intermittent fasting diet explained 2019

Delicious avo on toast. Something you might break your fast with. Photo: Envato Elements

Intermittent Fasting: What you need to know

Intermittent fasting is the latest diet to claim the spotlight in the fitness world. What is it, how does it work and should you do it

intermittent fasting diet explained 2019

Delicious avo on toast. Something you might break your fast with. Photo: Envato Elements

Note: The information below does not constitute medical advice. And, should you wish to make a drastic lifestyle change, it is vital to first discuss such decisions with an accredited and trusted medical professional.

Paleo, Keto, Atkins, South Beach, The Zone and now Intermittent Fasting. All these diets that have been touted as the next best thing in health and fitness.

We took a look at the research surrounding this method of eating to find out the basics, benefits and methods around this newly popular diet.

What is Intermittent Fasting?

Precision Nutrition defines Intermittent fasting (IF) as the practice of occasionally going for extended periods without eating.

IF has been around for ages within the health and fitness world. It initially became a popular topic of conversation in 2012 by BBC broadcast journalist Dr Michael Mosley’s TV documentary Eat Fast, Live Longer and book The Fast Diet.

As interest has piqued again within the fitspo world, IF is back in the spotlight.

Firstly, we all do some form of IF. We just call it to sleep. Although this seems like it is the latest diet craze, we humans are familiar with this dieting protocol.

We use fasting when we sleep. During times of food scarcity, humans have had to fast. Religious practices also have utilised fasting througout the years

Only recently has it become a dieting tool for those in the fitness world who are looking to keep in shape.

IF can be summarised as:

“The time from your last meal at night until your first meal the next day (assuming a typical sleep-wake cycle) makes up your “fasting” interval. And the time from your first meal of the day until your last meal makes up your “feeding” interval.”

Precision Nutrition

For example, if you eat supper at 19:00 and then breakfast at 7:00, you have fasted for 12 hours. Your “eating window” would also be 12 hours.

Why do Intermittent Fasting?

Harvard T.H. Chan School of Public Health says many people believe IF is a way to lose weight and reduce health markers associated with diseases such as blood pressure and cholesterol levels.

“Proponents of the diet believe that the stress of intermittent fasting causes an immune response that repairs cells and produces positive metabolic changes (reduction in triglycerides, LDL cholesterol, blood pressure, weight, fat mass, blood glucose).”

Harvard T.H. Chan School of Public Health

But here’s the thing about intermittent fasting research

There hasn’t been a lot of research done on humans however those done on animals have shown very strong supportive results.

However research and science are slowly catching up. Based on anecdotal evidence, those who follow the protocol swear by its claims for fat loss, mental clarity and improved sleep.

We’ve even written about it on these pages before with a writer sharing her first-hand experience of intermittent fasting.

“I follow a 16/8 fast every day. I’m semi-strict. Meaning, some days I will breakfast a bit earlier, other days I will fast much longer. Most days, I fast for at least 14 hours. I’ve fully adapted, having followed this lifestyle for almost a year. I train (both running and strength) in the mornings and always train fasted – with the help of a black coffee.”

Antoinette Muller

How to fast

Think of Intermittent Fasting as a lifestyle and not a diet.

IF appeals to those who already do some sort of fasting due to lifestyle habits and ingrained eating preferences. If you skip breakfast, prefer to eat later in the day or prefer bigger meals to smaller regular snacks throughout the day, this method may appeal to you.

Here are some of the more popular IF protocols.

Alternate Day Fast

On this plan, you would fast every other day. For example, Monday-Wednesday-Friday consists of fasting, while alternate days have no food restrictions.

You should make good, healthy food choices on the days you eat even though you can eat what you want without any restictions.

“The Every Other Day Diet” by Dr Krista Varady, is the most popular version of this fasting protocol.

Meal skipping

This is a random protocol based on our evolutionary background thus the rules are pretty flexible.

“As humans evolved to get their food and exercise randomly, so should we.”

Precision Nutrition

This protocol also follows other nutritional guidelines like eating a paleo type dieting (think evolutionary food). You would randomize calorie intake throughout the week and skip some meals throughout the day based on what suits you.

Eat stop eat

Created by Brad Pilon, this entails 24 hours of full fasting one to two times a week, based on the days of your choosing.

For example, you would eat breakfast on Monday, and don’t eat again until breakfast on Tuesday. If you prefer you could eat dinner on Wednesday, and then don’t eat again until dinner on Thursday.

Alternatively, the 5:2 diet approach advocates no food restriction five days of the week. On the other two days you would cycle with a 400-500 calorie diet.


This protocol is self-crowned as “The birthplace of Intermittent Fasting.”

Martin Berkhan created Leangains . This brand of time restricted feeding follows a daily set feeding and fasting window. This usually based around an 8/16 hour split.

This protocol has more rules than others such as specific nutrient timing strategies around exercise times as well as an emphasis on certain macronutrients above others.

Warrior Diet

This protocol is based on 20 hours of fasting and a four-hour eating window.

The Warrior Diet is based on the eating patterns of ancient warriors, who consumed little during the day and then feasted at night.

The Warrior Diet: Review and Beginner’s Guide

It was created in 2001 by Ori Hofmekler, a former member of the Israeli Special Forces, who transitioned into the field of fitness and nutrition.

The common practice is to set feeding windows towards the end of the day as it’s more convenient for family dinners and after-work training sessions.

However, modifications can be made based on individual and scheduling differences.

Individualisation and self-experimentation is key

Precision Nutrition, John Berardi, says that we are  about 5-7 years from knowing what exactly IF does in humans (and why), and a good 10-12 years from knowing which IF protocols are “best.”

“Because the research is so spotty, no one really knows which type of fasting is best for different goals – whether that’s fat loss, muscle preservation, disease prevention, or longevity. And because noone knows whether IF offers any additional physiological benefits at all, everyone is just guessing.”

Dr John Berardi

He experimented with various protocols and tracked his results to identify which worked best for his lifestyle.

He suggests you do the same.

Pitfalls and dangers of intermittent fasting

Harvard T.H. Chan School of Public Health cautions that there are various pitfalls. Individuals with the following conditions should abstain from intermittent fasting:

  • Diabetes
  • Eating disorders that involve unhealthy self-restriction (anorexia or bulimia nervosa)
  • Use of medications that require food intake
  • Active growth stage, such as in adolescents
  • Pregnancy, breastfeeding

There are few unanswered questions and uncertainty around whether IF is beneficial to weight loss.

“Although certain benefits of caloric restriction have been demonstrated in animal studies, similar benefits of intermittent fasting in humans have not been observed. It is unclear that intermittent fasting is superior to other weight loss methods in regards to the amount of weight loss, biological changes, compliance rates, and decreased appetite.”

Harvard T.H. Chan School of Public Health

The same unclarity surrounds the numerous health benefits that are reported in animal studies.

For now, given my propensity to getting super hangry, I am going to stick to a steady supply of snacks.

Research does show that once your body becomes adapted to this routine that hangry uncomfortable feeling will disappear or at least be easier to manage.

You aren’t starving

Monique Tello, Contributing Editor of the Harvard Health Blog who had previously lambasted the concept of IF here has looked at recent research and changed her stance.

New research is suggesting that not all IF approaches are the same, and some are actually very reasonable, effective, and sustainable, especially when combined with a nutritious plant-based diet. So I’m prepared to take my lumps on this one (and even revise my prior post).

Monique Tello

Her opinion changed based on a study done at University of Alabama. The study took a  small group of obese men with prediabetes and put them into two groups.

Group one  was participating in :

“early time-restricted feeding,” where all meals were fit into an early eight-hour period of the day (7 am to 3 pm). Group two had their meals spread out over 12 hours (between 7 am and 7 pm).

Harvard Health Blog

Although both groups did not lose weight, the eight-hours group had dramatically lower insulin levels. They had significantly improved insulin sensitivity, as well as significantly lower blood pressure.

The eight-hours group also had significantly decreased appetite.

The  Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy (CALERIE) study, which followed participants on calorie-restriction diet for 2 years, shows the same findings.

“Those in the calorie-restriction group had reduced risk factors (lower blood pressure and lower cholesterol) for age-related diseases such as diabetes, heart disease, and stroke. They also showed decreases in some inflammatory factors and thyroid hormones. There is some evidence that lower levels of these measures are associated with longer lifespan and diminished risk for age-related diseases. Moreover, in the calorie-restricted individuals, no adverse effects (and some favorable ones) were found on quality of life, mood, sexual function, and sleep.”

Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy (CALERIE)

Is it for you?

So it seems my hangry excuse is not a valid reason… At the end of the day what it comes down to is how does it fit into your lifestyle? Is it sustainable?

We know the best diet is one that you can be consistent with. Since there is insufficient evidence to recommend any type of calorie-restriction or fasting diet, the National Institute of Ageing suggests that you make sure that whatever you try provides you with a safe level of nutrition.

You should talk with your healthcare provider about the benefits and risks before making any significant changes to your eating pattern.