What Is Intermittent Fasting?
Intermittent fasting (IF) is an eating pattern that cycles between defined fasting and eating windows. Unlike diets that prescribe what to eat, IF prescribes when to eat. The primary mechanism for fat loss is calorie restriction — a compressed eating window naturally reduces total intake for most people. Research comparing IF to continuous calorie restriction with matched calories finds equivalent weight loss and metabolic outcomes.
Intermittent Fasting Protocols Compared
| Protocol | Fast | Eat | Difficulty | Best for |
|---|---|---|---|---|
| 16:8 (Leangains) | 16h | 8h | Easy | Most people; best adherence |
| 18:6 | 18h | 6h | Moderate | Smaller appetite; lower calorie needs |
| 20:4 (Warrior Diet) | 20h | 4h | Hard | Aggressive fat loss; not for athletes |
| OMAD (23:1) | 23h | 1h | Extreme | Experienced fasters only |
| 5:2 (Fast Diet) | 2 days at 500–600 kcal | 5 days normal | Moderate | Non-daily restriction; flexible schedule |
Metabolic Milestones During Fasting
As fasting duration increases, the body progresses through distinct metabolic states:
| Hours | State | What happens |
|---|---|---|
| 0–4h | Fed | Insulin elevated, glycogen replenishing, glucose as primary fuel |
| 4–12h | Post-absorptive | Blood glucose normalises, glycogen begins depleting, free fatty acids rise |
| 12–16h | Fat burning | Liver glycogen depleted, fat oxidation increases significantly |
| 16–24h | Ketosis onset | Ketone production begins as liver converts fatty acids to ketones |
| 24h+ | Autophagy | Cellular recycling of damaged proteins and organelles accelerates |
How to Calculate Macros on Intermittent Fasting
Macro targets during IF are the same as for any structured diet — the eating window just compresses when you consume them:
- Protein: 1.6–2.2 g/kg bodyweight — prioritise this. Muscle preservation during fasted periods depends on meeting daily protein targets.
- Fat: 25–35% of total calories. Fat is satiating and supports hormonal function during caloric restriction.
- Carbohydrates: Fill remaining calories. Prioritise around training if you exercise within your eating window.
This calculator sets protein at 1.8 g/kg and fat at 25% of calories, with carbohydrates making up the balance — a conservative, muscle-preserving macro split for IF.
5:2 Diet — Calorie Maths
The 5:2 protocol restricts calories to 500 (women) or 600 (men) on 2 non-consecutive days per week, with no restriction on the other 5 days. The resulting average daily deficit depends on your TDEE:
Avg daily deficit = 2 × (TDEE − 500) ÷ 7
Example: TDEE 2,000 kcal → 2 × 1,500 ÷ 7 ≈ 429 kcal/day deficit
At this deficit, fat loss is approximately 0.4 kg/week — slower than daily 500 kcal restriction but more flexible and easier to sustain long-term.
Frequently Asked Questions
What is the best intermittent fasting protocol for weight loss?
16:8 is the most evidence-backed and sustainable protocol. It restricts eating to an 8-hour window, which naturally reduces calorie intake for most people without requiring calorie counting. 5:2 produces similar results in clinical trials. The best protocol is the one you can adhere to consistently.
How many calories should I eat during intermittent fasting?
Your calorie target is your TDEE (total daily energy expenditure) adjusted for your goal. For fat loss, subtract 500 kcal from TDEE to lose approximately 0.5 kg (1 lb) per week. This calculator computes your TDEE from your age, sex, weight, height, and activity level using the Mifflin-St Jeor formula.
What macros should I eat on intermittent fasting?
Protein is the most important macro during IF. Aim for 1.6–2.2 g/kg of bodyweight per day to preserve muscle mass during the fasting period. Fat should be around 25% of total calories, with carbohydrates filling the remainder. This calculator sets protein at 1.8 g/kg as a conservative muscle-preserving target.
Does intermittent fasting put you in ketosis?
Ketosis typically begins 16–24 hours after your last meal, when liver glycogen is fully depleted and the body switches to fat oxidation. 16:8 fasting alone rarely produces full ketosis since the fasting window (16 hours) just reaches the threshold. OMAD (23:1) and extended fasts reliably achieve ketosis.
When does fat burning start during fasting?
Fat oxidation increases after approximately 12 hours of fasting, when glycogen stores begin to deplete. The body gradually increases free fatty acid utilization from 12 hours onward. Full fat-burning mode (measurable by respiratory quotient) is established by 16–18 hours.
What is autophagy and when does it start?
Autophagy is a cellular cleanup process where cells break down and recycle damaged components. It begins to increase measurably after 24 hours of fasting and peaks around 48 hours. Short daily fasting windows (16:8) produce minimal autophagy compared to extended fasts.
Can I build muscle with intermittent fasting?
Yes, but it requires a caloric surplus and adequate protein (1.8–2.2 g/kg) concentrated in the eating window. Studies comparing IF to non-IF with matched calories and protein show equivalent muscle gain. The main challenge is consuming enough calories and protein in a compressed eating window.
How many calories can I eat on 5:2 fasting days?
The original 5:2 protocol specifies 500 kcal for women and 600 kcal for men on fasting days, approximately 25% of average TDEE. On the 5 non-fasting days, you eat at your normal TDEE. The weekly caloric deficit from 2 restricted days averages to approximately 200–300 kcal/day — slower but consistent fat loss.
Is intermittent fasting safe for women?
IF is generally safe for most healthy women but may affect menstrual cycle regularity if calories are too low or fasting is too aggressive (OMAD, 20:4). 16:8 is the safest starting point. Women with PCOS, history of disordered eating, or who are pregnant or breastfeeding should consult a doctor before starting IF.
What breaks a fast?
Any food or caloric drink breaks a fast by triggering an insulin response. Black coffee and plain water do not break a fast. Technically, anything above 0–5 kcal can interrupt the fasted metabolic state. Artificial sweeteners are debated — some may cause an insulin response in certain individuals.