Intermittent Fasting Protocols: 16:8, 5:2, OMAD, ADF Compared

Intermittent fasting (IF) is the deliberate timing of meals to create periods of low insulin and low nutrient availability. The appeal is simple: fasting triggers a cascade of cellular repair processes — autophagy, mitochondrial biogenesis, NAD+ elevation, ketosis — that eating suppresses. By cycling between fed and fasted states, you can theoretically harvest the regenerative benefits of fasting without the muscle loss and risks of prolonged starvation.

The problem is that "intermittent fasting" is an umbrella term covering at least six distinct protocols, each with different effects, trade-offs, and compliance profiles. 16:8 is not the same as 5:2, which is not the same as OMAD or alternate day fasting. This guide compares them head-to-head and explains how to choose the right one for your goals — whether that's weight loss, autophagy, NAD+ elevation, or simply the convenience of fewer meals.

Why fasting works: the biology

When you stop eating, several things happen in a predictable sequence:

1. 12 hours in: Liver glycogen is largely depleted. Insulin drops. The body begins shifting toward fat oxidation.
2. 16 hours in: Ketone bodies (beta-hydroxybutyrate, acetoacetate) rise in blood. Autophagy — the cellular recycling process — begins to accelerate. NAD+ levels rise as cellular energy demand shifts.
3. 24 hours in: Ketones are clearly elevated. Autophagy is in full swing. Hepatic DNL (de novo lipogenesis) is shut down. Inflammation markers begin to drop.
4. 48 hours in: Growth hormone rises (protecting lean mass). Ketones plateau at 2–4 mM. Insulin is at its lowest. AMPK (the cellular energy sensor that activates repair) is maximally active.
5. 72+ hours in: Stem cell regeneration begins (in animal models). Immune cell turnover accelerates. Insulin signaling is profoundly suppressed. This is where prolonged fasting protocols live — and where medical supervision becomes important.

The downstream benefits are real: improved insulin sensitivity, lower inflammation, mild weight loss (mostly from caloric restriction, but with metabolic advantages), and in animal models, lifespan extension of 15–40% depending on protocol and species. In humans, the lifespan evidence is observational and harder to prove, but the healthspan benefits are well-established.

The six main protocols compared

16:8 — the foundational protocol

16:8 is where most people should start. You fast for 16 hours (typically overnight, skipping or delaying breakfast) and eat all your meals within an 8-hour window (typically 12 PM to 8 PM, or 10 AM to 6 PM). It's simple, sustainable, socially tolerable, and delivers the bulk of the metabolic benefits of intermittent fasting.

The evidence for 16:8 is solid. A 2023 meta-analysis of 23 RCTs found that 16:8 produced modest weight loss (1–4% of body weight over 8–12 weeks), reduced waist circumference, lowered fasting insulin, and improved LDL cholesterol. The effects aren't dramatic — most of the weight loss comes from eating ~200–500 fewer calories per day, because the eating window naturally limits intake — but the metabolic improvements exceed what you'd expect from weight loss alone, suggesting a direct fasting effect.

For most adults, 16:8 is the sweet spot: enough fasting to trigger mild ketosis and autophagy, low enough friction to sustain for years. If you do nothing else from this guide, do 16:8.

18:6 and 20:4 — tighter windows

Once 16:8 feels easy (usually after 2–4 weeks of adaptation), some people tighten the window to 18:6 or 20:4. The added fasting hours produce more ketosis, slightly more autophagy, and slightly more weight loss — but the marginal benefits diminish, and the social and compliance costs rise. A 20:4 window (eat only between, say, 2 PM and 6 PM) means you'll skip most social meals and need to be careful to get enough protein and calories in four hours.

If your primary goal is fat loss, 18:6 is a reasonable next step after 16:8. Beyond that, you're typically better off focusing on what you eat (lower carbohydrate, higher protein, whole foods) than on squeezing more fasting hours out of the day.

5:2 — two low-calorie days per week

5:2, popularized by Michael Mosley, takes a different approach: you eat normally 5 days per week and restrict to ~500 calories on 2 non-consecutive days (e.g., Monday and Thursday). The advantage is that you never have to fast for a full day — and the "normal" days are unrestricted, which some people find easier psychologically.

The evidence for 5:2 is comparable to 16:8 for weight loss and metabolic markers. Some people find it easier to comply with; others find the low-calorie days miserable and binge on the normal days. The main risk is using 5:2 as a license to eat poorly on the 5 normal days, which defeats the purpose.

5:2 pairs well with a Mediterranean-style diet on normal days. See our longevity diet guide for what "eating well" looks like.

OMAD — one meal a day

OMAD (one meal a day) is the most aggressive daily IF protocol — you eat all your calories in a single meal, typically within 1–2 hours, and fast for the remaining 22–23 hours. Proponents love the convenience, focus, and deep ketosis. Critics warn — correctly — that getting adequate protein (1.0+ g/kg/day), fiber, micronutrients, and total calories in a single meal is difficult, and that OMAD can lead to under-eating, muscle loss, and disordered eating patterns.

OMAD is best suited to highly motivated, keto-adapted people who plan their one meal carefully. For most readers, it's overkill — 16:8 delivers 80% of the benefits with far less risk. If you want to try OMAD, do it under the guidance of a nutritionist, and make sure your one meal contains 30–50 g of high-quality protein, plenty of vegetables, healthy fats, and adequate total calories.

Alternate day fasting

Alternate day fasting (ADF) means eating freely one day and either water fasting or eating ~500 calories the next, in alternation. The research, much of it from Krista Varady's lab at the University of Illinois, shows ADF produces 3–7% weight loss over 8–12 weeks and improves LDL, triglycerides, and insulin sensitivity comparably to daily caloric restriction.

The downside is that fasting days are genuinely hard. Most people report significant hunger, fatigue, and irritability on fasting days for the first 2–3 weeks, after which it becomes more tolerable. ADF is a legitimate protocol for aggressive fat loss but requires real commitment.

Prolonged fasting (3–5 days)

Prolonged water fasts of 3–5 days, or fasting-mimicking diets (FMD) like ProLon, push the body into a different metabolic state: deep ketosis, maximal autophagy, immune cell turnover, and (in animal models) stem cell-based regeneration. Valter Longo's research suggests 3–4 cycles per year of a 5-day FMD is sufficient to capture most of the regenerative benefits.

Prolonged fasting carries real risks: electrolyte imbalances, hypoglycemia, gallstones, refeeding syndrome (rare but serious), and muscle loss. It's also contraindicated for people with eating disorders, type 1 diabetes, pregnancy, low BMI, or certain other conditions. Do not attempt a 5-day water fast without medical supervision. The FMD approach (ProLon or Longo's published DIY recipe) is much safer and delivers most of the same benefits.

For more detail on FMD specifically, see the dedicated section in our longevity diet guide.

Benefits: autophagy, insulin, NAD+, weight

The four most-cited benefits of intermittent fasting, ranked by evidence strength:

1. Improved insulin sensitivity

IF reduces fasting insulin, lowers HOMA-IR (a marker of insulin resistance), and improves post-meal glucose response. This is the best-established benefit in humans, with effect sizes that rival or exceed caloric restriction and exercise.

2. Weight management

Most IF protocols produce 1–8% weight loss over 8–12 weeks, depending on protocol and adherence. The bulk of the loss comes from eating fewer calories in a smaller window, but the metabolic advantages (lower insulin, more fat oxidation) compound the effect.

3. Autophagy induction

Autophagy — the cellular recycling process that clears damaged proteins and organelles — is suppressed by eating and activated by fasting. In animal models, autophagy is clearly elevated after 16–24 hours of fasting. In humans, direct measurement is harder, but indirect markers (lower mTOR, higher AMPK, ketone elevation) all point in the right direction.

4. NAD+ elevation

NAD+ (nicotinamide adenine dinucleotide) is a coenzyme essential for mitochondrial function, DNA repair (via PARP enzymes), and sirtuin activation. NAD+ levels fall with age. Fasting raises NAD+ by reducing NADH (which competes with NAD+) and activating NAMPT (the rate-limiting enzyme in NAD+ salvage). This is one mechanism by which IF and NAD+ precursors like NMN may complement each other.

Risks and contraindications

Intermittent fasting is generally safe for healthy adults, but it's not for everyone. Avoid IF (or consult a physician first) if you:

• Have a history of eating disorders
• Are pregnant, breastfeeding, or trying to conceive
• Have type 1 diabetes or take insulin (severe hypoglycemia risk)
• Take medications that require food (some antibiotics, NSAIDs, etc.)
• Are underweight (BMI <18.5) or have a history of malnutrition
• Have adrenal fatigue or HPA axis dysregulation
• Are an adolescent still growing

Common side effects during the first 1–3 weeks of adaptation include hunger, irritability, fatigue, headache, and reduced exercise performance. These typically resolve as the body becomes keto-adapted. If side effects persist beyond 4 weeks, the protocol isn't working for you — adjust or stop.

Women in particular should be cautious with aggressive IF protocols. Some evidence suggests women's bodies are more sensitive to caloric restriction and stress, and aggressive IF can disrupt menstrual cycles and thyroid function. Many women do better with 14:10 or 16:8 than with OMAD or ADF.

How to choose the right protocol

Practical guidance, based on goals:

• New to IF, want general health benefits: Start with 16:8 (12 PM–8 PM eating window). Try it for 4 weeks. This is the right starting point for 80% of people.
• Already doing 16:8, want more fat loss: Try 18:6 for 4 weeks. If you feel good and progress continues, you can keep it. If you're hungry, fatigued, or losing muscle, go back to 16:8.
• Hate daily restriction: Try 5:2. Two low-calorie days per week may suit you better than a daily window.
• Want a periodic reset: Try a 5-day fasting-mimicking diet (ProLon or DIY) 2–4 times per year, on top of a baseline 16:8.
• Experienced faster seeking deep autophagy: Consider a 3-day water fast under medical supervision, once or twice a year. Do not attempt this casually.

Whatever you choose, prioritize protein intake in your eating window (1.2–2.0 g/kg/day depending on activity) and stay hydrated with water and electrolytes (sodium, potassium, magnesium) on fasting days.

Fasting-friendly sleep and tracking support

Two practical tools help with IF compliance and safety:

Sleep support during fasts

Many people find that fasting — especially in the first 1–2 weeks — disrupts sleep. Cortisol rises on fasting days, which can cause early waking or fragmented sleep. Two evidence-based supplements help: magnesium glycinate (relaxes muscles, supports GABA) and apigenin (a flavonoid from chamomile that mildly promotes sleep onset without sedation). Both are calorie-free and don't break a fast.

For more options, see our apigenin guide and magnesium guide.

HRV and recovery tracking

If you're going to fast regularly, track your heart rate variability (HRV). HRV measures autonomic nervous system balance; a sudden drop in HRV during a fast is a signal you're over-stressed and should refeed. The Oura Ring 4 is one of the best consumer HRV trackers and integrates fasting-friendly metrics like overnight resting heart rate and readiness scores.

For a deeper look at how wearables support longevity protocols, see our best longevity wearables guide.

The bottom line

Intermittent fasting is one of the highest-leverage lifestyle interventions available — it's free, it works, and the evidence is solid. But the right protocol matters. For most readers, 16:8 is the right starting point: 80% of the benefits, 20% of the friction. If you want more, tighten the window or layer in periodic FMD cycles. Avoid OMAD and prolonged water fasting unless you're experienced and have medical supervision.

Pair IF with a Mediterranean-style diet (see our longevity diet guide) and consistent sleep (see our sleep optimization guide) for compounding benefits. And for the complete beginner's framework that ties IF together with exercise, supplements, and tracking, see our beginner longevity protocol.