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Intermittent fasting: methods, effects and how to start properly

16:8, 14:10 or 5:2, and does it even do anything? Intermittent fasting works by making you eat less, not through the clock. Find out which method suits you, which times make sense and what's allowed during the fasting phase.

Gregor AßlaberJoerg Hauke
Gregor Aßlaber, Editor, Saturo · ✓Fachlich geprüft von Joerg Hauke, Nutrition expert
Aktualisiert 20 October 2026 · 15 Min. Lesezeit · 41 Quellen
In 30 seconds
Frau am Morgen in heller Küche mit Glas Wasser und schwarzem Kaffee während der Fastenphase
Fasting in the morning: water and black coffee are allowed, your first meal comes later.

The key points in 30 seconds

Fasting window at 16:8
16hrs

Eight hours of eating, sixteen hours of break. Sleep counts too, so in everyday life you usually only skip breakfast or dinner.

Studies compared
99

That's how many randomised trials the BMJ analysed in 2025: intermittent fasting leads to about the same weight loss as classic calorie reduction, not more.

Getting started
12hrs

Start with a twelve-hour break overnight and build up over one to two weeks. That's the advice from Barmer and the Austrian health portal (Gesundheitsportal Österreich).

During the fasting phase
Water, tea, coffee

Anything without calories is allowed: water, unsweetened tea, black coffee. Milk, sugar, juice and alcohol end the fasting phase.

01 Understand

What is intermittent fasting and how does it work?

Intermittent fasting means you eat within set time windows and deliberately take breaks in between. The 2024 international expert consensus defines it as repeated fasting periods of up to 48 hours. The method doesn't dictate what you eat, only when. That's exactly what makes it so simple, and it also explains why it's no miracle cure.

How does intermittent fasting work?

Intermittent fasting works because you usually eat less in a shorter eating window. The fasting phase itself isn't a metabolic trick: a 2025 study by the German Institute of Human Nutrition found no better insulin response at the same calorie intake, whether people ate early or late. What matters is your energy balance. If you eat as much in eight hours as you used to in fourteen, you won't lose weight. If the break means you skip the late snack and the second breakfast, you will. The clock is a tool against mindless snacking, nothing more, but nothing less either. The calorie calculator shows you how much you need per day.

Partial fasting and dinner cancelling: what do they mean?

Partial fasting isn't a technical term but a catch-all: sometimes it means fasting days with few calories like 5:2, sometimes skipping one meal, sometimes giving up sugar or alcohol. So ask what's actually meant. Dinner cancelling, on the other hand, is clearly defined: on two to three evenings a week you skip dinner, have only water or unsweetened tea in the evening, and your eating break lasts at least 14 hours. The DGE lists it as a gentle option that fits well with a family breakfast.
First meal around midday: scrambled eggs, wholemeal bread and vegetables after the fasting phase
The first meal after 16 hours: high in protein and with vegetables. If you break your fast with white bread and sweet things, you'll be hungry again an hour later.
02 Methods

Intermittent fasting methods: 16:8, 14:10, 12/12 and 5:2

The numbers always describe fasting hours to eating hours, or fasting days to eating days. Only three forms are scientifically well defined: time-restricted eating (16:8 and similar), 5:2 and alternate-day fasting. Everything else is common labelling with no studies of its own. The table shows what's behind each one and who it suits.

Intermittent fasting 16:8: how the most popular method works

With 16:8 you eat for eight hours a day and fast for sixteen, for example from 12 to 8 pm or from 8 am to 4 pm. Ulm University Hospital calls 16:8 the most beginner-friendly daily option, because sleep covers most of the break. In the TREAT study, 116 adults ate between 12 and 8 pm for twelve weeks with no calorie target: they lost 0.9 kg, while the control group with three fixed meals lost 0.7 kg. So 16:8 only works if the break also saves calories. Four to five fasting days a week are enough for many people to stick with it.
At a glance

Eleven methods compared

Fasting to eating ratio, who the method is meant for and what studies say about it.

Fasting : eatingWho it's forEvidence
12:1212 hrs : 12 hrs, dailyGetting started, sleep counts tooHardly studied; the benefit is the structure
14:1014 hrs : 10 hrs, dailyGetting started, intermediate stepMinimum break for dinner cancelling (DGE)
16:816 hrs : 8 hrs, daily or on 4 to 5 daysBeginners (Ulm University Hospital, Barmer)Under 1 kg in 12 weeks without a calorie target (TREAT 2020)
18:618 hrs : 6 hrs, dailyExperienced fasters, not for starting outNo official guidance; Barmer advises against more than 16 hrs
20:4 (Warrior Diet)20 hrs : 4 hrs, one large mealNot for anyone as a starting pointBarmer: red flag, hard to get enough nutrients
OMAD (23:1)One meal a dayNot recommendedMore hunger, higher blood pressure and LDL (Stote 2007)
5:22 fasting days a week with 500 or 600 kcalIf you prefer fasting by the dayHeidelberg: same as a daily diet after 50 weeks
Alternate-day fastingEvery other day around 25% of your needsExperienced fasters, very restrictive38% drop-out vs 29% on a daily diet
Eat-Stop-Eat1 to 2 × 24 hrs a weekExperienced fastersTerm used inconsistently, no studies of its own
Dinner cancelling2 to 3 evenings without dinner, at least 14 hrsGentle startDGE option, no studies of its own
Partial fastingNo fixed rulesCatch-all termNot a technical term, means different things depending on the source

Expert definitions exist only for time-restricted eating, 5:2 and alternate-day fasting (Cell Metabolism consensus 2024, DGE 2018). Calories on 5:2 fasting days according to the Verbraucherzentrale consumer advice centre (500 kcal for women, 600 kcal for men). Study figures from TREAT 2020, Schübel 2018, Trepanowski 2017, Stote 2007.

14:10 and 12/12 intermittent fasting: are 12 hours enough?

Yes, twelve hours is enough to get started, but probably not as a long-term way to lose weight. The Austrian health portal (Gesundheitsportal Österreich) already counts an eating break of at least twelve hours as intermittent fasting, and Barmer recommends starting with exactly that. Almost everyone can manage twelve hours without eating overnight: dinner at 19:00, breakfast at 7:00. The benefit is the structure, and above all the late-night snack disappears. 14:10 is the sensible next step, for example 8:00 to 18:00 or 10:00 to 20:00, and it matches the minimum break for dinner cancelling.

5:2 intermittent fasting and alternate-day fasting

With 5:2, you eat normally on five days and only around 500 kcal (women) or 600 kcal (men) on two non-consecutive days; the DGE puts this at about a quarter of your needs. The Heidelberg HELENA study with 150 participants showed 5.2% weight loss with 5:2 and 4.9% with daily calorie reduction after 50 weeks, so no real difference. Alternate-day fasting (every second day is a fasting day) is the toughest form: in a one-year study, 38% dropped out, compared with 29% on a daily diet, with the same result on the scales.

18:6, 20:4 and OMAD: when it gets too much

From 18 hours of fasting, you leave the range that experts recommend. Barmer advises not stretching the break beyond 16 hours and calls 20:4 a red flag, because it's hard to fit in all your nutrients within four hours. For one meal a day (OMAD), there is a controlled study in people of normal weight: same calories, but noticeably more hunger, higher blood pressure and higher LDL cholesterol than with three meals. Observational data also link very short eating windows with more cardiovascular deaths (chapter 7). Fasting longer isn't better, just harder.

Which intermittent fasting method is best?

The best intermittent fasting method is the one you can stick to, because in studies all forms lead to similar weight loss. The network meta-analysis in the BMJ 2025 (99 studies, 6,582 adults) found, compared with eating normally, 1.7 kg less with 16:8, 2.1 kg with classic calorie reduction and 3.4 kg with alternate-day fasting; the advantage of alternate-day fasting had disappeared in studies lasting 24 weeks or more. For everyday life, that means: 16:8 or 14:10 if you like fixed times of day, 5:2 if you'd rather be strict on two days and eat freely the rest of the time.
03 Understand

What happens during 16 hours without food

Three stages from your last meal to breaking the fast. Two of them are well supported by evidence; the third is often made out to be bigger than the data allow.

1

Blood sugar and insulin drop

After your last meal, your blood sugar rises and insulin moves it into your cells. After a few hours that's done and your insulin level drops. Now your body turns to its reserves.

2

Stores empty, fat gets used

Your liver releases stored glucose, while fat tissue supplies fatty acids. You still only lose weight if you take in less over the day than you burn.

3

Autophagy: proven in cells, barely measured in humans

During fasting, cells clear out damaged components. This is well documented in animal studies. In humans, there are no measurements of how much actually happens, and there's no hour threshold. Anyone promising autophagy from hour 16 is quoting mice.

04 Getting started

Intermittent fasting for beginners: how to start

The most common mistake at the start is wanting too much too soon: 16 hours from day one, plus exercise on an empty stomach and an empty fridge in the evening. It's better to start in stages, with fixed times and a plan for your first meal.

Starting intermittent fasting: the first week

Start with a twelve-hour break, and sleep counts towards it: dinner by 19:00, breakfast from 7:00. That's Barmer's advice, and the Austrian health portal already counts this break as intermittent fasting. After three to four days, you move one meal by an hour, and after a week by two: you've reached 14:10. If you feel good, go to 16:8 in week two. Fix your times in advance and drink enough during the fasting phase; the DGE recommends around 1.5 litres a day. Headaches in the first few days are often down to a lack of fluids. The planner works out your window:
Interactive

When is your eating window?

Choose your method, whether you're pushing breakfast back or bringing dinner forward, and your time. The planner shows your window across 24 hours and how much of the break falls during sleep.

Method

Most popular daily option

What are you moving?
First meal at
Going to bed at
Your day
First meal12:00
Last meal20:00
Eating window
8 Std.
Fasting time
16 Std.
of which asleep
8 Std.
Last meal before sleep
3 Std.

Your window: 12:00 to 20:00. 8 hours eating, 16 hours break, of which 8 hours asleep. There are 3 hours between your last meal and bedtime.

Guide values, not a diagnostic tool: sleep duration assumed to be 8 hours; gap between last meal and sleep based on the Verbraucherzentrale (3 to 4 hours, an estimate without a study); method notes based on Barmer and Universitätsklinikum Ulm. If you have diabetes, heart disease or take regular medication, talk to your doctor first.

Which time is best? Skipping breakfast or dinner

The early window has better data behind it, the late one fits more easily into daily life. In a 14-week study with coaching, participants who ate between 7:00 and 15:00 lost 2.3 kg more than those with a window of twelve hours or more. In a crossover study, eating late increased hunger at the same calorie intake. Counterpoint: the ChronoFast study found no metabolic advantage for eating early or late at the same calorie intake. In practice, that means: if you eat dinner with family or friends, skip breakfast and stick with it. If you're hungry in the morning, bring dinner forward.
Hunger in the first few days

Hunger comes in waves and passes after 15 to 20 minutes; a big glass of water or unsweetened tea helps you get through it. In a 28-day study, hunger on fasting days went down from week to week. If it's still strong after two weeks, your window is too tight or your first meal is too small.

Intermittent fasting daily plan for 16:8

This is what a day looks like with an eating window from 12:00 to 20:00, the time frame used in the TREAT study:

  • 7:00 Get up, big glass of water, black coffee or tea
  • 10:00 Drink again; hunger comes and goes
  • 12:00 First meal, high in protein: eggs, quark, pulses or fish with vegetables and wholegrain
  • 15:30 Small snack if needed: fruit, nuts, yoghurt
  • 19:30 Dinner, then just water or herbal tea
  • 20:00 Window closed, 16 hours of break until 12:00

If you have breakfast instead of dinner, everything moves forward: 8:00 to 16:00, last meal around 15:30. You don't need an app for this, the planner above is enough.
Early dinner in daylight: vegetables, salmon and water on the table
Early window, for example 10:00 to 18:00: dinner moves forward, and sleep covers the rest of the fasting time.
05 Rules

What's allowed when you're intermittent fasting?

The rule is simpler than all the questions suggest: during your fasting window, anything without calories is allowed. Water, mineral water, unsweetened tea, black coffee. Anything with sugar, milk, fat or alcohol ends the fast. Everything in between is a grey area, and the traffic light shows you where.

Intermittent fasting and coffee: what's allowed?

Black coffee is allowed during your fasting window, with around 2 kcal per cup. That's according to Barmer, AOK and the DGE, who classify coffee without milk and sugar as calorie-free. A splash of milk (30 ml) adds 16 to 21 kcal, a teaspoon of sugar 21 kcal, a splash of oat drink around 20 kcal. Strictly speaking, that ends the fast, though no studies test exactly this. The often-quoted 50 kcal rule has no evidence behind it. Our recommendation: black or with lemon in the morning, and save the milky coffee for your eating window. After 6 pm, go for tea instead, or your sleep will suffer.
Interactive

Does this break my fast?

Type in a drink or snack. Green is practically calorie-free, yellow is a grey area with few calories or unclear research, and red ends your fasting window.

  • Water tap water, mineral water, sparkling water 0 kcal per 250 mlFirst choice. Around 1.5 litres a day, and feel free to drink more during your fasting window.
  • Herbal tea, unsweetened peppermint, camomile, fruit tea 0 to 3 kcal per 250 mlAllowed without sugar, honey or milk, even in the evening.
  • Green tea, black tea, unsweetened matcha 3 kcal per 250 mlContains caffeine; allowed unsweetened during your fasting window.
  • Black coffee filter coffee, Americano 2 kcal per cupAllowed according to Barmer, AOK and DGE. After 6 pm, go for tea instead.
  • Espresso 3 kcal per shotSame as black coffee, with nothing added.
  • Coffee with milk 1.5% splash, 30 ml 16 kcal per cup1.5 g lactose, 1.3 g protein. Strictly speaking this breaks the fast, but there are no studies on it.
  • Coffee with whole milk 3.5% splash, 30 ml 21 kcal per cupSame as 1.5%, plus 1 g fat. Milky coffee belongs in your eating window.
  • Coffee with oat drink barista, 30 ml 20 kcal per cupContains sugar from the oat starch, and barista versions also have added oil.
  • Coffee with soya drink, unsweetened 30 ml 12 kcal per cupThe lowest-calorie milk alternative, but not calorie-free.
  • Coffee with sugar 1 tsp 21 kcal per cup5 g pure sugar, and your insulin responds. Ends your fasting window.
  • Lemon juice in water 1 tbsp 3 kcal per 250 mlPractically energy-free; the EU limit for “energy-free” is 4 kcal per 100 ml.
  • Apple cider vinegar in water 1 tbsp 3 kcal per 250 mlAlways diluted, to protect your tooth enamel. No proven fasting benefit, but it doesn't break your fast either.
  • Cola Zero, diet cola sweetener, sugar-free fizzy drink 0 kcal per canCalorie-free, and sweetener doesn't trigger an acute insulin response. DGE: it keeps you used to sweet tastes.
  • Cola fizzy drink, sweetened iced tea 139 kcal per can35 g sugar. Clearly red.
  • Orange juice apple juice, smoothie, spritzer 94 kcal per 200 mlFruit sugar is still sugar. Save it for your eating window.
  • Milk 1.5% or 3.5%, cocoa 94 to 128 kcal per 200 mlWith 7 g protein, it's a small meal.
  • Clear vegetable broth no extras 12 kcal per 250 mlCommon in fasting cures, but very salty. For daily 16:8, strictly speaking not a water substitute.
  • Bone broth chicken broth 33 to 45 kcal per cup5 to 10 g protein: a small meal, not a fasting drink.
  • Sugar-free chewing gum 5 kcal per pieceContains sugar alcohols. Gets saliva flowing and makes you hungrier.
  • Beer lager, 0.5 l 190 to 216 kcal per 0.5 lAlcohol has 7 kcal per gram and boosts your appetite.
  • Alcohol-free beer wheat beer, 0.5 l 125 kcal per 0.5 l18 g sugar. Alcohol-free doesn't mean calorie-free.
  • Wine red, white, sparkling, 0.2 l 166 kcal per 0.2 lDGE 2024: there is no risk-free amount.
  • Cucumber, fruit, bread, yoghurt any food from 10 kcalEvery bite with calories ends the fast, vegetables included. Belongs in your eating window.
  • Protein shake meal replacement from 110 kcal per servingProtein triggers insulin. As a meal, it belongs in your eating window.
Calories per serving from USDA FoodData Central (SR Legacy) and manufacturer data (Weihenstephan, Oatly, Alpro, Coca-Cola, Erdinger). Servings: cup 200 ml, splash 30 ml, 1 tsp sugar 5 g. Green means practically calorie-free, yellow is a grey area with no proven threshold, and red provides a meaningful amount of energy. No study tests whether individual drinks cancel out the effect of intermittent fasting.

Intermittent fasting and alcohol

Alcohol always breaks your fast: half a litre of lager delivers around 200 kcal, a glass of wine (0.2 l) around 166 kcal. Alcohol has 7 kcal per gram, almost as much as fat, and since 2024 the DGE no longer sees any risk-free amount. In your eating window it counts towards your daily total, and according to lab studies it also makes you hungrier: if you drink before eating, you eat more. After 16 hours on an empty stomach, it also hits you faster. If you do drink, have it with food, and alcohol-free beer is no fasting alternative at 125 kcal per 0.5 l.

Can you take a break from intermittent fasting at the weekend?

Yes. Intermittent fasting isn't a rigid rulebook, it's a tool for your weekly balance, and that stays largely intact with five set days and two free ones. 5:2 only has you fasting on two days anyway, Barmer advises adapting the plan to your daily life, and Helmholtz researcher Stephan Herzig says: longer breaks from eating on four days are better than none at all. What matters is that your two free days don't turn into making up for it. The same goes when you pause intermittent fasting for holidays, celebrations and illness: take a break, pick it up again on your next usual day, and don't fast extra to compensate.

What should you eat on intermittent fasting?

In your eating window, the normal rules of good nutrition apply, and that's exactly the weak spot of many approaches: the DGE criticises that intermittent fasting gives hardly any guidance on food choices. In practice, that means two to three meals, three to five hours apart, each with a protein source, vegetables and wholegrain. Protein keeps you full and protects your muscles; the DGE puts it at 0.8 g per kilogram of body weight a day, and 1.0 g from age 65. You'll find more under protein requirements. Don't break your fast with white bread and sweet things, or the hunger will come back quickly.
Man in a modern kitchen with a Saturo shaker, with the Shape Shake pack next to it
One meal in your window can be a meal replacement: a fixed 203 kcal, 17 g protein, 11 g fibre. It ends the fasting phase like any other meal.

If you eat lunch at the office and can't plan your first meal, a meal replacement can cover it: a meal with a fixed calorie count, protein, fibre and 26 vitamins and minerals. Studies only exist for the fasting days of 5:2 and alternate-day fasting, not for the 16:8 eating window. So the shake is a meal like any other, not a fasting product.
Shape Shake
From the Saturo range

Shape Shake: meal replacement with a fixed 203 kcal

30 g of powder in 300 ml of unsweetened soya drink makes 203 kcal with 17 g protein, 11 g fibre and 26 vitamins and minerals. Replacing two of your main meals a day with a meal replacement like this, as part of a calorie-restricted diet, contributes to weight loss; replacing one of your main meals a day helps you maintain your weight after weight loss. The shake only does its job as part of a calorie-restricted diet that includes other foods, and with enough fluids throughout the day.

203kcal per serving
17 gProtein
26Vitamins & minerals
11 gFibre per servingmore than a third of the 30 g a day
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06 Effect

Losing weight with intermittent fasting: what the studies show

Most people search for intermittent fasting to lose weight, and the data is now pretty solid: over 100 randomised studies, running for up to twelve months. The result is unspectacular and useful at the same time. It works, about as well as classic calorie reduction, and some people find it easier.

How much weight do you lose with intermittent fasting?

Without a calorie target, a realistic figure is one to two kilograms in twelve weeks, and noticeably more with coaching and a deficit. The 2025 BMJ analysis reports, compared with eating normally, 1.7 kg for 16:8, 2.1 kg for daily calorie reduction and 3.4 kg for alternate-day fasting. In the 2022 NEJM study, 139 people with obesity lost 8.0 kg in twelve months when they limited their diet to 8 am to 4 pm, and 6.3 kg without a time window; the difference was not significant. For what a realistic pace and a sensible deficit look like, read our article Losing weight.
5:2, two fasting days a week −5.2%

Weight after 50 weeks (HELENA, DKFZ Heidelberg)

Daily calorie reduction −4.9%

Weight after 50 weeks, 20% less every day

The cards show the German HELENA study by the DKFZ Heidelberg with 150 participants: after twelve weeks, 5:2 was ahead (7.1% vs 5.2%), but not after 50 weeks.

How long should you do intermittent fasting?

Intermittent fasting is meant as a long-term way of eating, not a short-term cure: studies run from eight weeks to twelve months, and the DGE describes it as a long-term form of diet. Getting used to it takes a few days to a few weeks. The scales usually show the first kilos in week one or two, and part of that is water. After that, the weekly balance is what counts, and after three to six months the curve flattens, because your body burns less at a lower weight. If you only use intermittent fasting as a starting phase, you carry on afterwards with the eating rhythm that comes easily to you.

The clock doesn't replace the plate. If you eat well within eight hours, you lose weight and stay full. If you make up in eight hours for what you missed over sixteen, you put weight on.

Joerg Hauke, Nutrition expert, expert review of this article

Why am I not losing weight despite 16:8?

Because the window alone doesn't guarantee a deficit. In the TREAT study, participants on 16:8 without a calorie target lost 0.9 kg in twelve weeks, barely more than the control group. Typical reasons: big portions to break the fast, snacks in the window, milky coffee and juices during the fasting phase, alcohol in the evening, everything different at the weekend. Ulm University Hospital describes the mistake of eating too much at the end of the window out of fear of hunger; some people even gain weight this way. Writing everything down for two weeks, drinks included, usually shows where the calories are hiding.

Weeks 1 to 12: what realistically happens

  1. Week 1 Getting used to it

    Hunger comes in waves, often with headaches and tiredness, usually down to a lack of fluids. The scales drop fast, but that's mostly water from your carbohydrate stores.

  2. Weeks 2 to 4 The rhythm is set

    The teething troubles settle down and hunger on fasting mornings eases. Now you'll see whether the window saves calories: around 0.25 to 0.5 kg a week if it does.

  3. Weeks 5 to 8 An honest balance

    Without a deficit, the scales stall; that's the moment to check your portions and drinks. With a deficit and enough protein, muscle mass is largely preserved.

  4. From week 12 Long-term or switch

    Without a calorie target, you've now reached 1 to 2 kg, with coaching more. If you stick with it, you treat the window as a habit, not a diet; if you hate it, change method.

Intermittent fasting, muscle and exercise

Intermittent fasting costs a little more fat-free mass than classic calorie reduction: 0.2 kg in one meta-analysis, and in the TREAT study the lean mass index measurably dropped in the 16:8 group. Two things counter this: enough protein (1.2 to 2.0 g per kilogram when you're training, according to a DGE working group) and strength training. In trained men, strength and muscle mass were maintained on 16:8 for eight weeks. Schedule your training in your eating window or shortly before it, and eat a protein-rich meal afterwards; for what belongs on your plate, see Eating after exercise. High-intensity training on an empty stomach is not for getting started.
07 Health

Is intermittent fasting healthy? Pros and cons

For healthy adults who are overweight, intermittent fasting is as safe in studies as other diets: across 15 studies with 1,365 participants, tiredness and headaches were no more common than in the control groups. The benefits depend on weight loss, not on the clock. And for some groups, the rule is: not without medical supervision.

Blood sugar, blood pressure and heart: what's proven

Blood sugar, blood pressure and blood fats improve in line with the weight you lose, and no further: in the BMJ analysis, HbA1c and HDL did not differ between fasting, calorie reduction and normal eating. A small study of eight men with prediabetes found better insulin levels with an early window without any weight loss; the larger ChronoFast study in 2025 couldn't confirm this. For the heart there is only observational data: eating windows under eight hours were linked to more cardiovascular deaths in US data; the authors don't rule out confounding factors, and studies with hard endpoints are missing. That's why windows of eight hours or more make sense.

Intermittent fasting for women: 14 or 16 hours?

The idea that women should only fast for 14 hours is a precaution, not a study finding. In a twelve-month study with an 8-hour window, testosterone, DHEA and SHBG stayed unchanged in women, and a 2026 review found no hormonal changes in women with obesity. What hasn't been studied is the menstrual cycle in lean women: If your period stops or exhaustion persists, that's a sign to stop, whatever the number of hours. For the menopause, there's no reliable data either for or against. During pregnancy and breastfeeding, intermittent fasting is off the table. In practice: start with 14:10, move to 16:8 if you feel good, and go back if your cycle is disrupted.
Diabetes and medication

With sulfonylureas, glinides or insulin, fasting raises the risk of low blood sugar. The German Diabetes Society only allows intermittent fasting for type 2 diabetes under medical supervision with an adjusted dose. For heart failure, the German Heart Foundation advises against true fasting.

Whether intermittent fasting suits you depends less on the method than on your situation. The list names the groups for whom experts advise against it or require a medical check first.
Self-check

Is intermittent fasting right for you?

Tap whatever applies to you. If nothing applies, there's nothing stopping you from starting with 12:12 or 14:10.

Not a diagnostic tool. The list summarises the exclusion and consultation groups from Gesundheitsportal Österreich, Barmer, AOK, the German Diabetes Society and the German Heart Foundation. There is no safety data for pregnant or breastfeeding women, teenagers and people with eating disorders; the precautionary principle applies.
08 Look it up

Frequently asked questions about intermittent fasting

When does autophagy kick in?

It hasn't been measured in humans. The 16 hours often quoted come from animal and cell studies. A 2026 review found that human studies mostly looked at static markers; actual autophagy activity was barely measured and was often unremarkable when it was. Anyone who names a fixed number of hours is promising more than the evidence supports.

Is vegetable broth allowed during intermittent fasting?

Clear vegetable broth has around 12 kcal per cup and is common in fasting programmes. For daily 16:8, it's strictly speaking a grey area: it technically breaks the fast, but you'd hardly notice. Bone broth with 5 to 10 g of protein, on the other hand, is a small meal and belongs in your eating window.

Which is better: skipping breakfast or skipping dinner?

Studies lean slightly towards the early window: 2.3 kg more weight loss in 14 weeks, and less hunger when eating early. With equal calories, though, the ChronoFast study found no metabolic benefit. Choose the version that suits your daily routine and stick to it on five days a week.

How many days a week should you do intermittent fasting?

Daily is the form that has been studied for 16:8; five days with two days off is a good compromise for everyday life. 5:2 only has you fasting on two days anyway. More important than the number of days is that your days off don't turn into catch-up eating.

Does intermittent fasting make you tired?

It's possible at first, but in studies it wasn't more common than with other diets: in 15 studies with 1,365 adults, tiredness didn't occur more often with intermittent fasting. Tiredness in week one is usually down to not drinking enough or a first meal that's too small. If it lasts for weeks, loosen your window and get it checked by a doctor.

Is intermittent fasting good for belly fat?

There's no targeted belly fat effect. Intermittent fasting reduces body fat as far as it creates a calorie deficit, and the belly is often where that shows first. In meta-analyses, waist circumference dropped marginally more than with a daily diet; in practice, the difference is negligible.

What mistakes are common with intermittent fasting?

Eating too much to break the fast out of fear of hunger, snacks and milky coffee during the fasting period, not drinking enough, too little protein, stepping up to 18:6 or 20:4 too quickly, and doing everything differently at weekends. Most of these eat into your deficit without you noticing.

What do doctors say about intermittent fasting?

The German Diabetes Society sees it as an option for type 2 diabetes under medical supervision, without recommending a particular form. The DGE rates the effect as comparable to normal calorie reduction. Prof. Hans Hauner (Heart Foundation): It hasn't been proven that this way of eating is healthier than others.

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Gregor Aßlaber
Gregor AßlaberAuthor · Editor at Saturo

Has been writing for Saturo since 2019 about nutrition, nutrients and how to get properly nourished in everyday life.

Joerg Hauke
Joerg HaukeFachliche Prüfung · Nutrition expert

Hat diesen Artikel am 20. Oktober 2026 auf fachliche Richtigkeit geprüft. The content does not replace medical advice.

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