What is Intermittent Fasting? Does it Work?

If you like to read fitness/nutrition magazines or websites, chances are you have come across the term ‘intermittent fasting.’ What does it mean, exactly? Can it really help you lose fat while maintaining muscle? Is fasting even safe?

Read on for more.

What it is:

Google ‘intermittent fasting’ (IF) and you will get close to five million links. Finding the next “game changing” diet or eating lifestyle always generates a ton of interest, especially from people looking to lose fat.

From a recent study, scientists believe that IF is the key to future weight loss programs. Why?

The 5:2 diet – also known as intermittent fasting – requires two non-consecutive days of “fasting” each week, when the dieter is to consume no more than 500 calories if they are a woman and 600 if they are a man.

There are no dietary restrictions for the remaining five days, which are often referred to as the “feeding” days.

The diet has gone viral following the BBC documentary Eat, Fast and Live Longer by doctor and journalist Michael Mosley, which screened in the UK last year.

There are three primary ways that fasting benefits humans:

Increased insulin sensitivity and mitochondrial energy efficiency – Fasting increases insulin sensitivity along with mitochondrial energy efficiency, and thereby retards aging and disease, which are typically associated with loss of insulin sensitivity and declined mitochondrial energy.

Reduced oxidative stress – Fasting decreases the accumulation of oxidative radicals in the cell, and thereby prevents oxidative damage to cellular proteins, lipids, and nucleic acids associated with aging and disease.

Increased capacity to resist stress, disease and aging – Fasting induces a cellular stress response (similar to that induced by exercise) in which cells up-regulate the expression of genes that increase the capacity to cope with stress and resist disease and aging.

For more in depth reading on intermittent fasting, I’d recommend these sources:

Common fasting myths:

Martin Berkhan busts a lot of the most well-known ones here.

Myth: Eat frequently to “stoke the metabolic fire”.

One of the biggest ones – we are all taught to eat a big breakfast to get our metabolism “firing” in the morning. And we are taught to avoid food at night so our body doesn’t have to burn calories while sleeping. Both of these thoughts are false.

Myth: Fasting tricks the body into “starvation mode”.

Another big one related to fasting. A lot of people I have mentioned IF to seem to think that fasting leads the body store to instantly store fat.

The metabolic rate is actually increased in short-term fasting. For some concrete numbers, studies have shown an increase of 3.6% – 10% after 36-48 hours (Mansell PI, et al, and Zauner C, et al). This makes sense from an evolutionary perspective. Epinephrine and norepinephrine (adrenaline/noradrenaline) sharpens the mind and makes us want to move around.

Desirable traits that encouraged us to seek for food, or for the hunter to kill his prey, increasing survival. At some point, after several days of no eating, this benefit would confer no benefit to survival and probably would have done more harm than good; instead, an adaptation that favored conservation of energy turned out to be advantageous. Thus metabolic rate is increased in short-term fasting (up to 60 hours).

I don’t think anyone on IF fasts for more than 60 hours (and I wouldn’t recommend anything that crazy).

Myth: Skipping breakfast is bad and will make you fat.


Breakfast skipping is associated with higher body weights in the population. The explanation is similar to that of lower meal frequencies and higher body weights. Breakfast skippers have dysregulated eating habits and show a higher disregard for health. People who skip breakfast are also more likely to be dieting, thus by default they are also likely to be heavier than non-dieters. Keep in mind that most people who resort to breakfast skipping are not the type that sit around and read about nutrition. They are like most people dieting in a haphazard manner. The type to go on a 800 calorie-crash diet and then rebound, gaining all the weight (and then some) back.

This is the first thing that I tried out on myself – skipping breakfast. For a few days I was really hungry in the morning, but I didn’t miss it as much after a week or two. And I started to find that my energy levels were unaffected after about two weeks of it.

I didn’t skip my black coffee, though. 

How to implement IF:

The beauty of IF is there are so many different ways to implement it, depending on your work and fitness schedule. The first thing to do is to not view it as a “diet” with an end date. It represents an approach to eating – an eating lifestyle, if you will.

And not all fitness experts believe in it (relative to other ways to eat healthy).

Charles Poloquin is one of them.

Take Away: Intermittent fasting can produce fat loss, but it is NOT the best method. In fact, it puts you at risk of altering your hormones and circadian rhythms. You may compromise your body’s ability to regulate itself, leading to exhaustion and disease.

That being said, I wouldn’t recommend IF to people with a history of eating disorders, or people who have had past issues with over/under-eating. Having restrictive eating periods isn’t for everyone. It requires discipline, and like anything else it can be abused if not implemented correctly. It isn’t for high end athletes either, who need calories (fuel/energy) before, during, and after performing their sport.

  • You can do a full day fast once a week. This doesn’t work for some people (myself included).
  • You can do a full day fast twice a week. Again this doesn’t work for everyone.
  • You can do smaller fasts within each day. For example, Martin Berkhan’s Leangains program involves fasting for 16 hours (from nine at night to 1pm the next day), and then entering an eight hour “feeding window” each day. This means that you don’t eat breakfast (outside of coffee or tea). Here is an example training day on the Leangains program:

Sample setup

  • 11.30-12 AM or 5-15 minutes pre-workout: 10 g BCAA 
  • 12-1 PM: Training
  • 1 PM: Post-workout meal (largest meal of the day).
  • 4 PM: Second meal.
  • 9 PM: Last meal before the fast.

I won’t bother regurgitating entirely what Dr. Berardi discovered with his research – he tried out different IF protocols and shared his thoughts on what worked and what didn’t. To access the different eating schedules, check out the different chapters of the e-book (from the link above).

Dr. Berardi with some closing thoughts:

Intermittent fasting can be helpful for in-shape people who want to really get lean without following conventional bodybuilding diets, or for anyone who needs to learn the difference between body hunger and mental hunger. (And for the latter, I only recommend the Trial Fast.)

It’s a helpful tool and one I’ll continue to use periodically. But it’s not the end-all, be-all of nutrition or fitness. People have been getting in awesome shape — and staying in awesome shape — for decades without the use of intermittent fasting.

There are pros and cons to IF. If done consistently and correctly, there are benefits to it. But like anything else, if abused, it won’t have the desired outcome. If you think this is something that may work for you, give it a try for two weeks. Track what you eat, when you eat, and how you feel. Self-experimentation is often the best way to give diets and eating programs a full review (after you have done your research, of course).

Essential IF reading: