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The Ultimate Guide to Intermittent Fasting

Last updated: Oct 24, 2020 at 9:19AM | - Published on: Nov 1, 2018

Written by Sarah Neidler, PhD

Scientifically Reviewed by Raphael Sirtoli

What is intermittent fasting?

Fasting can often be a little intimidating to people, especially if you’ve never tried it before. But it doesn’t have to be daunting. Once you get the hang of it and learn a thing or two about it, you’ll be less worried and your body becomes accustomed to going extended periods without food. It actually starts to actually feel good!

So what exactly is intermittent fasting (IF)? It’s pretty simple — it refers to the idea of rearranging your eating schedule to allow your body to go anywhere from 12 to 16 hours without food throughout the 24 hour day.

A typical person usually consumes 3 meals per day — breakfast around 8-9am, lunch around 12-1pm, and dinner between 5-6pm. But when intermittent fasting, you eliminate regimented meal times and extend your “fasting window” between meals. Typically, a person who fasts will break their overnight fast around midday and then have a late dinner around 7-8pm. Alternatively, there is also the option of having an early breakfast and a late lunch, then fasting until the next morning.

Whether you eat earlier or later, the fasting window is generally about 14-16 hours — roughly three times longer than those who don’t fast. So what does this mean for health and performance? Does when you eat really matter just as much as what you eat? Let’s find out!

Nutrita and Intermittent fasting

Whatever diet or health goal you select with Nutrita, we aim to normalize your appetite and body composition. This means that, eventually, you’ll default to approximately 2 meals a day.

2 meals a day isn’t a magic number, it simply reflects an average of someone who eats when hungry and stops when full – unprompted by the clock or social pressures. Again, we’re talking about an average here, not a carved-in-stone rule.

Say you’ve been out hiking in the cold for many hours and feel like a third meal (or even a snack) – go for it! Are you pregnant? Well then obviously you don’t need to stick to a strict 2 meal a day eating schedule, go ahead and have more food.

Nutrita doesn’t pretend to anticipate your appetite better than your biology, a claim made by too many in the world of nutrition. All we do is provide the conditions for your biology to normalize its dysregulated appetite by helping you make better and better food choices.

The science behind it

Intermittent fasting isn’t a new thing. From an evolutionary standpoint, many people think we evolved the ability to fast or fast intermittently because we were bad hunter gatherers failing to find enough food – that’s not true. Quite the opposite actually!

We evolved to fast because the kind of food our unique hunting and gathering abilities allowed us to access was very high in calories and nutrients, like fatty ungulates – think mammoths or pigs. This meant our meals could keep us fully and nourished for longer.

More recently, as intermittent fasting has become increasingly popular, the number of studies revealing benefits have also increased.

There is 3 main benefits of intermittent fasting. It allow autophagy and clean mitochondria, accelerate weight loss and improve longevity.

How exactly does intermittent fasting work? We’re going to clear that up for you.

Your body operates a in a different way when you’re “feasting” than when you’re “fasting”. After you eat, your body takes some time to process what’s been ingested. Because there is a readily available energy source from your meal, your body will use that for fuel.

But when you’re in a fasted state, your body must use its energy stores. Where might they be?

You will first use up your liver glycogen (stored glucose) before you ramp up fat use from your stored body fat. When following a ketogenic diet specifically, liver glycogen stores are lower than when on a standard diet. This means that the only plentiful energy reserves remaining are from your body fat.

This ‘forced’ reliance on body fat is characteristic of intermittent fasting and keto.

Additionally, when we’re not taking in calories a specific period of time — that is, our fasting period — we force our bodies to not only turn to non-carb fuel sources, but also to ‘clean out’ the body — i.e. get rid of old, damaged or worn-out cells and structures. We can recycle some of these parts and make room for new, more youthful ones — a process called autophagy for the small parts and autolytic cannibalism for the bigger ones (like whole cells).

Intermittent fasting has also been touted for the long list of benefits it offers. From fat loss and increased mental clarity, to autophagy and improved insulin sensitivity, it’s no wonder intermittent fasting is taking the health world by storm.

But if you’re wondering just what exactly intermittent fasting can do for you and how it does it, keep reading — we’ll cover more benefits.

But like many other health care regimens, intermittent fasting doesn’t have to drastically change your life. It can be easily adapted to fit seamlessly into your lifestyle.

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Is there only one way to intermittent fast? Of course not.

Truth is, there are actually several ways of fasting, and the method you choose should (a) be achievable for you and (b) have good science behind.

The 16/8 Method — Fasting for 16 hours each day, this method restricts the eating window to a period of 8 hours. For some, however, the fasting window can range from 16-20 hours, further restricting the eating window.

Alternate Day Fasting — Involves rotating between eating normally and fasting. For example, eat normally Monday, Wednesday, Friday, and Sunday, and fast Tuesday, Thursday, and Saturday.

The 5:2 Method — Involves eating normally five days of the week and restricting calories to 500-600 the other two days.

24-Hours or OMAD — The 24-hour fasting or One Meal A DAY method is simply going 24-hours over a chosen period without food. For example, eating dinner at 7pm and not eating again until 7pm the following day.

Like most things in life, there are better and worse ways to do it. There’s also quite a bit of margin for you to experiment and see what precise pattern you feel best with. It’s important to listen to your body.

Never push yourself to a point where you feel as though something isn’t right – you can always restart the fast in a few days. If in doubt, start with smaller windows and gradually increase your fasting period as your body adjusts.

So what do we at Nutrita recommend? We suggest aiming for 2 meals a day on average.

This automatically sets you up for a 16/8 type of intermittent fasting. Some days you might feel satisfied with one meal, whilst on others you might think a few more bites of bacon with a handful of macadamia nuts will hit the stop.

We don’t recommend the 5:2 Method because we don’t want people to associate food with discipline. We don’t do that with drinking water after all, so why is food any different? We encourage people to eat when hungry until and stop when full.

This works when we fix our biology with smart food choices, not by exerting our limited will power.

Did you know?

A case report from 1973 tells us that a 27 year old morbidly obese Scottish man weighing 207 kg (456 lbs) fasted for 382 days [1]!

What’s maybe even more amazing is that “prolonged fasting in this patient had no ill-effects” — not exactly though outcome people would expect.

While this example is extreme, it doesn’t tell us that, with respect to food, less is more. It does, however, reinforce the idea that humans are well adapted to go long periods without food. From this we can safely say that not only is intermittent fasting safe, it’s also the norm!

10 benefits of intermittent fasting

It doesn’t seem like going for long periods without food would be beneficial for your body, but let us tell you, there’s actually quite a long list of them. Here’s a rundown of some of the benefits of intermittent fasting on keto:

Accelerates fat loss. Not only are you likely naturally consuming fewer calories due to a restricted eating window, but you’re also burning stored body fat for fuel.

Improves glucose uptake efficiency and insulin sensitivity. Intermittent fasting helps to increase insulin mediated glucose uptake rates into tissues that can use it for fuel (e.g. muscle). It also helps to decrease/normalize biomarkers (insulin and glucose) associated with chronic diseases. [2]

Normalizes/decreases appetite. When running off of your main energy store, body fat, rather than incoming sugary foods, your body is using a stable and well regulated supply of energy [3]. Intermittent fasting often helps people eat less in the evening, too. This is time of day when our metabolic response to food tends to worse compared to when it’s still light out [4]. It may therefore be healthier to increase food consumption during the day when our bodies are better able to metabolize and use energy.

Culls gut microbiota. Although no one knows what a ‘healthy microbiota’ looks like, fasting initially culls our gut flora, reduces excess gut permeability and increases overall diversity (somewhat counterintuitively maybe) [5]. It’s analogous to a gardner removing pesky weeds to encourage the growth of their desired flowers.

Improves brain function. IF increases BDNF (brain-derived neurotrophic factor), a protein that’s involved in learning and memory, helping us make certain brain pathways faster and more efficient [6,7].

Recycles faulty mitochondria. Think of mitochondria as the powerhouse of your cells that unavoidably incur damage over-time, becoming less efficient energy producers. During a fasted state when ATP (energy) levels decrease, the body is stimulated to create more and better mitochondria. This means that when food is available it will be ‘burned more cleanly’ [8].

Stimulates autophagy. If you’ve never heard of the term autophagy, it’s essentially our bodies way of housekeeping. A lot of our basic cellular processes create waste that has to either be excreted from our bodies or recycled to improve the efficiency and function of our various organelles [9]. Autophagy is part of that sorting process and is alway ongoing, but strongly upregulated when fasting.

Reduces/normalizes oxidative stress and inflammation. Intermittent fasting may enhance the body’s resistance to oxidative stress and normalize the amount of damaging free radicals our mitochondria face. It also helps to reduce levels of pro-inflammatory cytokines and stimulate anti-inflammatory pathways which are too often understimulated [9,10].

Provides mental clarity. Fat is the most energy-efficient and energy-dense fuel available to our body – making our brains particularly happy! When our small glucose stores are low (they’re never empty) the brain is able to function at its best given its particularly fuel requirements. This leaves us mentally more clear and focused [11].

Improves fitness. Working out in a fasted state can have many positive effects on the body, such as stronger metabolic adaptations (increased training stimulus), higher sensitivity to growth factors (think muscle synthesis) and improved metabolic responses to post-workout meals, crucial for fast recovery [12, 13]. Just a note of caution: we’re talking about health here, not world-class athletic performance. So although fasted training certainly has a place, even amongst high level athletes, this doesn’t mean it’s optimal during all periods of the training (or competitive) cycle.

Intermittent fasting has a lot of positive effect of health. It lower blood sugar level, contribute to healthy brain function, accelerate weight loss and much more…

What about fasting and ketone levels?

One of the great things about intermittent fasting is that it helps us get into the keto sweet spot that much quicker.

But first, let’s refresh your mind on one of the basics of the keto diet — what are ketones?

Following a standard american diet, the body breaks down its major food source (carbs) into single sugar units called glucose which then use to run our body on. When you’re following a ketogenic diet, however, the amount of glucose your body receives is very limited, so in order for the body to function, it must rely on another source for energy — fat.

When you rely on lots of fat compared to glucose, some of that fat gets turned into ketones. These ketones are a large part of your alternate fuel supply on a keto diet or when fasting. The rest comes mainly from fatty acids, the breakdown products of triglycerides – what you’d call ‘fat’ when talking about fatty food.

So why combine a ketogenic diet with intermittent fasting? While fasting alone is great and provides many health benefits, it might not be enough to get your body to produce a desired level ketones if you’re refeeding on carb-heavy foods. When you follow a ketogenic diet, you’re restricting glucose intake that much more, allowing your body to get into ketosis much easier than if you were just following a standard diet and intermittent fasting.

When we follow a keto diet and naturally fall into an intermittent fasting pattern, believe it or not, fasting is actually easier! Our bodies are already turning fat into ketones for fuel.

So rather than switching between using lots glucose and then back to lots of fat, the body can rely on one main source of energy (fat/ketones) and flexibly use the other secondary source of energy (glucose) as the need arises [14].

This eliminates those unstable energy levels (think blood sugar spikes and dips). It also avoids imbalanced hunger hormones, and with those, the need to eat every few hours.

If you’re looking for more information on how to find out your ketone levels, refer to our article 3 Ways to Measure Ketones.

Will intermittent fasting burn muscle?

If you’re into any sort of fitness regimen, going an extended period of time without food may be a little nerve wracking. But contrary to popular belief, intermittent fasting has actually been shown to be beneficial for fitness. How?

Rather than looking at what you eat, intermittent fasting focuses on when you eat — something that is crucial to maintaining muscle mass and losing fat.

Several studies have shown that exercising in a fasted state is highly beneficial for fat loss. This is because it has a positive impact on your sympathetic nervous system (SNS) and increases your capacity to burn fat.

Amongst other things, the sympathetic nervous system is activated by two things: exercise and lack of food. When your tank is on empty and you exercise, you activate the SNS and turn on your body’s fat burning processes, which are controlled by the SNS.

Fasting also helps to increase the release of human growth hormone (HGH), or the hormone needed to induce muscle synthesis [15, 16]. So when fasting and exercise are combined, it maximizes the impact of fat and glycogen breakdown for energy.

But will you lose muscle? The answer is likely no. How much muscle you will build (or fail to build) has more to do with (a) amount and quality of your protein, (b) if you resistance train enough and (c) your sleep hygiene.

Studies have shown that exercise in both a fasted and non-fasted states yield much of the same results — minimal changes in both body mass and composition following aerobic exercise, specifically [17]. Another study showed that, in comparison to daily calorie restriction, intermittent calorie restriction (in the form of intermittent fasting) may be more beneficial for retaining lean body mass [18] .

And don’t worry about eating as soon as you’ve stopped exercising. The myth of having to eat within 30 minutes after you’ve exercised has been sold to us by…food and supplement companies. Sure, if you don’t eat for a day or two after exercising, that’s not optimal for muscle growth. So again, just eat when you’re hungry – which you very well should be after going for a run, lifting weights or swimming.

Is intermittent fasting dangerous?

For most people, intermittent fasting is a great way to improve your overall health and increase longevity without many side effects. But like any other health journey, it’s crucial you do your own research and talk with a medical professional before embarking on any serious dietary or lifestyle changes.

By forming your own opinion, you not only gain some knowledge and understanding of how to reach your goals safely and effectively, but you can also ask smart questions to people who know more than you and benefit from their wisdom.

It’s key to acknowledge and remember, however, that while intermittent fasting may be great for the vast majority of people nearly all the time, it isn’t for everyone at all times. There are certain populations where the risks associated with fasting may outweigh the benefits. Can you guess who?

  • If you’re pregnant
  • If you’re already undernourished (calories and/or nutrients)
  • If you’re an uncontrolled type 1 diabetic (we’ll cover this more below)
  • If your exercise load is unusually high (and already a departure from a healthy lifestyle)

Is intermittent fasting good for diabetics?

As previously mentioned, caution should be taken when trying intermittent fasting if you suffer with specific health conditions — type 1 diabetes included. That’s not to say, however, that people with diabetes cannot fast because it is a great tool. It simply means, let your doctor know and have your meds adjusted by a health professional accordingly.

While more studies must be conducted on human subjects, many studies on rodents have yielded positive results and promising results for humans. Here are some ways intermittent fasting may help you if you have diabetes: [19, 20, 21, 22].

  1. Improve fasting glucose levels
  2. Improve body fat percentage
  3. Improve post-meal blood sugar levels
  4. Improve circulating lipid levels
  5. Increase insulin sensitivity
  6. Improve mitochondrial function
  7. Decrease inflammation
  8. Reduce leptin levels (making you less ravenous)
  9. Improved insulin production from the pancreas

Studies have shown intermittent fasting for patients with type 2 diabetes to have a positive effect on insulin resistance — the main concerns for diabetes. Cells become insulin resistant when they’re forcibly stuffed with glucose and fatty acids, but periods of fasting allow for this extra glucose and fat to be burned off in due time. The result? Cells once again become responsive to insulin [23].

Like many other diseases, diabetes responds extremely well to an intermittent fasting way of eating. But because diabetics are given terrible nutritional advice and are on multiple medications, it’s important to stand on the side of caution and consult your medical professional before experimenting willy nilly with intermittent fasting.

Talk and listen to your doc, but don’t take what they have to say as gospel. Especially when it comes to ‘the average doc’ and their knowledge of diabetes. Consult doctors for seconds opinions, especially those who can prove they’ve read the up to date scientific literature (no, official guidelines don’t count).

At Nutrita we pride ourselves on quality information; we produce it because we do our best to follow the science, not the trail of diplomas on someone’s wall.

Let us help you: intermittent fasting keto meal plan sample

With intermittent fasting, it’s all about timing. But that’s not to say there’s only one time period you should eat and one time period you should fast.

If you’re following the standard 16:8 fasting plan, you’ll generally break your fast around noon or early afternoon. Here, we want our meals to be as nutrient-dense as we possibly can. To help you understand eating patterns and times, here’s a sample of what an intermittent fasting day may look like on a 12pm-8pm eating window.

12pm: Grass-fed beef burgers

Grass Fed Organic Raw ground beef with spices

Extremely easy to prep ahead, grass-fed burgers are a simple but delicious go-to option for breaking your fast. Enjoy on top of a bed of dark leafy greens with a simple homemade dressing like olive oil and apple cider vinegar for a tasty meal packed with a complete range of amino and fatty acids. To add a little extra fat and flavour, top your burgers with avocado and a homemade aioli (roasted garlic is great!).


  • ½ pound ground grass-fed beef liver
  • ½ pound ground grass-fed beef
  • ½ teaspoon garlic powder
  • ½ teaspoon cumin powder
  • Sea salt and pepper to taste
  • Olive oil cooking oil
  • Cooking oil: butter


  1. Mix together all ingredients together in a bowl and form together desired-size patties.
  2. Heat butter over skillet on medium-high heat.
  3. Cook burgers in skillet until desired doneness.
  4. Store in a container in the fridge and use within 4 days.

Note: you can substitute beef for any other type of ground meat that you prefer (chicken, turkey, bison, lamb, etc.)

7:00pm: Salmon with Roasted Vegetables

Grilled Salmon Fish Fillet With Vegetables Mix. Top View On Dark

Seafood is a fantastic source of omega-3 fats and is packed with nutrients. Serve alongside some of your favorite vegetables if you like them**, but don’t worry if you don’t. If you do, try roasting them in coconut or avocado oil and add your favourite spices for a quick and delicious dinner.


  • 1 pound salmon (or other fish of choice like rainbow trout)
  • 2 tablespoons fresh lemon juice
  • 2 tablespoons ghee, coconut, or avocado oil
  • 4 cloves garlic, finely diced**
  • 1-2 cups of vegetables per person (broccoli, cauliflower, brussel sprouts, zucchini, peppers, asparagus — whatever you prefer)**


  1. Preheat oven to 400°F.
  2. Mix together lemon juice, ghee (or fat of choice), and garlic.
  3. Place salmon in foil and pour lemon and ghee mixture over the top.
  4. Wrap salmon with the foil and place on a baking sheet.
  5. Bake for 15 minutes or until salmon is cooked through.
  6. If your oven size allows, you can roast your vegetables alongside salmon on a separate baking sheet.

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Written by Sarah Neidler, PhD

Sarah Neidler did her PhD in cancer research at the Beatson Institute for Cancer Research in Glasgow. She has a strong interest in nutrition and the ketogenic diet and believes that they are beneficial for the treatment and prevention of chronic conditions, such as heart disease and diabetes. She loves cooking, reading, sewing, Yoga, and CrossFit.

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