Intermittent Fasting: The Complete Guide to Every Protocol (16:8, 5:2, OMAD)

Intermittent fasting has moved from fringe biohacking into mainstream nutrition science over the past decade. The research is genuinely compelling — but there's also a lot of hype and confusion about what IF actually is, how to do it correctly, and whether it's the right approach for your body and lifestyle. This guide cuts through both.

What Is Intermittent Fasting?

Intermittent fasting (IF) is a structured eating pattern that alternates between periods of eating and deliberate fasting. It's not a diet in the traditional sense — it doesn't specify what you eat, only when you eat.

The key metabolic effect exploited by IF is the shift from glucose metabolism to fat metabolism that occurs during prolonged fasting. After roughly 12 hours without food, liver glycogen stores are depleted and the body increasingly relies on fat oxidation and ketone production for energy.

The Main Intermittent Fasting Protocols

16:8 — The Most Popular Protocol

Fast for 16 hours, eat within an 8-hour window. The most common version is skipping breakfast and eating from noon to 8 PM, though windows can be adjusted to suit your schedule.

18:6

A stricter variant of 16:8. Eat within a 6-hour window, fast for 18. Popular among those who've adapted to 16:8 and want to increase the fasting benefit. Example: 1:00 PM – 7:00 PM.

20:4 ("Warrior Diet")

A 4-hour eating window with a 20-hour fast. Typically involves one large meal in the evening. This is aggressive and not recommended for beginners — it can make adequate protein and micronutrient intake very difficult.

5:2

Eat normally for 5 days a week, restrict to approximately 500 calories (women) or 600 calories (men) on 2 non-consecutive days. Less strict in terms of daily timing but requires significant calorie restriction on fast days.

OMAD (One Meal a Day)

The extreme end of IF — a single meal within a 1-2 hour window, with a 22–23 hour fast. Difficult to maintain adequate protein and micronutrient intake. Not appropriate for most people.

Eat Stop Eat (24-hour fasts)

One or two 24-hour fasts per week. For example, finishing dinner at 7 PM Monday and not eating until 7 PM Tuesday. The remaining days follow normal eating patterns.

What Happens in Your Body During a Fast

What Does the Research Actually Show?

The evidence for intermittent fasting is real but often overstated in popular media. Here's an honest summary:

Weight Loss

IF produces weight loss primarily because it reduces total calorie intake. When matched calorie-for-calorie to continuous calorie restriction in clinical trials, the weight loss outcomes are broadly similar. IF's advantage is that many people find it easier to maintain than daily calorie counting.

Metabolic Health

Studies show improvements in insulin sensitivity, fasting glucose, blood pressure, and triglycerides — all meaningful cardiovascular risk markers. These effects appear even independent of weight loss in some studies.

Autophagy

Autophagy is the cellular "self-cleaning" process by which damaged proteins and organelles are broken down and recycled. It's upregulated during fasting. The 2016 Nobel Prize in Physiology (awarded to Yoshinori Ohsumi for autophagy research) sparked enormous public interest in IF. However, the clinical translation of autophagy research is still emerging — we know it's important; we don't yet have precise human data on optimal fasting durations to maximise its benefit.

Muscle Retention

A common concern: does fasting cause muscle loss? Research suggests that IF with adequate protein intake (1.6–2.2g per kg of body weight) preserves muscle mass at least as well as continuous calorie restriction. Resistance training during IF is particularly effective for maintaining lean mass.

What Breaks a Fast?

Technically, any caloric intake ends a fast. However, in practice, the question is whether something significantly raises insulin or disrupts the key metabolic benefits you're seeking.

• Water, sparkling water: Does not break a fast ✓
• Black coffee: Does not meaningfully break a fast. May enhance fat oxidation. ✓
• Plain tea (no milk or sweetener): Does not break a fast ✓
• Coffee with milk: Small amounts are borderline. A splash (<30 mL) is generally considered acceptable by most practitioners.
• Artificial sweeteners: Technically zero calories, but may trigger an insulin response in some individuals. Approach with caution if metabolic effects are your goal.
• Bone broth: Contains protein and some calories — technically breaks a strict fast, but has minimal caloric impact.
• Apple cider vinegar: Negligible calories. Does not meaningfully break a fast.
• Gum: If sugar-free, generally accepted as fine.

Who Should Be Careful With Intermittent Fasting

• People with a history of disordered eating: Rigid eating windows can reinforce unhealthy relationships with food.
• Pregnant or breastfeeding women: Increased nutrient demands make fasting inappropriate.
• People with type 1 diabetes or on insulin: Fasting requires careful medical supervision to prevent hypoglycaemia.
• Those with adrenal fatigue or high cortisol: Extended fasting can worsen cortisol dysregulation.
• People on certain medications: Some medications must be taken with food. Check with your GP.

Frequently Asked Questions

Can I exercise while fasting?

Yes. Many people exercise fasted without issue, particularly with low-to-moderate intensity cardio and strength training. High-intensity training on an extended fast may be suboptimal for performance. If training hard, consider placing your training near the start of your eating window.

Will I lose muscle on IF?

Not if you eat adequate protein within your eating window. Aim for 1.6–2.2g of protein per kg of body weight spread across your eating window. Resistance training is the most powerful signal to maintain muscle during any calorie-restricted protocol.

Why am I not losing weight on 16:8?

Most likely because total calorie intake hasn't decreased. IF works by restricting the eating window, which naturally reduces food intake for many people — but if you compensate by eating more during the window, the caloric deficit disappears. Track your food intake for a week to check.

How long does it take to adapt to intermittent fasting?

Most people experience hunger, irritability, and low energy in the first 1–2 weeks. Adaptation typically takes 2–4 weeks. Starting with a 12-hour fast and gradually extending to 14, then 16 hours over several weeks reduces the adjustment difficulty significantly.