Intermittent fasting: 16/8, 18/6, OMAD, 5:2 -- which protocol to choose?

If you've already been practicing 16/8 for a while, you've probably come across more advanced protocols: 18/6, OMAD (one meal a day), 5:2. Before trying one, it helps to understand what they actually involve -- and above all, to understand that none is inherently superior to the others. These are tools, not requirements.

The 4 protocols, factually

16/8 -- 16 hours fasting, 8 hours eating. This is the most common and moderate of the advanced protocols: most people can fit it into their life without major disruption, simply skipping one meal (often breakfast).

18/6 -- The eating window narrows to 6 hours. This is a more demanding version of 16/8: less time to eat means more planning is needed to hit your protein, fiber, and micronutrient targets across fewer meals.

OMAD (One Meal A Day) -- A single meal per day, covering your entire day's calorie and nutrient needs. This is the most extreme variant among common intermittent fasting protocols.

5:2 -- Near-normal eating 5 days a week, with significantly reduced calorie intake on 2 non-consecutive days. Unlike the other protocols, restriction isn't daily but concentrated into two spaced-out days.

More extreme doesn't mean more effective

This is the single most important point of this article: a protocol that restricts your eating window more tightly isn't superior for weight loss. What determines your weight loss remains your total calorie balance over the week -- not how narrow your eating window is. Moving from 16/8 to OMAD won't magically "unlock" anything if your weekly calorie intake stays the same.

What these advanced protocols mainly change is the level of constraint. The narrower the window, the harder it becomes to cover your protein, fiber, and micronutrient needs across fewer eating occasions -- and the higher the risk of discomfort (fatigue, irritability, trouble concentrating) for many people.

These protocols are preferences, not requirements

There's no universally "ideal" protocol. Some people feel better with a wide window and several small meals; others prefer a more concentrated format. The right protocol is the one you can sustain long-term without sacrificing your quality of life, your training energy, or your protein targets. Nothing obliges you to progress toward a more extreme format -- staying on 16/8 long-term is perfectly valid, as covered in our intermittent fasting beginner's guide.

If you do consider a more restrictive format like 18/6 or OMAD, favor a gradual transition over an abrupt change, and monitor how your body responds over several weeks before deciding whether the format suits you.

5:2: a different relationship with restriction

5:2 works differently from daily protocols: it concentrates restriction into just two non-consecutive days, with near-normal eating the rest of the week. For some people, this approach is easier socially than a daily constraint. For others, the very-low-calorie days are hard to sustain. As with the other protocols, this isn't an inherently "healthier" format -- just a different distribution of the same weekly restriction. You can estimate your calorie needs with our calorie deficit calculator before spreading your intake across the week.

When to see a doctor

The most extreme protocols -- OMAD, extended fasts, very narrow windows -- are not appropriate for everyone and should never be attempted without professional guidance in the following situations:

• A history of disordered eating (anorexia, bulimia, binge eating): these protocols can reactivate dangerous restriction patterns.
• Pregnancy or breastfeeding: nutrient needs are continuous and don't fit a restricted eating window.
• Diabetes or any condition requiring regular food or medication timing: skipping meals can dangerously disrupt blood sugar or treatment effectiveness.
• Being underweight: concentrating your intake into a very short window makes it harder to reach a sufficient calorie intake.

In any of these cases, consult a doctor or dietitian before trying OMAD or extended fasting. This article describes common protocols; it is not personalized medical advice.

How Calerys helps

Whatever protocol you choose, the challenge is the same: covering your needs within a sometimes-reduced window. With Calerys, a WhatsApp message like "salmon 150g spinach eggs" instantly gives you calories, protein, and fiber, so you can quickly check that your single OMAD meal or your two 18/6 meals actually cover your day's needs.

Conclusion

16/8, 18/6, OMAD, and 5:2 are four different ways to distribute the same constraint: your total calorie intake. None is inherently better, and the most extreme formats aren't right for everyone -- nor are they necessary for progress. Pick a protocol you can sustain calmly, and get medical advice before trying OMAD or extended fasting if you're in an at-risk situation.