Intermittent Fasting for Weight Loss
Intermittent fasting can lead to weight loss, mainly because it usually means eating less food overall. Studies comparing it with a standard kilojoule-reduced diet generally find little or no difference in weight lost, so the method that suits you is the one you can keep up.
Everything here is general information, not medical advice. Outcomes vary between individuals, and a GP or accredited practising dietitian can tell you whether fasting suits your health.
On this page
- Key Points
- What Is Intermittent Fasting?
- Intermittent Fasting Methods Compared
- What the Evidence Says About Fasting and Weight Loss
- A Worked Example: The 5:2 Diet Over One Week
- How to Start Intermittent Fasting
- What to Expect in the First Weeks
- Who Should Not Try Intermittent Fasting
- Healthy Diet and Physical Activity Still Matter
- Medical Weight Loss for Overweight or Obesity
- Frequently Asked Questions
Key Points
Intermittent fasting sets times when you eat normally and times when you eat little or no food.
Common methods include 16/8, 12/12 and the 5:2 diet. No one method has been shown to be the best.
Weight loss seems to come mostly from eating less overall, and meal timing may not make a difference.
Intermittent fasting is not suitable for everyone, including people who are pregnant, underweight or living with an eating disorder.
During eating times, a healthy diet from the five food groups and regular physical activity still matter.
What Is Intermittent Fasting?
Intermittent fasting is a way of eating that limits food to set times. There are times when you eat normally, and other times when you eat little or no food (healthdirect). It is also called intermittent energy restriction.
Most diets focus on what to eat. Intermittent fasting sets no rules about which foods to eat or avoid, only about the timing or the amount. You can drink water at any time, including during a fasting period.
Intermittent Fasting Methods Compared
These are the most common schedules. Each one splits the day or the week into eating and fasting periods.
| Method | Pattern | Example |
|---|---|---|
| 12/12 | 12 hours fasting, then a 12-hour eating window | Eat from 7am to 7pm |
| 14/10 | 14 hours fasting, then a 10-hour eating window | Eat from 9am to 7pm |
| 16/8 | 16 hours fasting, then an 8-hour eating window | Eat from midday to 8pm |
| 18/6 or 20/4 | 18 or 20 hours fasting, then 6 or 4 hours eating | Eat from 2pm to 8pm on 18/6 |
| 5:2 diet | Usual eating on 5 days, 2,000 to 3,000 kJ on 2 days | You choose the 2 days |
| Eat-stop-eat | A 24-hour fasting period, 1 or 2 days a week | From one dinner to the next |
| Alternate day fasting | 24-hour fasts on set days each week | Plans differ between sources |
| Extended fasts | Several days of little or no food | Only with medical or dietitian supervision |
The 12/12, 14/10, 16/8, 18/6 and 20/4 schedules are all forms of time-restricted eating. The example times are illustrations only, and any window of the same length works the same way. On the 5:2 diet, 2,000 to 3,000 kJ is about 480 to 710 calories, at 4.2 kJ per calorie. Alternate day plans range from 1 or 2 up to 3 or 4 fasts of 24 hours a week, depending on the source.
There is no clear answer to which method is best. The 12/12 schedule is the simplest method, as it can mean a last meal at 7pm and the next meal at 7am. Many people find 16/8 the most sustainable to stick to over time.
What the Evidence Says About Fasting and Weight Loss
The idea behind fasting is that after a fasting period, people do not fully make up for the food they went without. That creates an energy gap, and the body draws on stored energy, mostly fat. Here is what Australian health sources say about the results:
It is about as good as other diets for weight. Many trials have compared intermittent fasting with standard energy-restricted diets, and most find only minor differences in weight lost (Dietitians Australia). The Better Health Channel reaches the same view.
Timing may not be the key. Some research has found that the benefits come from eating fewer calories, and that the timing of meals may not make a difference (healthdirect).
Nutrients still count. Recent research found that people may lose weight with intermittent fasting without getting the same health benefits as some other diets. Diets that cut kilojoules while still meeting nutrient needs can give more health benefits.
Regain is common. Most behavioural weight loss methods, intermittent fasting included, lead to short-term weight loss followed by weight regain.
Other benefits are not settled. Some studies have found lower blood pressure, better cholesterol levels, lower inflammatory markers and a lower resting heart rate. It is not clear whether these come from the fasting pattern or from eating less overall. Claims about living longer rest mostly on animal studies.
So intermittent fasting is one way to eat less, not a special route to fat loss. Results vary between individuals: some people do well with it, and some do not, as with any weight loss diet. For what a realistic pace looks like, read how much weight you can realistically lose in a month.
A Worked Example: The 5:2 Diet Over One Week
The average adult needs about 8,700 kJ (about 2,070 calories) a day. This example uses that figure to show how the energy gap on a 5:2 week can grow or shrink.
| Week | Energy eaten | Gap against 8,700 kJ a day |
|---|---|---|
| Usual eating, 7 days | 60,900 kJ | None |
| 5:2, usual intake on 5 days | 48,500 kJ | 12,400 kJ a week |
| 5:2, 10% more on 5 days | 52,850 kJ | 8,050 kJ a week |
The 5:2 rows use 2,500 kJ on each of the 2 lower days, the middle of the 2,000 to 3,000 kJ range. On average, the gap is about 1,770 kJ a day with usual eating on the other 5 days, and about 1,150 kJ a day if those days creep up by 10%. This is arithmetic only, your own needs may be higher or lower, and outcomes vary between individuals.
The same logic applies to 16/8. Eating the same food in fewer hours does not change the total. To work out a target for your body, see how many calories a day you need to lose weight and what a calorie deficit is and how to work out yours.
How to Start Intermittent Fasting
Talk to your doctor before you start, to check that it is right for you. Your doctor may need to adjust the doses of any regular medicines. Then:
Start simple. Cutting out night-time snacking is one way to begin trialling intermittent fasting.
Pick a method that fits your life. There is no one right way, and it is fine to change methods if you need to. Think about how the schedule will affect the people you live with and your social life.
Eat normally in your eating window. When a fasting period ends, focus on vegetables, fruit, wholegrains and lean sources of protein.
Drink plenty of water. Your body also gets fluid from food, so you may need more water while fasting to avoid dehydration.
Two pitfalls can undo the plan. The first is eating more on the other days or hours, which shrinks the energy gap, as the worked example shows. The second is choosing a schedule you cannot keep up: being able to sustain any diet is key, so a strict schedule you cannot follow for a long time may not suit you. If your weight stops changing, read why weight loss can stall in a calorie deficit and what to do about a weight loss plateau.
What to Expect in the First Weeks
When you first start intermittent fasting, you will feel hungry, and you may get headaches. Hunger is the main side effect. You may also feel weak, and your brain may not work as well as usual. These effects may only be temporary, and your body is likely to get used to fasting over time.
Stop intermittent fasting and speak to your doctor if you are:
feeling weak, anxious or irritable
unable to concentrate
unable to do your usual activities
Who Should Not Try Intermittent Fasting
Intermittent fasting is not suitable for everyone, and it can be harmful for some people. Australian health sources advise against it if you:
are underweight
have, or have had, an eating disorder
are pregnant or breastfeeding
are a child or teenager
are an older adult
take a daily medicine that must be taken with food
Talk to your doctor first if you take medicines for diabetes or are having treatment for cancer. Some diabetes medicines lower blood glucose, and without food they can work too well and put a person at risk of hypoglycaemia (low blood glucose). The doses of diabetes and other medicines may need to change (Diabetes Australia). The effect of intermittent fasting is also unknown for people with some conditions, including established coronary artery disease, heart failure and chronic kidney disease.
Intermittent fasting could be a trigger for someone vulnerable to an eating disorder. If food or your weight is causing you distress, the Butterfly Foundation's National Helpline is open 7 days a week, 8am to midnight (AEST/AEDT), on 1800 33 4673.
Healthy Diet and Physical Activity Still Matter
Intermittent fasting changes when you eat, not what a healthy diet looks like. During your eating times, the Australian Dietary Guidelines recommend a wide variety of nutritious foods from the five food groups every day: vegetables and legumes, fruit, grain foods (mostly wholegrain), lean meats, fish, eggs, tofu, nuts, seeds and legumes, and milk, yoghurt and cheese or their alternatives (mostly reduced fat). The guidelines also advise drinking plenty of water and limiting foods and drinks with saturated fat, added salt, added sugars and alcohol.
Australia's physical activity guidelines for adults recommend moderate to vigorous activity for 30 minutes or more on most days, plus muscle-strengthening activities on 2 or more days a week. For ways to add movement to your day, read how many steps a day can support weight loss, and for the full picture, how to lose weight.
Medical Weight Loss for Overweight or Obesity
Some people living with overweight or obesity may need a more intensive approach, such as a very low energy diet or weight loss medication, and some people living with obesity may need bariatric surgery.
Weight loss medication is used together with a healthy diet and physical activity, not instead of them. It has side effects, and any medical treatment is a decision for a doctor or nurse practitioner after an individual assessment. Outcomes vary between individuals. To learn more, read how medical weight loss programs work.
Time-restricted eating suits some people and not others, and for adults living with overweight or obesity, Burst Health provides medical weight loss plans online, reviewed by an Australian-registered doctor or nurse practitioner: check whether you are eligible.
Frequently Asked Questions
How much weight can you lose with intermittent fasting?
There is no set amount. Intermittent fasting leads to weight loss through an energy gap, because people do not fully make up for the food they went without. Eating more on the other days or hours shrinks that gap, as the worked example shows. Some people do well with it and some do not. Outcomes vary between individuals, so a GP or dietitian can help set realistic expectations.
How many days a week should you do intermittent fasting to lose weight?
There is no set number of days. Some methods, such as 16/8, are followed every day, while the 5:2 diet uses 2 lower-energy days a week and eat-stop-eat uses 1 or 2. No method has been shown to be the best for weight. The right choice is a schedule you can sustain alongside a healthy diet.
Should a fasting period last 12, 14 or 16 hours?
There is no clear answer, and no number of hours has been shown to work best. There is no one right way to do intermittent fasting, so choose a method that feels right for you and fits your daily life, and change it if you need to. Talk to your doctor first if you take regular medicines.
Is fasting from 7pm to 7am good?
It can be a simple place to start. A 7pm to 7am gap is the 12/12 schedule, the simplest form of time-restricted eating. According to healthdirect, having regular meals and avoiding late-night food or lots of snacks seems to be good for your health. Intermittent fasting is still not suitable for everyone, so check with your doctor first.
What happens after 7 days of intermittent fasting?
Hunger is likely in the first days, and headaches are common too. These effects may only be temporary, as the body is likely to get used to fasting over time. Weight changes over one week vary between individuals. The section on the first weeks above lists the signs that mean you should stop and speak to your doctor.
Does intermittent fasting burn belly fat?
Not on its own. You cannot choose where your body loses fat, and belly fat can reduce as part of overall fat loss. Overall fat loss comes from gradual, lasting changes to diet, physical activity and sleep. All physical activity helps burn body fat and keep muscle. For the full picture, see how to lose belly fat.
Medical weight loss plans, online
In three steps.
- 1Answer a quick questionnaireOnline, no appointment, no in-person visit.
- 2A practitioner reviews itAn Australian-registered doctor or nurse practitioner decides whether treatment is suitable for you.
- 3Start your planIf treatment is suitable, you start your plan with the practitioner supporting you throughout.
References
healthdirect. Body mass index (BMI) and waist circumference.
Better Health Channel (Victorian Government). Weight loss: common myths.
Eat for Health (NHMRC). Australian dietary guidelines 1 - 5.
This article is general information only and does not replace individual medical advice. Please speak with a registered Australian healthcare practitioner about your own circumstances.