Intermittent Fasting
A pattern of cycling between eating and fasting windows. Moderate evidence supports short-term weight loss comparable to standard calorie restriction.
Intermittent Fasting Key Facts
Moderate evidence from multiple RCTs
Several randomised controlled trials have compared intermittent fasting to continuous calorie restriction. Results consistently show similar weight loss outcomes. A 2024 meta-analysis in Nature Reviews Endocrinology confirmed IF produces 3-8% body weight loss over 8-12 weeks.
Generally safe for healthy adults
Intermittent fasting is generally well-tolerated by healthy adults. Common side effects include initial hunger, irritability, and difficulty concentrating during the fasting window. These typically improve within 1-2 weeks of adaptation.
3-8% body weight in 8-12 weeks
Clinical trials show IF produces weight loss comparable to standard calorie restriction (eating less every day). The primary mechanism is reduced total calorie intake due to a shorter eating window, not metabolic changes from fasting itself (Cienfuegos 2022).
Partially aligned
The NHMRC does not specifically endorse or discourage intermittent fasting. It can be compatible with the Australian Dietary Guidelines if the foods consumed during eating windows come from all five food groups. The pattern of eating matters less than the quality.
Moderate - some social adjustment needed
The 16:8 method (skip breakfast, eat 12pm-8pm) is the easiest to adopt. No foods are restricted and no calorie counting is mandatory. The main challenge is social meals that fall outside your eating window.
What is the Intermittent Fasting?
Intermittent fasting (IF) is not a diet in the traditional sense - it does not prescribe what to eat, but when to eat. The approach cycles between periods of eating and fasting, with the most popular method being the 16:8 protocol (fasting for 16 hours, eating within an 8-hour window). Other methods include 5:2 (eating normally 5 days, restricting to 2,000-2,500 kJ on 2 days) and alternate-day fasting.
Research suggests IF produces weight loss comparable to standard calorie restriction, primarily by reducing total calorie intake through a shorter eating window. Some studies suggest additional metabolic benefits including improved insulin sensitivity and reduced inflammation markers, though these findings are not consistent across all trials.
How the Intermittent Fasting Works
During fasting periods, insulin levels drop and the body increasingly relies on stored fat for energy. After approximately 12 hours of fasting, glycogen (stored glucose) becomes depleted and the body upregulates fat oxidation. This metabolic switch is well-documented in human studies. However, the primary mechanism for weight loss is simpler: eating within a restricted time window typically leads to consuming fewer total calories, even without deliberate calorie counting. Studies comparing IF to calorie-matched continuous restriction show similar weight loss, suggesting the calorie deficit - not the fasting itself - drives fat loss (Cienfuegos 2022).
Common IF protocols: - 16:8: Fast 16 hours, eat within 8 hours (e.g., 12pm-8pm) - 5:2: Eat normally 5 days, restrict to 2,000-2,500 kJ on 2 non-consecutive days - Alternate-day fasting (ADF): Alternate between normal eating days and fasting/very low calorie days - OMAD (One Meal A Day): 23:1 fasting ratio - more extreme and less studied
Macronutrient Breakdown
IF does not restrict macronutrients. You can follow any macronutrient ratio within your eating window.
Aim for at least 1.2g/kg body weight to preserve muscle mass, especially if in a calorie deficit.
No fat restriction. Focus on food quality within your eating window.
The key principle is when you eat, not what you eat. However, food quality still matters for health outcomes and satiety.
Intermittent Fasting in Australia
Intermittent fasting can be compatible with the Australian Dietary Guidelines if the foods consumed during eating windows include foods from all five food groups. The NHMRC has not specifically evaluated IF protocols, but acknowledges that various meal timing approaches can support healthy eating patterns. Dietitians Australia notes that IF may suit some individuals but is not suitable for everyone, particularly those with a history of disordered eating. In the Australian context, the 16:8 method (skipping breakfast, eating 12pm-8pm) aligns well with typical work schedules and social dinner times.
Multiple randomised controlled trials support intermittent fasting for short-term weight loss (8-12 weeks). However, most studies show IF is not superior to standard calorie restriction - it simply provides an alternative approach that some people find easier to follow. Long-term adherence data (beyond 12 months) is limited.
Research & Evidence
Cienfuegos S, Corber S, Gabel K, et al.
Effect of Intermittent Fasting on Reproductive Hormone Levels in Females and Males: A Review of Human Trials
Nutrients
Finding: Review of human trials found IF produces weight loss primarily through calorie reduction rather than metabolic changes from fasting itself. Reproductive hormone effects were minimal in most studies.
Varady KA, Cienfuegos S, Ezpeleta M, Gabel K
Clinical application of intermittent fasting for weight loss: progress and future directions
Nature Reviews Endocrinology
Finding: Comprehensive review found time-restricted eating (16:8) produces 3-5% weight loss over 8-12 weeks. Alternate-day fasting produces 3-8% weight loss. Results comparable to calorie restriction.
de Cabo R, Mattson MP
Effects of Intermittent Fasting on Health, Aging, and Disease
New England Journal of Medicine
Finding: Landmark NEJM review found IF triggers a metabolic switch from glucose to ketone-based energy, with evidence for improved stress resistance, reduced inflammation, and delayed ageing in animal models. Human evidence is promising but less conclusive.
Liu D, Huang Y, Huang C, et al.
Calorie Restriction with or without Time-Restricted Eating in Weight Loss
New England Journal of Medicine
Finding: RCT of 139 participants over 12 months found that time-restricted eating (8-hour window) did not provide additional weight loss benefit beyond standard calorie restriction. Both groups lost similar amounts of weight.
Lowe DA, Wu N, Rohdin-Bibby L, et al.
Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters
JAMA Internal Medicine
Finding: RCT found 16:8 time-restricted eating without calorie counting led to modest weight loss (-0.94kg over 12 weeks) but also significant lean mass loss, raising concerns about muscle preservation.
Source data from published peer-reviewed studies. Links open in a new tab to external medical databases.
Intermittent Fasting Pros and Cons
Pros
No food restrictions
IF focuses on when you eat, not what you eat. You can enjoy all foods within your eating window, making it more flexible than most diets.
Comparable to calorie restriction for weight loss
Multiple RCTs show IF produces similar weight loss to standard dieting, giving people an alternative approach if daily calorie counting does not suit them.
Source: Liu 2022
Simplifies meal planning
Fewer meals to prepare each day. Many people find eating 2-3 meals in an 8-hour window easier to manage than 5-6 small meals throughout the day.
May improve insulin sensitivity
Some studies suggest fasting periods improve insulin sensitivity and reduce fasting insulin levels, which may benefit people with prediabetes.
Source: de Cabo & Mattson 2019
Free to follow
No special foods, supplements, meal plans, or subscriptions required. Works with any budget and any cuisine.
Flexible scheduling
The eating window can be adjusted to fit your lifestyle. Night-shift workers, for example, can shift their window accordingly.
Cons
Not superior to standard calorie restriction
When calories are matched, IF does not produce more weight loss than eating throughout the day. The benefit is behavioural, not metabolic.
Source: Liu 2022
Risk of lean mass loss
One RCT found that 16:8 without resistance training led to significant lean mass loss alongside fat loss. Combining IF with resistance training and adequate protein is important.
Source: Lowe 2020
Initial hunger and irritability
The first 1-2 weeks can involve significant hunger, difficulty concentrating, and mood changes as the body adapts to a new eating schedule.
May trigger overeating
Some people compensate for the fasting period by overeating during their window, negating the calorie deficit. This is more common with more extreme protocols like OMAD.
Not suitable for everyone
People with diabetes (especially on insulin or sulfonylureas), pregnant/breastfeeding women, those with a history of eating disorders, and adolescents should avoid IF without medical supervision.
Social challenges
Skipping breakfast with family, declining morning tea at work, or adjusting dinner times can create social friction.
Who is the Intermittent Fasting For?
May be suitable for
- Healthy adults looking for a structured approach to reduce calorie intake
- People who do not enjoy calorie counting and prefer time-based rules
- Those who naturally skip breakfast and want to formalise this pattern
- People who have plateaued on standard calorie restriction and want a different approach
Not recommended for
- Children and adolescents (growing bodies need regular nutrition)
- Pregnant or breastfeeding women
- People with type 1 diabetes or type 2 diabetes on insulin/sulfonylureas (risk of hypoglycaemia)
- Those with a history of anorexia, bulimia, or binge eating disorder
- People with adrenal insufficiency or other conditions requiring regular food intake
Medical Considerations
Important - Read Before Starting
- If you take diabetes medication (insulin, sulfonylureas), fasting can cause dangerous blood sugar drops - consult your doctor before starting
- Blood pressure medication may need dose adjustment if IF lowers your blood pressure
- If you experience dizziness, fainting, or persistent nausea during fasting, stop and consult a healthcare professional
- Women trying to conceive should be aware that extreme fasting protocols may affect reproductive hormones in some individuals
Intermittent Fasting Foods
Foods to Eat
Protein (prioritise)
- Lean meats
- Fish and seafood
- Eggs
- Greek yoghurt
- Legumes
- Tofu and tempeh
Complex carbohydrates
- Brown rice
- Sweet potato
- Oats
- Quinoa
- Wholegrain bread
Healthy fats
- Avocado
- Olive oil
- Nuts and seeds
- Oily fish (salmon, mackerel)
Vegetables
- All vegetables - aim for variety and colour
Fruit
- All fruit in moderation, especially berries
During fasting window
- Water
- Black coffee
- Herbal tea
- Sparkling water (no sweeteners)
Foods to Avoid
During fasting window
- Anything with calories (even small amounts break the fast)
- Milk in coffee
- Diet drinks with artificial sweeteners (may trigger insulin response - debated)
General (for weight loss)
- Ultra-processed foods
- Sugary drinks
- Excessive alcohol
- High-calorie low-nutrient snacks
Sample 7-Day Intermittent Fasting Meal Plan
| Day | Breakfast | Lunch | Dinner | Snacks | ~kJ |
|---|---|---|---|---|---|
| Monday | Black coffee (fasting) | Grilled chicken salad with avocado, mixed leaves, olive oil dressing (12pm) | Salmon fillet with sweet potato and steamed broccoli (7pm) | Greek yoghurt with berries (3pm) | 7,500 kJ |
| Tuesday | Black coffee (fasting) | Tuna and brown rice bowl with edamame and cucumber (12pm) | Lean beef stir-fry with vegetables and rice noodles (7pm) | Apple with almond butter (3pm) | 7,800 kJ |
| Wednesday | Black coffee (fasting) | Chicken and vegetable soup with wholegrain bread (12pm) | Baked barramundi with roast vegetables (7pm) | Handful of mixed nuts (3pm) | 7,200 kJ |
| Thursday | Black coffee (fasting) | Egg and avocado on sourdough toast with tomato (12pm) | Lamb stir-fry with Asian greens and jasmine rice (7pm) | Carrot sticks with hummus (3pm) | 7,600 kJ |
| Friday | Black coffee (fasting) | Greek salad with grilled prawns and olive oil (12pm) | Homemade chicken and vegetable curry with brown rice (7pm) | Protein smoothie - banana, protein powder, milk (3pm) | 7,900 kJ |
| Saturday | Black coffee (fasting) | Brunch - 3 eggs, smoked salmon, avocado, sourdough (11am) | BBQ steak with corn on the cob and garden salad (6pm) | Dark chocolate (2 squares) and strawberries (3pm) | 8,200 kJ |
| Sunday | Black coffee (fasting) | Roast chicken with roast vegetables and gravy (12pm) | Leftovers or light meal - soup and toast (6pm) | Fruit and cheese platter (3pm) | 7,400 kJ |
This is a sample Intermittent Fasting meal plan for illustration. Adjust portions based on your individual calorie needs. Consult an Accredited Practising Dietitian for a personalised plan.
Suggested Australian Products
Black coffee
StapleAny
Coffee is a natural appetite suppressant and is allowed during the fasting window. Black only - no milk, sugar, or sweeteners.
Herbal tea variety pack
StapleT2, Twinings, or Lipton
Herbal teas (peppermint, chamomile, ginger) help manage hunger during fasting windows without breaking the fast.
Protein powder
SupplementTrue Protein or Bulk Nutrients
Useful for hitting protein targets within a shorter eating window. Australian brands offer good value.
Sparkling water
StapleSodaStream or San Pellegrino
Carbonated water can help with hunger pangs during fasting. Zero calories, allowed during fasting window.
Meal prep containers
ConvenienceSistema or Decor
Having pre-prepared meals ready for your eating window prevents poor food choices when breaking the fast.
How to Get Started
- 1Choose your IF method - 16:8 (most popular and easiest) is recommended for beginners
- 2Pick your eating window based on your lifestyle (e.g., 12pm-8pm suits most office workers)
- 3Start gradually - try a 12:12 fast for the first week, then extend to 14:10, then 16:8
- 4During fasting hours, drink water, black coffee, or herbal tea to manage hunger
- 5Break your fast with a balanced meal containing protein, vegetables, and healthy fats - avoid breaking with sugary or ultra-processed food
- 6Prioritise protein (at least 1.2g/kg body weight) to preserve muscle mass
- 7Include resistance training 2-3 times per week to prevent lean mass loss
- 8Track your eating window rather than calories for the first 2 weeks to build the habit
- 9Adjust your window timing if your initial choice causes problems (e.g., shift to 10am-6pm if evening events are common)
- 10Reassess after 4 weeks - if you are not losing weight, you may need to also track calories within your eating window
Common Mistakes to Avoid
Watch out for these
- Overeating during the eating window - a compressed eating time does not mean unlimited food. Total calories still matter.
- Not eating enough protein - IF increases the risk of muscle loss if protein intake is inadequate. Aim for 1.2-1.6g/kg body weight.
- Skipping resistance training - combining IF with no exercise increases lean mass loss (Lowe 2020).
- Breaking the fast with junk food - the quality of what you eat still matters for health, energy, and satiety.
- Starting too aggressively - jumping straight to 20:4 or OMAD before adapting to 16:8 leads to higher dropout rates.
- Obsessing over the exact minute of the fast - a few minutes either way does not matter. Consistency over perfection.
- Using IF as an excuse to avoid addressing food quality - "I can eat anything in my window" leads to poor nutrition.
Related Calculators
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Protein Intake Calculator
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Find a Dietitian Near You
Get personalised advice on the Intermittent Fasting from an Accredited Practising Dietitian in your area.
Intermittent Fasting and Weight Loss Medication
Intermittent fasting can be combined with GLP-1 receptor agonist medications (Wegovy, Mounjaro, Ozempic), though appetite suppression effects may stack, potentially leading to very low calorie intake. If you are on weight loss medication, discuss IF with your prescribing doctor to ensure adequate nutrition.
Frequently Asked Questions
What is the best intermittent fasting schedule?
The 16:8 method (fasting 16 hours, eating within 8 hours) is the most studied and easiest to sustain. Most people eat from 12pm to 8pm, effectively skipping breakfast. The 5:2 method (normal eating 5 days, very low calories on 2 days) is an alternative that suits people who prefer not to fast every day. The best schedule is the one you can consistently follow.
What are the best intermittent fasting times?
For the 16:8 method, the most common eating windows are 12pm-8pm (skip breakfast), 10am-6pm (early dinner), or 11am-7pm (compromise). Research suggests earlier eating windows (e.g., 8am-4pm) may have additional metabolic benefits due to alignment with circadian rhythms, but this is difficult for most people socially. Choose the window that fits your lifestyle and social commitments.
Can I drink coffee during intermittent fasting?
Yes - black coffee is allowed during the fasting window. It contains minimal calories (2-5 per cup) and does not meaningfully affect the fasting state. Do not add milk, sugar, cream, or sweeteners. Coffee may also help suppress appetite during the fasting period.
Does intermittent fasting slow metabolism?
Short-term IF (up to 12 weeks) does not appear to significantly reduce metabolic rate beyond what is expected from weight loss itself. Prolonged severe calorie restriction from any method can reduce metabolic rate (adaptive thermogenesis), but this is related to the size and duration of the calorie deficit, not the fasting pattern specifically.
Is intermittent fasting better than counting calories?
For weight loss specifically, research shows they produce similar results when calorie intake is matched (Liu 2022). IF may be easier for people who dislike tracking calories, while calorie counting may suit people who prefer flexibility in meal timing. Neither is objectively better - the most effective approach is the one you can sustain long-term.
Will intermittent fasting cause muscle loss?
It can if you do not take precautions. One study found 16:8 IF without resistance training led to significant lean mass loss (Lowe 2020). To preserve muscle: eat at least 1.2-1.6g protein per kg body weight, perform resistance training 2-3 times per week, and avoid extreme calorie deficits.
This website is for informational and research purposes only. We are not medical professionals and nothing on this page constitutes medical advice, diagnosis, or treatment. Always consult a qualified doctor, Accredited Practising Dietitian, or specialist before making any changes to your diet.
The information on this page is based on published peer-reviewed research and Australian dietary guidelines. Individual results may vary. Data sourced from the NHMRC, Dietitians Australia, and published clinical studies. Last reviewed 2026-04-01.
Sources
- Cienfuegos S, Corber S, Gabel K, et al. (2022). Effect of Intermittent Fasting on Reproductive Hormone Levels in Females and Males: A Review of Human Trials. Nutrients. DOI: 10.3390/nu14112343 PMID: 35684143
- Varady KA, Cienfuegos S, Ezpeleta M, Gabel K (2022). Clinical application of intermittent fasting for weight loss: progress and future directions. Nature Reviews Endocrinology. DOI: 10.1038/s41574-022-00638-x PMID: 35194176
- de Cabo R, Mattson MP (2019). Effects of Intermittent Fasting on Health, Aging, and Disease. New England Journal of Medicine. DOI: 10.1056/NEJMra1905136 PMID: 31881139
- Liu D, Huang Y, Huang C, et al. (2022). Calorie Restriction with or without Time-Restricted Eating in Weight Loss. New England Journal of Medicine. DOI: 10.1056/NEJMoa2114833 PMID: 35443107
- Lowe DA, Wu N, Rohdin-Bibby L, et al. (2020). Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters. JAMA Internal Medicine. DOI: 10.1001/jamainternmed.2020.4153 PMID: 32986097