it stays high all the time, the storeroom practically never closes. A whole fasting day can keep this switch on release for longer than a few hours' break.
RCT, n=107 At the Genesis Prevention Centre in Manchester, a team led by Harvie compared two routes to 25 percent less energy. 107 premenopausal women either ate about 2,700 kilojoules, around 650 kilocalories, on two days a week, or cut back every day. After six months the weight loss was similar, 6.4 and 5.6 kilograms. Fasting insulin and insulin resistance fell moderately in both groups, slightly more with the fasting days.
For you, this means: the added benefit for insulin was measurable, but small.
Harvie MN et al. Int J Obes (Lond). 2011. DOI: 10.1038/ijo.2010.171RCT, secondary analysis, n=43 The Chicago group later looked only at the participants in their one-year study who were already insulin resistant. In them, fasting insulin fell by 52 percent with alternate day fasting and by 14 percent with daily calorie reduction. Insulin resistance dropped by 53 versus 17 percent, even though both groups lost a similar amount of weight.
For you, this means: if your insulin is high, whole fasting days might offer a lever of their own. However, this is a post hoc analysis of 43 people.
Gabel K et al. Obesity (Silver Spring). 2019. DOI: 10.1002/oby.22564A meta-analysis from Maastricht with 28 trials fits this picture. There, alternate day fasting improved the HOMA index, a measure of insulin resistance, more than daily calorie reduction. For weight and fat mass there was no difference. Fat-free mass fell by around 0.2 kilograms more with the fasting forms taken together.
RCT, n=36 At the University of Bath, a team led by Templeman tested three patterns in lean, healthy adults over three weeks. Fasting every other day with 150 percent of energy needs on eating days lowered body fat by 0.74 kilograms. The same saving, spread evenly across every day, lowered it by 1.75 kilograms. The researchers found no fasting-specific benefits for metabolism or blood vessels.
For you, this means: if you are lean and metabolically healthy, whole fasting days do not automatically bring more. In this study they even came off worse for fat loss.
Templeman I et al. Sci Transl Med. 2021. DOI: 10.1126/scitranslmed.abd8034Whole fasting days are not a turbo boost for the scale. They are more like a different gear. Where they show an advantage, it lies mainly in insulin and in people whose metabolism is already under pressure.
What does this mean for you? If your main goal is weight, choose the form you can sustain for many months. If your fasting insulin or your HOMA index is raised, whole-day fasting may be an option to discuss with your doctor. You can read how fasting and insulin resistance are connected in Fasting and blood sugar and in Recognising insulin resistance.
And now you know why more restriction on paper does not automatically mean more progress in life.
Autophagy and ketones: what lies behind the deeper effect
In short: Whole fasting days lengthen the time your body spends running on fat and ketones instead of sugar. This shift usually only begins beyond twelve hours without food. In a study from Graz, ketones rose under alternate day fasting even on eating days. That more autophagy also takes place is biologically plausible, but has hardly been measured directly in humans.
Autophagy sounds like spring cleaning for your cells. And if you're going without for a whole day, it should at least pay off on the inside too. That's understandable.
Imagine your metabolism as a hybrid car. First you drive on the battery, the sugar store in your liver. Only when it runs low does the second engine kick in: fatty acids and the ketones made from them.
Review A team led by Anton from the University of Florida and the National Institute on Aging describes this moment as the metabolic switch. It occurs where the liver's sugar stores run empty and fatty acids are mobilised, typically beyond twelve hours without food. The authors propose using blood ketones as a marker of whether, and how strongly, this switch is flipped.
For you, this means: with 16:8 you reach this zone for only a few hours. A fasting day keeps you in it considerably longer.
Anton SD et al. Obesity (Silver Spring). 2018. DOI: 10.1002/oby.22065RCT, 4 weeks A team at the University of Graz led by Stekovic and Madeo had healthy, middle-aged adults without obesity follow strict alternate day fasting for four weeks. On average they took in 37 percent less energy. The ketone beta-hydroxybutyrate rose, even on eating days. In people who had already been fasting this way for more than six months, the researchers found, among other things, a lower level of the inflammatory marker sICAM-1, lower LDL and lower T3, the active thyroid hormone. No side effects occurred in this study.
For you, this means: on alternate day fasting your body can stay in fat-burning mode for longer. A falling T3 can be an energy-saving adaptation. In my view, it's worth paying attention to whether you feel cold or tired more often.
Stekovic S et al. Cell Metab. 2019. DOI: 10.1016/j.cmet.2019.07.016And autophagy? A review from Mashhad in Iran led by Bagherniya concluded that fasting and calorie restriction upregulate autophagy in many tissues and organs. However, a large part of the underlying evidence comes from animal and cell models. In humans, autophagy in living tissue can still hardly be measured directly. Which fasting duration triggers how much cellular cleanup in humans therefore remains open. You can read more in Autophagy: when does it start? and in Recognising ketosis during fasting.
Measured in humans: Longer breaks from food raise ketones and the share of fat burning. Under alternate day fasting, ketones stayed raised even on eating days in one study.
Mechanistically plausible, open in humans: that whole fasting days trigger more autophagy than 16:8, and that this leads to a noticeable health benefit.
My assessment: Autophagy is a nice side thought, but not a solid main reason to switch.
Autophagy is not a switch you flip at hour 24. It's more like a dimmer whose setting no one can yet read reliably in humans. If you switch to whole fasting days only for autophagy, you're building on the least certain argument.
And now you know why the deeper effect of whole fasting days is well described for metabolism, but remains an open question for cellular cleanup.
The demands compared: hunger, daily life and hormones
In short: Whole fasting days demand more of you than 16:8 on three levels. Hunger on fasting days was clearly noticeable in some studies, and in one study it stayed raised for three weeks. Your daily life has to be planned around the fasting days. And hormonal axes such as the thyroid, stress axis and reproductive axis respond measurably to longer breaks from food, often temporarily.
It's 5 pm on a fasting day. You've barely eaten since the evening before. Your head is clear, but your patience is wearing thin. There's cake in the office. At home it smells of dinner.
This is exactly where it's decided whether whole-day fasting suits you. Not in the lab, but in these hours.
Hunger: does the body get used to it?
Intervention study, n=16 At the Pennington Biomedical Research Center in Louisiana, 16 adults without obesity fasted every other day for 22 days with no calories at all. Hunger rose on the first fasting day and stayed raised throughout. Fasting insulin fell by 57 percent, and fat burning increased. The researchers themselves suggested allowing a small meal on fasting days to make the approach more sustainable.
For you, this means: in people without obesity, hunger on zero-calorie days did not settle down during this period.
Heilbronn LK et al. Am J Clin Nutr. 2005. DOI: 10.1093/ajcn/81.1.69Intervention study, n=16 A study from Chicago with 16 adults with obesity showed a different picture. On fasting days they ate an average of 501 kilocalories, about a quarter of their needs. Hunger on fasting days eased from the second week onwards and stayed low after that. On eating days they did not overeat, and their daily step count stayed the same.
For you, this means: with a small meal on the fasting day, your body can get used to the rhythm.
Klempel MC et al. Nutr J. 2010. DOI: 10.1186/1475-2891-9-35In a one-year study from Oslo with 112 adults, both groups lost a similar amount of weight, 8.0 and 9.0 kilograms. But the group with two fasting days per week, 400 kilocalories for women and 600 for men, reported more hunger: 4.7 versus 3.6 points on a numerical scale. In a small pilot study from Colorado, on the other hand, 93 percent of participants kept up eight weeks of zero-calorie fasting every other day. So hunger is not destiny, but it is a cost you need to factor in.
Daily life: fasting days need a calendar
16:8 can usually be built into a day without anyone noticing. A fasting day, on the other hand, wants the whole week planned around it. Business dinners, family celebrations, sport and shift work clash with fasting days more often than with a postponed breakfast.
The studies reflect this. In the one-year study from Chicago, participants ate more than planned on fasting days. In the successful study from Colorado, everyone received an intensive support programme with group meetings. That is more structure than most people have available in daily life.
Hormones: your body keeps count
Through the lens of Clinical Psychoneuroimmunology, a fasting day is more than a calorie gap. It is a signal that all four major control systems of your body read.
Nervous system
Hunger activates alertness and drive. That can give you focus. But in a nervous system that is already tense, a fasting day can also amplify restlessness, irritability or poor sleep.
Hormonal system
The thyroid, stress axis and reproductive axis respond to longer breaks from food. In studies, T3 and the number of LH pulses fell, and with longer fasting cortisol rose. These are mostly adaptations, not damage.
Metabolism
Insulin falls, ketones rise, the share of fat burning increases. This is where whole fasting days are strongest.
Immune system
In the Graz study, an age-associated inflammatory marker was lower in people with long fasting experience. Whether this leads to a clinical benefit is open.
Physiology study, n=8 At the University of Virginia, a team led by Bergendahl examined eight healthy men on a normal eating day and on the fifth day of a water fast. On the fasting day they released 1.8 times as much cortisol over 24 hours. The peak shifted from the morning to the early afternoon.
For you, this means: your body also reads longer fasting as stress. A single fasting day is much shorter than five days and has hardly been studied in this respect. If your stress axis is already running at its limit, I still see this as a reason for caution.
Bergendahl M et al. J Clin Endocrinol Metab. 1996. DOI: 10.1210/jcem.81.2.8636290RCT, n=12 At the National Institute of Child Health and Human Development, normal-weight women fasted for three days in the first half of their cycle. On the last fasting day the number of LH pulses fell, the rhythmic signals the pituitary gland sends to the ovaries. Follicle maturation, ovulation and cycle length, however, stayed unchanged.
For you, this means: your reproductive system registers breaks from food. In healthy, normal-weight women, however, it proved robust in this study.
Olson BR et al. J Clin Endocrinol Metab. 1995. DOI: 10.1210/jcem.80.4.7714088A review by the Chicago group looked at premenopausal women with obesity. On intermittent fasting, they had lower androgens and more SHBG, the transport protein for sex hormones. Oestrogen, prolactin and the pituitary's regulatory hormones did not change. In lean, athletic young men, testosterone fell without muscle mass or strength suffering. With raised androgens, for example in PCOS, this might even be of interest. However, the authors stress that there are still very few studies. You can read how to adapt fasting to your cycle in Fasting, women and the cycle.
Demands compared
My assessment based on the studies cited and my clinical experience. These are not measurements, just a rough guide.
| Area | 16:8 | 5:2 | Alternate Day |
|---|---|---|---|
| Hunger on fasting days | low | medium | high |
| Planning in daily life | low | medium | high |
| Signal to stress and hormonal axes | low | medium | high |
| Risk with glucose-lowering medication | medium | high | high |
Load is not a bad thing. Muscles grow through load, and your metabolism also learns through stimuli. What matters is whether your system reads a fasting day as a training stimulus or as yet another threat. That depends less on the method than on whatever else is weighing on you in your life right now.
If you're feeling exhausted right now or sleeping badly, read Fasting with exhaustion and Intermittent fasting and sleep first.
And now you know why the same form of fasting can be a training stimulus for one person and an additional stressor for another.
Who the jump from 16:8 to 5:2 may be worth it for
In short: Whole fasting days may be worth it if 16:8 is running steadily for you, you sleep well, your energy is reliable and a metabolic goal such as raised insulin is in the foreground. They are less likely to suit you if you are already struggling with 16:8 or have cravings, sleep problems, exhaustion or cycle disorders. If at all, I see 5:2 as the gentler entry point compared with alternate day fasting.
Maybe you're looking for a clear yes or no here. No article can give you that. But there are signs you can orient yourself by.
Good conditions for a careful trial
- 16:8 is running steadily. For weeks, without a struggle, without cravings and without evening binges.
- Your sleep holds up. You mostly sleep through the night and wake up reasonably rested.
- Your weight is normal or raised, not underweight.
- If you menstruate: your cycle is regular.
- There is a metabolic goal, such as raised fasting insulin, a raised HOMA index or a lot of belly fat.
- Your daily life has room for two set days a week without business dinners, family celebrations or hard training.
- No glucose-lowering medication, or your treatment has been adjusted to fasting days by your doctor.
- For you, food is not a matter of control, guilt or compensation.
Better not right now
- You are already struggling with 16:8 or constantly thinking about food during the fasting window.
- You have bouts of cravings or eat well beyond your hunger after fasting phases.
- You sleep badly or wake up at night with a pounding heart.
- You feel exhausted, burnt out or constantly cold.
- Your cycle has become irregular or has stopped.
- You train very intensely or do hard physical work.
- Your iron stores or other nutrient stores are low.
For some of these points you'll find dedicated articles: Cravings during intermittent fasting, Intermittent fasting and iron deficiency, Fasting and exercise and Muscle loss during fasting.
5:2 or alternate day: where to start?
If you want to take the step, I see 5:2 as the gentler entry point. Two fasting days leave you five days of room for your life. Alternate day fasting did show a slightly larger effect on weight in studies. But in the one-year study from Chicago, the dropout rate was also highest in this group.
There are also middle paths. An earlier or slightly shorter eating window, a single calm fasting day per week or a multi-day fasting mimicking diet a few times a year may suit some people better. Which path makes sense depends on your goal and your daily life.
The question is not whether you can manage one fasting day. Almost anyone can. The question is whether you can manage it twenty times without your sleep, your mood or your relationship with food paying the price.
In the end, it's not about the most elegant method. It's about freedom: food that nourishes you instead of occupying you. A form of fasting that keeps you thinking about food all day takes exactly this freedom away from you, no matter how good its studies look.
And now you know why suitability says more about you than about the method.
When whole-day fasting is not a good fit: pregnancy, eating disorders, diabetes and medication
In short: During pregnancy and breastfeeding, with a current or past eating disorder, with type 1 diabetes, with underweight, and for children and adolescents, whole fasting days are not a solo experiment. If you take insulin, sulfonylureas or SGLT2 inhibitors, every fasting day needs to be planned with your doctor. The same applies to serious conditions of the heart, kidneys or liver.
This section is the most uncomfortable one. But it is also the most important.
Whole fasting days are not a solo experiment if any of these points applies to you
- Pregnancy and breastfeeding: During pregnancy, metabolism responds much faster to breaks from eating. While breastfeeding, energy needs are higher.
- Eating disorder, current or past: Strict fasting days can set off old patterns of control and compensation again.
- Type 1 diabetes: There is a risk of hypoglycaemia and ketoacidosis, and good studies are largely lacking.
- Glucose-lowering medication: Insulin and sulfonylureas increase the risk of hypoglycaemia, SGLT2 inhibitors the risk of ketoacidosis.
- Underweight, children and adolescents: Here both reserves and data are lacking.
- Other medication and serious conditions: Blood pressure medication, diuretics, medicines that must be taken with a meal, and conditions of the heart, kidneys or liver should be discussed with your doctor beforehand.
If you experience dizziness, a racing heart, confusion, severe weakness or nausea with vomiting on a fasting day, stop fasting and get medical help.
Pregnancy: the body switches into fasting mode faster
Physiology study As early as 1982, a team led by Metzger and Freinkel showed how quickly metabolism in late pregnancy responds to a break from eating. When the overnight fast without breakfast was extended from twelve to 18 hours, blood sugar in women in their third trimester fell further, while free fatty acids and ketones rose. In non-pregnant women, the values stayed largely stable over the same period. The researchers advised pregnant women not to skip even breakfast.
For you, this means: if even a skipped breakfast noticeably shifts metabolism, a whole fasting day is not an option during pregnancy.
Metzger BE et al. Lancet. 1982. DOI: 10.1016/s0140-6736(82)91750-0Eating disorders: when control becomes the problem
In a Canadian survey of 2,762 adolescents and young adults, 47.7 percent of women and 38.4 percent of men had done intermittent fasting in the past year. Those who fasted showed symptoms of eating disorders more often. A cross-sectional study shows a correlation, not causation. Whether fasting encourages the symptoms or whether people with disordered eating fast more often remains open.
In a study from Chicago with 59 adults with obesity, binge eating and depressive symptoms decreased over eight weeks of alternate day fasting. At the same time, restrictive eating behaviour increased. For many people this shift may be harmless. If you have had an eating disorder, it is exactly this pull towards more control that is the risk.
Diabetes and medication: plan fasting days together
RCT, n=41 In Wellington, New Zealand, people with type 2 diabetes fasted on two days per week with 500 to 600 kilocalories for twelve weeks. All of them took metformin or other glucose-lowering medication. Although the medication was reduced, fasting roughly doubled the rate of hypoglycaemia, with a relative risk of 2.05. Weight, HbA1c and quality of life nevertheless improved.
For you, this means: fasting days and glucose-lowering medication need to be planned together, not tried out one after the other.
Corley BT et al. Diabet Med. 2018. DOI: 10.1111/dme.13595In a one-year study from Adelaide with 137 people with type 2 diabetes, 5:2 was equivalent to daily calorie reduction for HbA1c. Medication with a risk of hypoglycaemia was adjusted at the start according to a predefined protocol. Even so, in the first two weeks 35 percent of those taking sulfonylureas or insulin had episodes of low or high blood sugar, across both groups. You can read more in Fasting with type 2 diabetes.
With SGLT2 inhibitors there is an additional risk. A team led by Peters described 13 episodes of ketoacidosis or ketosis with barely raised blood sugar in nine people, seven of them with type 1 diabetes. Because blood sugar was almost normal, the danger was recognised late. Fasting days push metabolism further towards ketones. In my view, this combination is therefore only conceivable with close medical supervision.
A review by Grajower and Horne describes the evidence on fasting with type 1 or type 2 diabetes as almost non-existent. The authors consider fasting justifiable only under medical supervision and with self-monitoring of glucose. With type 1 diabetes, for me this means: no fasting day without your diabetes team by your side.
A contraindication is not a verdict on your willpower. It is information that your body has other priorities right now. For many of these situations there are gentler paths, such as a moderate eating window with medical support.
And now you know why the same fasting days can be a tool for one person and a risk for another.
What is well supported and what remains open
Supported by randomised trials and meta-analyses: 5:2 and alternate day fasting can lead to weight loss about as well as a daily calorie reduction. Alternate day fasting showed a small lead in shorter studies. In type 2 diabetes, 5:2 was equivalent for HbA1c, but hypoglycaemia occurred more often with medication.
Supported by smaller studies: In insulin resistance, whole fasting days might lower insulin more strongly. Hunger on fasting days can ease with a small meal, while on zero-calorie days it persisted in one study. Longer breaks from food can change T3, LH pulses and cortisol.
Mechanistically plausible, open in humans: that whole fasting days trigger more autophagy, and that this leads to a benefit for health or lifespan.
My assessment: Whole-day fasting can be a powerful tool for selected people with a metabolic goal and a stable foundation. For many others, a well-timed eating window is the more sustainable choice.
Three levers if you want to try whole fasting days
There's no need to rush. These three directions can help you find your bearings. They do not replace medical advice.
Check the foundation first
See whether 16:8 has been running for a few weeks without a struggle and whether your sleep holds up. A baseline before you start makes sense, for example fasting insulin or the HOMA index, ferritin and thyroid values. That way you can see later whether anything is changing, instead of just guessing.
Start gently, not with a zero-calorie day
In many studies, a fasting day meant about a quarter of energy needs, usually around 400 to 650 kilocalories. A small, protein-rich meal can make the fasting day more manageable. 5:2 leaves you more room than fasting every other day. Put fasting days on quiet days without hard training and without dinner plans or lunch dates. You can read how to ease back into eating afterwards in Breaking your fast well.
Set your stop signals in advance
Write down how you will notice that it is not a good fit: several bad nights, a change in your cycle, cravings on eating days, thoughts circling around food, a drop in performance or constantly feeling cold. After a few weeks, review with your doctor whether your values and how you feel are moving in the direction you want.
And now you know why the wisest fasting day is the one you planned beforehand and reviewed calmly afterwards.
Frequently asked questions about the 5:2 diet and alternate day fasting
What is the 5:2 diet?
On the 5:2 diet, you eat normally on five days a week and very little on two non-consecutive days. In studies, the amount on fasting days was usually about 400 to 650 kilocalories, roughly a quarter of energy needs. In a study from Oslo it was 400 kilocalories for women and 600 for men. The other five days have no set calorie target. So unlike 16:8, 5:2 does not change your day, but your week.
What is alternate day fasting?
Alternate day fasting means fasting every other day. Fasting days with 0 to 500 kilocalories alternate with eating days with no set target. That adds up to three to four fasting days per week. If you eat nothing at all on the fasting day, around 36 hours pass between two meals. It is the strictest of the common forms of fasting and in studies showed the largest effect on weight, by a small margin, but also a high dropout rate.
Is 5:2 better than 16:8 for losing weight?
On average, not clearly. In a network meta-analysis of 99 trials in the BMJ, only alternate day fasting showed a small lead over daily calorie reduction, on average 1.29 kilograms. Compared with time-restricted eating such as 16:8, the lead was 1.69 kilograms. In trials lasting 24 weeks or more, there were no longer any differences between the diet types. What usually matters most, therefore, is which form you can sustain over months.
How many calories can I eat on a fasting day?
In most studies, a fasting day meant about a quarter of energy needs, usually around 400 to 650 kilocalories. In a study from Chicago, people with obesity ate an average of 501 kilocalories on fasting days, and their hunger eased from the second week onwards. On fasting days with no calories at all, hunger stayed raised for three weeks in another study. A small, protein-rich meal can therefore make the fasting day more manageable. If you have pre-existing conditions or take medication, discuss the amount with your doctor.
Does whole-day fasting bring more autophagy than 16:8?
Biologically that is plausible, but it has not been shown in humans. The shift from sugar to fat metabolism typically begins beyond twelve hours without food, and a fasting day keeps you in this zone for longer. That fasting upregulates autophagy in many tissues is shown mainly by animal and cell models. In humans, autophagy in living tissue can still hardly be measured directly. Autophagy is therefore not a solid main reason to switch.
Does the body get used to hunger on fasting days?
That depends on the form. In a study from Chicago with around 500 kilocalories on fasting days, hunger eased from the second week onwards and stayed low after that. In adults without obesity who ate nothing at all on fasting days, by contrast, hunger stayed raised for 22 days. In a one-year study from Oslo, the group with two fasting days per week reported more hunger than the group with daily reduction. So hunger is not destiny, but it is a cost you need to factor in.
Is whole-day fasting suitable for women?
It can suit many women, but not all of them. In a study with normal-weight women, the number of LH pulses fell on the last of three fasting days, but ovulation and cycle length stayed unchanged. In premenopausal women with obesity, androgens fell on intermittent fasting, while oestrogen stayed unchanged. If your cycle is irregular, or you are very lean, exhausted or sleeping badly, I see a moderate eating window as the better choice. During pregnancy and breastfeeding, whole fasting days are out of the question.
Do I lose muscle with whole-day fasting?
Almost any weight loss involves losing some fat-free mass too. A meta-analysis from Maastricht with 28 trials found that the fasting forms taken together lowered fat-free mass by around 0.2 kilograms more than a daily calorie reduction. In lean people, fasting every other day lowered body fat less in one study than the same saving spread across every day. Enough protein on eating days and strength training can help you preserve your muscles.
What side effects can whole-day fasting have?
In a study from Chicago with 59 adults with obesity, over eight weeks of alternate day fasting 17 percent reported constipation, fewer than 20 percent dizziness and fewer than 15 percent general weakness. In a study from Graz, no side effects occurred in healthy people. In my practice I often see hunger, irritability, feeling cold or restless sleep on fasting days. If you get a racing heart, confusion or severe weakness, stop fasting and get medical help.
Who should not do whole-day fasting?
Whole fasting days are not a solo experiment during pregnancy and breastfeeding, with a current or past eating disorder, with type 1 diabetes, with underweight, or for children and adolescents. If you take insulin, sulfonylureas or SGLT2 inhibitors, every fasting day needs to be planned with your doctor. In one study in type 2 diabetes, fasting days roughly doubled the rate of hypoglycaemia, even though the medication was reduced. With conditions of the heart, kidneys or liver, and with medicines that must be taken with food, you should also seek medical advice beforehand.
Should I start with 5:2 or with alternate day fasting?
In my view, 5:2 is the gentler entry point. Two fasting days per week leave five days of room for family, work and training. Alternate day fasting did show a slightly larger effect on weight, but in a one-year study 38 percent of the fasting group dropped out, compared with 29 percent with daily reduction. Start with fasting days of around a quarter of your energy needs and decide in advance how you will notice that it is not a good fit.
More from the Fasting Guide
- Intermittent fasting 16:8: what happens in your body
- Fasting for beginners: the gentle start
- Autophagy: when does it start?
- Recognising and measuring ketosis during fasting
- What breaks a fast?
- Breaking your fast well: the first meal
- Cravings during intermittent fasting
- Fasting, blood sugar and insulin resistance
- Fasting with type 2 diabetes
- Fasting and cholesterol
- Muscle loss during fasting
- Fasting and exercise: training on an empty stomach
- Fasting, women and the cycle
- Intermittent fasting for women over 40
- Fasting during menopause
- Fasting with Hashimoto's
- Fasting with exhaustion
- Intermittent fasting and sleep
- Intermittent fasting and iron deficiency
- Fasting and the gut microbiome
- Fasting with rheumatic disease
- Fasting and lifespan
- Fasting mimicking diet (FMD) explained
- Buchinger therapeutic fasting
- Water fasting: opportunities and risks
- Fasting during Ramadan
- Fasting as detox: autophagy instead of waste deposits
Where this topic touches other areas
Whole-day fasting touches on insulin, weight, thyroid, hormones and muscles. If you want to go deeper, these neighbouring topics from other guides are worth a look.
Recognising insulin resistance
How you can recognise a persistently high insulin level.
InsulinStrength training with insulin resistance
How muscle work can support glucose metabolism.
Weight lossEating little and still not losing weight
Possible causes beyond the calorie balance.
MenopauseLosing weight during menopause
Why it can become harder in midlife.
ThyroidWhen too little T4 becomes T3
Why the active thyroid hormone can be low.
HormonesExercise, hormones and the cycle
How training and your cycle can interact.
MusclesMuscle loss with age
Why muscle mass matters more with the years.
Weight lossStopping weight loss injections
What can come next after GLP-1 therapy.
Sources
All studies were checked in PubMed for authors, year, journal and DOI. Key papers were also cross-checked via Consensus and verified via their DOI. The study type is shown in square brackets so you can judge the strength of the evidence yourself.
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- Klempel MC, Bhutani S, Fitzgibbon M, Freels S, Varady KA. Dietary and physical activity adaptations to alternate day modified fasting: implications for optimal weight loss. Nutr J. 2010;9:35. DOI: 10.1186/1475-2891-9-35 [Pathophysiology, intervention study without a control group, n=16 adults with obesity, 10 weeks]
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- Catenacci VA, Pan Z, Ostendorf D, Brannon S, Gozansky WS, Mattson MP, et al. A randomized pilot study comparing zero-calorie alternate-day fasting to daily caloric restriction in adults with obesity. Obesity (Silver Spring). 2016;24(9):1874-83. DOI: 10.1002/oby.21581 [RCT, pilot study, n=26 adults with obesity, 8 weeks]
- Bergendahl M, Vance ML, Iranmanesh A, Thorner MO, Veldhuis JD. Fasting as a metabolic stress paradigm selectively amplifies cortisol secretory burst mass and delays the time of maximal nyctohemeral cortisol concentrations in healthy men. J Clin Endocrinol Metab. 1996;81(2):692-9. DOI: 10.1210/jcem.81.2.8636290 [Pathophysiology, randomised order of eating day and fasting day, n=8 healthy men, 5 days of water fasting]
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- Cienfuegos S, Corapi S, Gabel K, Ezpeleta M, Kalam F, Lin S, et al. Effect of Intermittent Fasting on Reproductive Hormone Levels in Females and Males: A Review of Human Trials. Nutrients. 2022;14(11):2343. DOI: 10.3390/nu14112343 [Review, analysis of human trials]
- Metzger BE, Ravnikar V, Vileisis RA, Freinkel N. “Accelerated starvation” and the skipped breakfast in late normal pregnancy. Lancet. 1982;1(8272):588-92. DOI: 10.1016/s0140-6736(82)91750-0 [Pathophysiology, metabolic study in pregnant women in the third trimester and non-pregnant women]
- Ganson KT, Cuccolo K, Hallward L, Nagata JM. Intermittent fasting: Describing engagement and associations with eating disorder behaviors and psychopathology among Canadian adolescents and young adults. Eat Behav. 2022;47:101681. DOI: 10.1016/j.eatbeh.2022.101681 [Real world, cross-sectional study, n=2,762 adolescents and young adults]
- Hoddy KK, Kroeger CM, Trepanowski JF, Barnosky AR, Bhutani S, Varady KA. Safety of alternate day fasting and effect on disordered eating behaviors. Nutr J. 2015;14:44. DOI: 10.1186/s12937-015-0029-9 [Intervention study without a control group, n=59 adults with obesity, 8 weeks]
- Corley BT, Carroll RW, Hall RM, Weatherall M, Parry-Strong A, Krebs JD. Intermittent fasting in Type 2 diabetes mellitus and the risk of hypoglycaemia: a randomized controlled trial. Diabet Med. 2018;35(5):588-594. DOI: 10.1111/dme.13595 [RCT, n=41 adults with type 2 diabetes, 12 weeks]
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