Intermittent fasting: the definitive guide

by | Diet Style

Intermittent fasting guide

The intermittent fasting craze has taken the fitness and nutrition world by storm.

It is definitely a valid strategy for weight loss, but is it better than others?

How many hours of fasting can be done?

I will summarise the main scientific evidence and key findings for you to review so that you can make an appropriate and informed decision regarding fat loss and health improvement.

Let’s remember that in the science of nutrition, everything depends on many, many factors. So what works for some may not work for others. For that reason, and regardless of what you believe about this particular topic, I suggest that you keep an open mind when reading this Guide on Intermittent Fasting.

3 strategies for fat loss

When it comes to nutritional interventions and fat loss, there are three strategies that are often used, sometimes deliberately and sometimes unconsciously:

  1. Dietary restriction
  2. Calorie restriction
  3. Restricting timing intake.

Each factor can be manipulated independently of the others to optimise the desired results.
Any diet or nutritional approach you have heard of employs one or more of these strategies.

Dietary restriction means selecting what you eat or do not eat. For example, avoiding red meat or dairy.

Calorie restriction means limiting the amount you eat (in calorie terms). This strategy is usually the most popular and is what most people mean by “dieting” when it comes to weight loss.

Restricting the timing of intake refers to when to eat and when not to eat.

Talking about intermittent fasting and similar interventions is confusing because most people, and often many practitioners, do not know exactly what the term refers to.

Intermittent fasting (intake restriction over time) falls into the latter category.

What is fasting?

The first important distinction is that fasting is not synonymous with starvation.
Fasting is the voluntary and controlled abstinence from consuming food.

This prescribed abstinence can be for different reasons. Originally for spiritual reasons, you have probably also heard of fasting as a political protest (Mahatma Gandhi went on hunger strikes for more than 3 weeks for social justice). 

Today IF is a very popular strategy for weight and fat loss and to improve overall health.
When we refer to fasting, we usually mean fasts where it is okay to consume water, or even teas, but not foods or beverages that provide calories, sweeteners or the like.
And when we talk specifically about intermittent fasting, we are actually talking about eating a time-restricted diet on a regular basis in order to improve health.

4 Intermittent fasting protocols

There are 4 protocols that are the most popular when it comes to intermittent fasting:

The first is alternate day fasting (ADF). 

It alternates between a day of eating and a day of fasting.
Within the fasting days, you can eat nothing or only a few calories; usually 1000 or 500 calories allowed.
On non-fasting days, you can eat ad libitum, i.e. without restriction.

The second is the 5:2 method. 

In this protocol, you eat for five days a week and fast for two. Generally, these two days are consecutive.

The third protocol is to fast for at least 24 hours (with some frequency). 

This could mean, for example, doing a 72-hour fast once every three months, or a 5-day fast once a year.
A true extended fast like this would only include water, tea and minerals with zero calories.

The fourth protocol is time-restricted feeding (TRF).

This approach is also the most widespread and has the most proponents.
Time-restricted feeding is when you limit the hours during which you eat each day. In other words, there is a feeding window.
Most people carry out this approach in their daily lives, whether they know it or not. 
They normally stick to approximately an 8/16 window, which means they will eat for 16 hours and fast for 8 hours, while sleeping, a little before and a little after.
But for it to truly be intermittent fasting, proponents of this approach stress that more hours of the day must be spent without food.
The most popular protocol is 16/8 (16 hours fasting and 8 hours intake) but it can also be 18/6 (18 hours fasting and 6 hours intake) or even 20/4 (20 hours fasting and 4 hours intake).

Would you want to know which approach is right for you or your patients?
Get my IF assessment calculator

Fasting for weight and fat loss

Now we know what Intermittent Fasting means and we explained its different forms. But the most important question to answer is: Does intermittent fasting work for fat loss?

Is it more effective than other approaches (diets)?

The key to understanding how to manipulate body composition is to know what will cause us to lose (or gain) mass.
Weight loss comes down to energy balance: calories in vs. calories out.
If you take in more calories than you “burn” or expend, you will gain weight. And if you take in fewer calories than you “burn” or expend, you will lose weight.
Over the years, dietary interventions for weight loss have focused primarily on continuous energy restriction (CER). 
However, relatively recently, studies on fasting for body composition manipulation started to be published.

Scientific studies say so…

A recent review by Rynders and colleagues, published in 2019 (1), concluded that, based on the data available so far, “intermittent fasting does not appear to produce greater weight loss than continuous energy restriction (CER)”. That is, as long as the deficit achieved is similar between the two approaches.

But let’s not forget that, “adherence to weight loss interventions is the greatest predictor of success, and some people may respond better to continuous calorie restriction; while others will respond more favourably to intermittent fasting or time-restricted eating”. 

In other words, intermittent fasting is another calorie restriction strategy that can be more adherent (or not) and therefore achieve the goal of fat loss more effectively and efficiently.

A meta-analysis and systematic review also published in 2019 (2) concluded that intermittent fasting improves glycemic control and insulin resistance with a reduction in BMI, a decrease in leptin level and an increase in adiponectin concentration, which are generally considered favorable changes for most people.

To keep it simple, what they found was that this approach works as long as a calorie deficit is created and the person sticks to the plan.

A systematic review (3) covering forty studies on the subject found that when Calorie Matching, although intermittent fasting may provide several physiological and psychological health benefits, it does not provide any additional benefit when it comes to fat loss compared to continuous energy restriction (CER).

At the end of the day, as I mentioned earlier, it is the calorie deficit that produces the weight loss when fasting, not the act of fasting itself.

Intermittent Fasting is not for everyone

This approach is discouraged for people who currently have an eating disorder or have a history of an eating disorder.
This type of restriction may stimulate the onset of the eating disorder, or if the eating disorder is in a stage of stagnation or remission, it may cause some of the destructive thoughts and behaviours to return.
It is also contraindicated in populations with malnutrition or deficiencies of certain nutrients, as well as for athletes practising certain sports where large amounts of energy are required continuously, type 1 diabetics, pregnant or lactating women, and people who require food to take their medication.
Finally, it is not recommended for children under the age of 18 or over the age of 70.

So what is the recommendation?

Evidence-based medicine aims to make treatment decisions based on the best available evidence in the scientific literature. It is great to address issues such as infectious diseases because infections are acute and complications are generally simple to treat, usually with an antibiotic or antiviral. All these characteristics make the measurement of variables relatively straightforward.

The problem, however, is that most health-related treatments in the real world are much more complicated, especially in nutrition.

Intermittent fasting works. But if you are looking to lose fat and improve body composition, it is not magical or superior to other approaches.

Using this protocol will depend on preferences, tastes, schedules and other factors that will vary from person to person.

Do you want to know if intermittent IF is for you? And if so, which protocol is right for you?
If your answer is YES, CLICK HERE

If you read this Intermittent Fasting Guide you should now be able to assess the pros and cons of this strategy and decide whether it is worth giving it a try. At the end of the day the correct answer is usually found after personal experimentation.


1. Effectiveness of Intermittent Fasting and Time-Restricted Feeding Compared to Continuous Energy Restriction for Weight Loss

The Effectiveness of Intermittent Fasting to Reduce Body Mass Index and Glucose Metabolism: A Systematic Review and Meta-Analysis.

3. Do intermittent diets provide physiological benefits over continuous diets for weight loss? A systematic review of clinical trials

About Me

Hi, I’m Nico. I’m a Registered Nutritionist and trained Dietitian. I can personally help you transform your short-term health and fitness goals into life-changing results. Whatever your goal is, I will teach you and give you evidence-based tools for you to work towards it and maintain those results in time.

Calculate your Macros

Start your health journey with my FREE Macro Calculator. Complete the QUIZ and receive a personalized NUTRITION GUIDE.


Ready to start? Learn more about my services and how we can work together.
If you have Health Insurance you may be eligible for cover.


If you want to start making some changes on your own, check out my tools, recipes, gym routines and moreAll evidence-based.