Intermittent fasting for weight loss: does it work?

What is intermittent fasting?

Intermittent fasting is a way of eating in which you plan regular periods with no food, or very little food, between periods of normal eating. The focus is on when you eat, while most other diets focus on what and how much you eat.

The idea is old. Fasting has been part of religious and cultural life for thousands of years. What is newer is the use of planned fasting schedules as a weight-loss method.

Researchers group the many versions into three families. The first limits eating to a set number of hours each day. The second uses one to three low-calorie days each week. The third alternates fasting days and normal days. Each family has its own rules, and each asks something different of your body and your routine.

Intermittent fasting is one of several possible routes to eating less. It is not a treatment in itself, and it is not required for weight loss. Many people lose weight and improve their health without ever skipping a meal. Read more about the full picture of lifestyle treatment for obesity.

The main methods: 16:8, 5:2 and other fasting schedules

The best-known methods are 16:8, a daily eating window, and 5:2, which uses two low-calorie days a week. The table compares them with the other common schedules, from the gentlest to the most demanding.

The numbers in names like 16:8 or 5:2 are a quick way to describe the pattern. In 16:8, the first number is the hours without food and the second is the eating window. In 5:2 and 4:3, the first number is normal days and the second is low-calorie days.

Some popular versions push much further, such as one meal a day or fasts of 24 hours or more. These have been studied far less, and Harvard’s diet review warns that long stretches without food can lead to overeating afterwards. We do not suggest them.

How does fasting for weight loss work?

Fasting for weight loss works mainly by helping people eat fewer calories over a week, not through a special effect on fat. When the eating window is shorter, there are fewer chances to snack, and many people simply eat less without counting.

A 12-month trial from the University of Illinois Chicago shows this clearly. Adults living with obesity who ate only between noon and 8 in the evening, without counting calories, ended up eating about 425 kcal a day less. A second group that counted calories cut about 405 kcal a day. Weight loss in the two groups was similar.

During a long fast your body does change its fuel mix. After the stored sugar in the liver runs low, it uses more fat and produces small molecules called ketones. The US National Institute on Aging describes this switch, but it also stresses that scientists do not yet know whether it brings extra health benefits in people, and that too many ketones can be harmful.

So the honest summary is this: if fasting helps you eat less without feeling deprived, it can support weight loss. If you make up for the fasting hours by eating more in the window, the scale will not move. Your weight also depends on factors well beyond the clock, such as your genes and biology, sleep, stress and medicines.

Intermittent fasting for weight loss: what the trials show

Randomised controlled trials, where people are assigned to a diet by chance, show that intermittent fasting leads to weight loss in the range of a few kilos, much like other calorie-reduced diets. The table lists some of the most-cited trials.

Three patterns stand out. First, most trials found similar weight loss with fasting and with daily calorie reduction. Second, a few found a modest advantage for fasting, of around 2 to 3 kg, often in programmes with a lot of extra support. Third, dropout rates are often high in both kinds of diet, which tells us that sticking with any plan is the real challenge.

Most of these trials lasted one year or less. We do not yet know much about what happens over five or ten years, which matters because weight often comes back after any diet ends.

Is fasting better than ordinary calorie reduction?

No. The best summaries of the evidence show that intermittent fasting works about as well as ordinary calorie reduction for weight loss, not better. It is one valid option among several.

The latest Cochrane review brought together 22 randomised trials with 1,995 adults living with overweight or obesity. Compared with standard dietary advice, intermittent fasting made little to no difference to weight loss: the average gap was about a third of a percentage point of body weight. The reviewers rated the certainty of this evidence as low, mainly because many trials were small or had design problems.

In the smaller group of six trials that compared fasting with no advice at all, the Cochrane team also found that fasting likely made little to no difference over the study period. Other trials, such as the 12-month Chicago study in the table, did find more weight loss than no change. This mixed picture is one reason to keep expectations modest.

A large network meta-analysis in the BMJ pooled 99 trials with 6,582 adults. Every fasting method and daily calorie reduction led to more weight loss than eating freely. Alternate-day fasting came out slightly ahead of daily calorie reduction, by about 1.3 kg, but in trials lasting six months or longer this edge was no longer seen.

The Harvard T.H. Chan School of Public Health reaches the same conclusion in its diet review: it is unclear that intermittent fasting beats other methods for weight loss, appetite or how easy it is to stick to. Their bottom line is that strong recommendations cannot be made yet.

Fasting is a tool, not a shortcut. If counting calories or eating three regular meals suits you better, that is an equally good choice. The approach you can keep up calmly, month after month, is the one that works.

Time-restricted eating and 16:8 in practice

Time-restricted eating means you eat your meals within a fixed window each day, usually 8 to 10 hours, and have nothing but calorie-free drinks for the rest. The 16:8 pattern is the most popular form.

The appeal is that you do not have to weigh food or count calories. You need a clock and a plan for your meals. For people who often snack in the evening, a clear "kitchen closed" time can help, and Harvard notes that a cut-off time may also ease reflux or poor sleep caused by late eating.

Does the timing of the window matter?

Possibly a little. In the trial by Jamshed and colleagues, people who combined calorie reduction with an early window, from 7 in the morning to 3 in the afternoon, lost about 2.3 kg more over 14 weeks than people eating over 12 hours or more. Their diastolic blood pressure and mood also improved slightly.

An early window does not fit every life, though. Shift work, family dinners and social meals all matter. A window that you can keep on most days is more useful than an ideal window you keep only twice a week.

What can you drink during the fasting hours?

In 16:8 trials such as TREAT, people were asked to take nothing with calories during the fasting hours. Water, sparkling water, and plain tea or coffee without milk or sugar fit that rule. Juice, soft drinks, milky drinks and alcohol all contain calories and end the fast.

Food quality still counts

An eating window says nothing about what goes on your plate. If the window is filled with fast food and sweets, the benefits are likely to be small. Meals built on vegetables, fruit, whole grains, beans, fish or other protein foods keep you fuller for longer, which makes the fasting hours easier. Our free 7-day eating plan shows what balanced days can look like.

The 5:2 approach in practice

With the 5:2 approach you eat your usual, balanced meals on five days and eat very little on two days that are not next to each other. In studies, the low days usually provided about 500 to 600 kcal.

Some people like 5:2 because five days a week feel normal, and they only need to plan carefully for two. Others find the low days hard, with hunger, low energy or headaches, especially at work or when caring for children.

A low day of 500 to 600 kcal is a very low intake. That is why we do not suggest starting with 5:2 on your own. If it appeals to you, discuss it first with your doctor or a dietitian, especially if you have diabetes, take medicines or have a physically demanding job.

A dietitian can help you plan low days that still give you enough protein, fibre and nutrients. On the normal days, the aim is ordinary healthy eating, not making up for the low days.

The 4:3 version, with three low days, produced modestly more weight loss than daily calorie reduction in one recent trial. Participants also had frequent group sessions and exercise advice, so it is hard to know how much came from the fasting itself.

Who should not fast, or must ask a doctor first?

Some people should not try intermittent fasting at all, and others need medical advice before they start. The Harvard diet review lists diabetes, eating disorders, medicines that must be taken with food, pregnancy, breastfeeding and the teenage growth years as reasons to avoid fasting.

Do not try fasting if you:

• are pregnant, trying to become pregnant or breastfeeding
• have now, or have had in the past, an eating disorder such as anorexia, bulimia or binge eating disorder
• are under 18; children and teenagers need regular meals for growth
• have type 1 diabetes, unless your diabetes team plans it with you

Talk to your doctor before you start if you:

• use insulin or other diabetes medicines, especially those that can cause low blood sugar (hypoglycaemia), such as sulfonylureas. Doses often need to change before fasting, and your doctor may ask you to check your blood sugar more often.
• take medicines that need to be taken with food or at fixed times. Your pharmacist can tell you which of your medicines this applies to.
• are older and feel frail, have lost weight without trying, have weak muscles or have fallen recently.
• have kidney or liver disease, or another long-term condition that is treated with medicines.
• have had bariatric surgery, as your eating pattern after surgery needs a specialist plan.
• have a physically heavy job, work night shifts or drive for a living, where dizziness or low concentration would be unsafe.

The diabetes point matters most. In the Australian 5:2 trial in people with type 2 diabetes, the medicines most likely to cause low blood sugar were reduced at the start under a medical protocol. Even so, about a third of participants who used sulfonylureas or insulin had a low or high blood sugar event in the first two weeks. Please never adjust these medicines yourself.

A short doctor visit checklist can help you prepare for that conversation. Some medicines can also affect weight; your doctor can tell you whether that applies to you.

Side effects of intermittent fasting

Side effects of intermittent fasting are usually mild. The ones reported most often are headache, tiredness, feeling sick and dizziness.

A recent meta-analysis of 15 randomised trials with 1,365 adults living with overweight or obesity found no clear increase in tiredness, headache or dropping out compared with control diets. There was a hint of more dizziness in people doing a late daily window without other support, but this result was not certain. The Cochrane review was also unsure about side effects, because few trials measured them well.

Two concerns go beyond comfort. The first is the risk to your relationship with food, covered below. The second is that in the 12-month alternate-day fasting trial by Trepanowski and colleagues, LDL cholesterol was higher at the end in the fasting group than in the daily calorie reduction group. If you already have raised cholesterol or heart disease, this is another reason to discuss fasting with your doctor.

If you use insulin or other medicines that can lower blood sugar, learn the signs of a hypo: shaking, sweating, a racing heart, confusion or sudden hunger. Treat it as your care team has taught you and contact them before fasting again.

Protecting muscle and strength while fasting

Any weight loss includes some loss of muscle and other lean tissue, and fasting is no exception. Keeping muscle strong matters for balance, joints and daily tasks, especially as you get older.

In the 12-week TREAT trial of 16:8, people in the fasting group lost a little more lean mass in their arms and legs than the comparison group. A recent scoping review of 20 trials in adults aged 45 and over found that differences in lean mass were generally small, but the trials were short and the authors could not draw firm conclusions for older people.

The US National Institute on Aging points out that most fasting trials have included people aged 60 or younger, so results may not apply to older adults. If you are older, start with the gentle step below and involve your doctor.

Three habits help protect muscle with any eating pattern:

• include a source of protein in each meal, such as eggs, dairy, fish, chicken, tofu, beans or lentils
• do muscle-strengthening exercise on at least two days a week; our Level Start workouts include seated strength moves
• keep walking; even short, regular walks help keep legs strong

Fasting, eating disorders and your relationship with food

For some people, strict rules about when they may eat lead to a stronger focus on food, cycles of restriction and overeating, or feelings of guilt. This is the main psychological risk of fasting.

Harvard’s review warns that long periods without food may raise the risk of overeating afterwards and of an unhealthy fixation on food. Doctors writing in Clinical Diabetes and Endocrinology have urged clinicians to consider the risk of disordered eating before suggesting intermittent fasting to a patient.

Research on this question is still young, and many people fast without running into these problems. What matters is how fasting feels for you, and whether you notice any of the signs below.

Signs that fasting is not good for you

• you think about food much more than before, or plan your day around the next meal
• you often eat far more than you planned at the end of a fast, and feel out of control
• you feel guilt or shame when you eat outside your window
• you want to make the fasting periods longer and longer
• your mood, sleep or social life suffer because of your eating rules

If you recognise these signs, stop fasting and go back to regular meals. You have not failed; this approach simply does not suit you. Talking with your doctor or a psychologist can help, and our page on psychological support explains what kinds of help exist.

A gentle way to try time-restricted eating

If you would like to try intermittent fasting and your doctor agrees, the gentlest start is a 12-hour overnight break. Most of those hours are spent asleep, so it asks little of your body, and it often removes late-evening snacking.

Talk to your doctor first, especially if anything in the list above applies to you. These steps are general ideas, not personal medical advice.

Weeks 1 and 2: notice and set a 12-hour break

1. For three days, simply write down the time of your first and last food or caloric drink. Many people are surprised how long their eating day is.
2. Choose a 12-hour break that suits your life, for example from 19:00 to 7:00 or from 20:00 to 8:00.
3. After your evening meal, close the kitchen. Brush your teeth, make a cup of herbal tea or go for a short walk to mark the end of the eating day.
4. Keep your normal, balanced meals. Do not try to eat less in other ways at the same time.

Weeks 3 and 4: check how it feels

1. Ask yourself: am I sleeping well, is my energy fine, do I feel calm around food?
2. If yes, you can keep the 12-hour break as your new routine. That alone is a good result.
3. If you feel fine and would like to go further, you could try 13 hours on a few days a week.

After week 4: decide, without pressure

1. Some people move gradually to a 10-hour window (14:10). Increase by 30 to 60 minutes at a time, and only if it still feels easy.
2. Only consider 16:8 if 14:10 has felt comfortable for several weeks and your doctor agrees. Many people never need to go that far.
3. If any of the warning signs above appear, go back to regular meals.

Weigh yourself no more than once a week, at the same time of day. A realistic pace is gradual; our page on safe weight loss explains what to expect. To get a sense of your own energy needs, you can use our calorie deficit calculator, but there is no need to count calories if the eating window already helps you eat a little less.

What to eat when your eating window opens

Within your eating window, the same principles apply as for any healthy eating pattern: plenty of vegetables and fruit, whole grains, protein foods and healthy fats, with fewer sugary drinks and highly processed snacks.

The first meal after a longer break deserves some thought. Many people find that a meal with protein and fibre, such as oats with yoghurt and fruit, eggs with wholegrain bread, or a bean and vegetable soup, keeps hunger steadier than something sweet.

If you get hungry between meals in your window, choose snacks that fill you up for a modest amount of energy. Our list of healthy snacks gives practical ideas with calories and protein.

A few habits make fasting easier to live with:

• plan your meals for the window in advance, so you do not reach the end of the fast with nothing ready
• keep your eating window similar on most days, including weekends, so your body gets used to the rhythm
• be flexible for birthdays, holidays and family meals; one later dinner does not undo anything
• avoid alcohol in the evening, which often leads to late snacking and poorer sleep

Sleep and stress also affect hunger and cravings. If your nights are short or your days are very stressful, read about sleep, stress and mental health before adding fasting rules on top.

Is intermittent fasting right for you?

Intermittent fasting may suit you if you are rarely hungry in the morning, often snack late in the evening, and like clear and simple rules. It is less likely to suit you if you get dizzy or shaky without food, have a demanding physical job, or find that strict rules make you think about food more.

The evidence gives you freedom. Since fasting and daily calorie reduction lead to similar results, you can choose the one that fits your health, your medicines, your family life and your taste. You can also combine elements, such as three regular meals with a 12-hour overnight break.

Whatever you choose, the building blocks of lasting change stay the same: good food, regular movement at the right level, walking, enough sleep and support from people around you. Fasting can be a small part of that, never a replacement for it.

Free help to lose weight with good food and walking

Obesity Foundation is a non-profit and everything we offer is free. We do not sell diets or fasting programmes. Our focus is good food, movement that suits your body, and walking, in a way that fits your daily life.

• Ask for your free eating plan: balanced meals that can fit a regular routine or a 12-hour overnight break, adapted to your taste and budget.
• Get the free workout videos: Level Start helps you build strength gently, with seated options and no jumping.
• Plan a free video meeting if you are unsure whether fasting is a good idea for you and want to talk it through.
• Become a member of the free Obesity Foundation Club for tips, encouragement and a place to keep track of your progress.

We are not doctors, and this page cannot tell you whether fasting is safe for you. Please talk to your own doctor before you change how or when you eat, especially if you take medicines.