Popular Conditions & Diets

The Rily guide to intermittent fasting

What really happens when you stop eating? How to fast the right way, and why it's all worth knowing about.

Diet feature

December 7, 2025

Contrary to the hype surrounding wellness culture, fasting isn't a fad - it's a natural metabolic pace that humans have practiced and evolved with for hundreds of thousands of years. For the overwhelming majority of our species' history, before we had fridges, supermarkets, delivery apps, and sneaky midnight snacks, our ancestors cycled between feasting and fasting. It wasn't some biohacking 'protocol,' it was just life.

In the last second of the evolutionary clock, however, modern living has broken that rhythm. The truly miraculous ability to eat almost constantly from morning to midnight, and through every season, means that time in a fasted state is the exception instead of the norm. The result of this normalized constant consumption is insulin resistance, constant cycles of hunger, and chronic low-grade inflammation. And all of it quietly chips away at our short-term well-being and long-term health.

Intermittent fasting might just offer us a chance to step back in time and reclaim some of the benefits of not eating that our predecessors were so familiar with. So if you've ever been curious about whether it's actually healthy, or what happens inside your body after 4, 12, or 20 hours without food, this guide will break it all down clearly.

By the end, you'll be clear on exactly how fasting works, which schedule will suit you best, how to get started, and how to fully reap the benefits- without burning out.

In this article, you'll learn:

What intermittent fasting is (and what it's not)
What happens when you fast, hour by hour
The real, proven benefits
Popular approaches to intermittent fasting
Who should not fast
How to begin, step by step
What breaks a fast
And, we'll answer some frequently asked fasting questions

Two Bowls of Veggie Packed Tofu Scramble — Veggie Packed Tofu Scramble (left) | Anti-Inflammatory Smoothie Bowl (right)

Veggie Packed Tofu Scramble (left) | Anti-Inflammatory Smoothie Bowl (right)

So, what is intermittent fasting, exactly?

Intermittent fasting ("IF") is unique in the world of diet culture for emphasizing what you eat less and when you eat it more. In a nutshell, it structures your day or week into periods of eating and periods of not eating.

There are two main categories:

Time-restricted feeding (TRF): This limits the 'feeding window' to a predetermined range. The most common form of TRF involves 16 hours of fasting followed by an 8-hour feeding window; however, there are numerous other variations.
Pattern-based fasting: This focuses less on dividing each day and more on the week as a whole. Examples include five days of eating and two days of significant caloric restriction, aka "5:2", and alternate-day fasting, which is exactly what it sounds like-fasting every other day.

We'll look more closely at these fasting methods shortly. For now, let's clear up a few common misconceptions:

Intermittent fasting is not starvation. It should be a structured, sustainable, and comfortable way to incorporate fasting into your lifestyle without making your lifestyle all about fasting.

Seasoned Ribs — Thai Style Pork Ribs (left) | Prosciutto Wrapped Asparagus Salad (right)

prosciutto salad1.jpg — Thai Style Pork Ribs (left) | Prosciutto Wrapped Asparagus Salad (right)

The fasting timeline: what happens in the body, hour by hour

The best way to understand how intermittent fasting actually works, and why it can be so beneficial, is to look at how your body's metabolism changes as you progress further from eating, and more deeply into a fasted state.

0-4 hours after eating: the fed state After a meal, your insulin naturally rises, working like a key in a lock to let the fresh supply of blood glucose flow into cells. Your body is focused on digesting food, storing nutrients, and building tissues (also known as anabolic growth). Excess energy is stored as glycogen for later use. Protein synthesis is at its peak, and with plenty of carbs to burn, ketones are basically at zero. A meal especially high in carbohydrates can extend the fed phase for 6-8 hours and delay the metabolic switch that follows.

4-12 hours after eating: the transition phase When blood glucose is removed from the blood and the cells are nourished and fueled, insulin levels decrease. The body begins to shift away from storage mode and into usage mode: liver glycogen starts to be broken down, and gluconeogenesis (the creation of new glucose from non-carbohydrate sources like amino acids and glycerol) increases gradually. At this point, ketones start to appear at trace levels. This is where we generally feel hunger, which is often as psychological as it is physiological. It's common to sit down to lunch a few hours after breakfast and say "I'm starving," but you are, most assuredly, not!

Broccoli Frittata Cups — Broccoli Chickpea Frittata Cups (left) | "Fried" Chicken Biscuit Sandwich (right)

Fried Chicken Biscuit Sandwich — Broccoli Chickpea Frittata Cups (left) | "Fried" Chicken Biscuit Sandwich (right)

Common approaches to intermittent fasting

When it comes to IF, there's no one best method: if anything, the best approach to IF for you will be the one that suits your biology and lifestyle. What you do not want to do is try to make your lifestyle conform to a fasting schedule!

The following have been well established in scientific literature and popular culture alike:

**16 / 8 ** The single most beloved way to fast intermittently, 16/8, as the name suggests, involves fasting for 16 hours and eating all meals within an 8-hour feeding window, typically between 12 and 8 p.m. Since half the fasting time will be spent asleep, this approach can be as simple as skipping breakfast, allowing for shared social lunches and dinners. This schedule has also been scientifically validated to lower blood pressure, naturally reduce caloric intake, and help with weight loss; all without ever counting calories.

20 / 4 This more restrictive approach, also known as "," as popularized by Ori Hofmekler, offers even more time in ketosis and theoretically provides slightly more of all the benefits of fasting. However, it may not be easy to fit into real life, and it is generally less easy to sustain. It typically involves one large meal and plenty of snacking, or two meals at either end of the four-hour feeding window. There is evidence that the Warrior Diet can indeed .

Gluten-Free Air Fryer Orange Chicken (left) | Thai Green Curry with Crispy Cod (right)

Gluten-Free Air Fryer Orange Chicken (left) | Thai Green Curry with Crispy Cod (right)

Mistakes people make when intermittent fasting

Any diet or eating pattern will have common pitfalls. If you're an existing or aspiring IF'er, here's what to watch out for:

Micronutrient-poor food inside the eating window. Intermittent fasting should not be seen as a licence to eat 'whatever' when you're not fasting. Moreover, high glycemic diets will make the fast itself much less pleasant. Low-carb diets and IF go together naturally: the common theme is an easy, gentle insulin arc, instead of a rollercoaster.
Treating fasting as punishment for indulgence. Over time, this can potentially result in an unhealthy relationship with food in general, even for those without a history of disordered eating. If you see that IF is leading to a cycle of bingeing and deprivation, it is likely a big red flag that it's not a good fit.
Not eating enough during the feeding window. Your body still needs calories, and 16/8 fasting (for example) naturally leads to a reduction of caloric intake. You may get so comfortable with this new arrangement that you actually under-eat during the feeding window.

Chocolate Peanut Butter Chia Pudding (left) | One Pot Pasta with Leek, Bacon & Peas (right)

Chocolate Peanut Butter Chia Pudding (left) | One Pot Pasta with Leek, Bacon & Peas (right)

The bottom line: is intermittent fasting a healthy choice?

Remember, even a popular IF pattern like 16/8 is by no means a cookie-cutter solution. Finding the exact strategy that suits you best may take some trial and error, and it might lead you to the discovery that IF, generally, isn't for you.

However, after noting the critical caveat that not everyone will enjoy or benefit from intermittent fasting, we can say with considerable confidence that intermittent fasting is generally healthy and effective.

This is not because it's magical, or because it 'hacks' the body, but because it realigns us with a metabolic pattern our bodies have evolved to recognize. The evidence appears strongest for cardiovascular health, reducing inflammation, promoting weight loss, and improving metabolic flexibility.

In a sense, intermittent fasting should be thought of not so much as a trend as a return to the type of feast-and-famine rhythm humans have followed for the vast majority of our history. When adapted to real life and approached with care and intention, it can be a surprisingly comfortable and sustainable way to balance health and freedom.

Almabruk, B. A., Alharbi, S. H., Alsaqer, F. S., Al Essa, A., Eid, H., Alqahtani, O., Badawood, M. A., Alzahrani, E. M., Alzahrani, E. M., Alshaikh, F. K., Alfaraj, R. M., Alarqan, H. H., Aljuaid, R., Al Sharari, A., & Alghamdi, M. A. (2024). The role of intermittent fasting on Metabolic syndrome: A systematic review and meta-analysis. Cureus. https://doi.org/10.7759/cureus.71623

Cienfuegos, S., Corapi, S., Gabel, K., Ezpeleta, M., Kalam, F., Lin, S., Pavlou, V., & Varady, K. A. (2022). Effect of intermittent fasting on reproductive hormone levels in females and males: A review of human trials. Nutrients, 14(11), 2343. https://doi.org/10.3390/nu14112343