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What Is Intermittent Fasting and Is It Healthy?



Intermittent fasting has been growing in popularity in the last few years thanks to the 5:2 diet and personalities like Dr Michael Mosley.

I decided that, as it’s January and people are inevitably trying new diets, I would explore the basics of intermittent fasting and look at some common questions people ask about it.  

What Is Intermittent Fasting?

Intermittent fasting (IF) is used in popular culture as an umbrella term for fasting of some description. So, when someone says they are doing IF, it doesn’t really tell us much more than they are restricting eating in some fashion.

IF can be broken down into two types.

The first type, and what I think most people mean by IF, is time-restricted eating. This means that a person eats all of their meals and snacks within a particular window of time each day (24hr period).

This type of restriction is based around our circadian rhythm. It can be broken down into variants like:

Feeding Windows – eating within a set window in a 24-hour day. A popular one is the 16:8 diet where you eat within an eight hour window then fast for sixteen hours. This can vary to a 18:6, 20:4 etc. People can choose between these depending on tolerance to not eating for prolonged periods and general lifestyle (i.e around work hours).

5:2 Diet – You eat one small meal (normally less than 500 calories) or nothing in a 24hr period. You do this two days a week and eat normally for the other five days.

Random Meal Skipping – Skip random meals during the week.

People do other variations of intermittent fasting but these are some of the most popular. 

The second type of IF is known as just fasting or doing a fast.

This normally constitutes not eating for 24hrs but can be longer. It doesn’t relate to our circadian rhythm. This lengthy fasting usually causes the body to start to utilise fatty acids (ketones) as energy and not glucose. This is due to the body’s ability to store only so much glucose in your liver and muscles. 

After eating your first meal, you start a cascade of metabolic processes in the body. This is called your metabolism (in a very simplified way). From that point, your metabolism is at its highest then slows down as the day goes on.

So, if you eat your first meal at 7am and then your next at 8pm, your metabolism at 8pm will be very low and you might store more calories as fat. Alternatively, if you eat your first meal at 2pm then 7pm, your metabolic window is reduced. So getting your calories in a smaller window may have beneficial effects.


Should I Do Intermittent Fasting? What are the Benefits?

Most people are doing IF to lose weight and, in this case, the answer is yes.

study found that if people skipped breakfast they lost more weight overall. Unfortunately, this is probably just down to the basic fundamentals of consuming less calories than you are burning. Also, getting a balanced diet is still crucial.

So the question then becomes ‘is IF a good strategy for you to eat less calories?’ Yes, possibly if you already have a fairly good, balanced diet.

Another possible reason to do IF is for general health. This is an area still under a lot of discussion and ongoing research. It does seem to benefit particular biomarkers and lipids. There is a lot of preliminary research on rats showing that IF might have great promise in slowing the ageing process and extending lifespan.

Note: Before we get too excited though, we have to wait until human trials have been conducted. Rats have a very different metabolism to us, so fasting for 24-48hrs is very different for them than for humans. A rat can lose 20% of its weight in 48hrs while in humans, we would lose 2-3% at most. To get the desired effects we might have to fast for a lot longer periods.

IF is thought to have this effect by releasing Ketones (like beta-hydroxybutyrate), which have been shown to have anti-ageing effects. 

It also causes something called autophagy. This when the body repairs damaged cells in the body and regenerates new/healthier cells. Being in a fast promotes this, like a spring cleaning process for the body.

Most people are going to find this too difficult to maintain though. Possibly (after speaking a doctor) trying to do a 24-48hr water-only fast could be beneficial for your health.


Common Questions

Can you have coffee on a fast/Will having a coffee break my fast? 

Different experts say different things. In the time-restricted sense of IF, especially if you do it for weight loss, having coffee shouldn’t be a problem. You do need to make sure it’s without milk or sugar. 

Caffeine starts chemical pathways in your body that negate the health benefits of fasting though, so if you’re in it for health rather than weight loss, it would be best left out.

Should I exercise when fasting?

It depends – if you are exercising for no longer than 60 min aerobically (like running, football, swimming), this shouldn’t make much of a difference. If you are an endurance athlete and consistently training longer than 60 mins, then there are some small improvements in performance by eating before you exercise.

Most of us train for less than 60 minutes though, so this shouldn’t be an issue. When training in a fasted state, your body uses fat more readily as an energy source. This is good if you are trying to lose fat and eating before exercises blunts this mechanism.

Note: If you’ve been fasting for a long period, say 16 hours, then do a high intensity workout, it may be beneficial to get some protein within that hour window after your training session. Normally you can get your protein within hours after doing your workout and it won’t hurt your recovery. After you’ve been fasting for prolonged period though, you may require protein sooner. 

Best way to break a fast?

Try not to binge. Just having a balanced meal (protein, carbs and fats) is going to be best. Sometimes you might get a stomach upset if you haven’t eaten for 24-48hours. Try sticking to relatively small meal rather than a large meal. 

IF shows great promise in promoting good health, longevity and weight loss. I think it makes sense that our bodies are well-adapted and potentially work optimally in a fast state. This is how we would have evolved over many of hundreds of thousands of years.

Like exercising, IF is a type of stress which can be good for us. We don’t have to stick to it rigidly and do it all the time. I think every now and then it can be a good thing. Especially given that, when we look around, most of the health problems we see in our society are due to overeating. A little restriction isn’t going to harm us.

Always remember to speak to your doctor before you embark on any fasting diet. Good luck! 

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