How To Avoid The 5 Most Common Side Effects Of Intermittent Fasting

There are 5 most common side effects of intermittent fasting that people commonly report when they start fasting. In this article, you’ll learn what they are, why they happen, how to manage them if you do get them and most importantly, how to minimise or even avoid these side effects altogether

Table of Contents

1. The first side effect of intermittent fasting is HUNGER!

Now, this might seem like an obvious one but important to mention as it’s not quite as straightforward as many people first think.

Many people don’t start because they are afraid of being hungry. They think that if they skip a meal that they will carry on getting more and more hungry until they eat again. This isn’t true.

It’s important that we understand what drives our hunger.

What Drives Our Hunger?

Pretty much everything in your body is controlled by nerves, hormones and often some combination of the two!

Hunger is no different. The process of digestion starts even before you put food in your mouth.

This is why your stomach rumbles when you see or smell food, or even if you hear the sound of food being prepared.

The signals travel from your eyes or your nose to your brain, which triggers the secretion of stomach acid, and the hormones that prepare your body for digestion.

This response is called the Cephalic Phase of Digestion. I’ll put a link down below to the Wikipedia article on this topic if you want to geek out and learn more.

But the Cephalic Phase Response isn’t only triggered by seeing or smelling food, there is also an element of it which is a learned response.

If you have always eaten breakfast at 7:30 am every morning, then eventually, your Cephalic Phase Response will start to happen, just before that time, whether the meal comes or not. 

The body learns to expect the meals it is used to having at the time it is used to having it. If you suddenly decide you’re going to miss that meal, this response doesn’t just completely go away.

However…What many people often think is that if you skip a meal, say, breakfast, you’re just going to get more and more hungry until you eat again, and that simply isn’t true.

The hunger typically lasts 20-30 minutes and then it passes. 

Why? Because the surge of hormones subsides and you don’t feel hungry anymore.

You may have waves of hunger until you eat again, but you won’t just keep on getting more and more hungry.

And the good news is that the learned response can be unlearned, but you will have to learn to ride these waves of hunger until your body unlearns this response.

Here are a few tips to help with the hunger when you’re just starting out.

1. Take It Slowly

You don’t have to jump straight into a long fast. Start with something shorter like just missing breakfast.

If missing breakfast is too much, just start shifting it later in the day.

Maybe you could have breakfast at your mid-morning break at work, or if that’s not possible, then just start by having breakfast a bit later on the weekend. Maybe you could make that a larger meal than you’d normally eat and call it brunch!

Another idea would be to just make your evening meal a bit earlier than you normally would and lengthen your fasting time that way.

2. Don’t be around food!

Go easy on yourself. If you’re fasting… don’t go triggering that Cephalic Phase response by surrounding yourself with food.

Don’t underestimate the power of this. The brain is very powerful. Infact… I can probably make you hungry in just a few seconds. 

Look at the image below! Mmmm…. Tasty! 

Did that make you start to feel hungry?

Let me know in the comments down below.

Just type “Hungry” or “Not Hungry”.

By the way, if you’re reading this mid fast, and that just made you really hungry! Sorry. Don’t worry… it will pass soon!

3. Avoid Boredom

Boredom is your enemy. Can lead you to focus and obsess around food. It can be really hard to keep yourself occupied.

Do something that distracts you that you don’t associate with eating.

Keep active – go out for a walk.

One really good way to avoid boredom is to subscribe to my Youtube channel!

Like I mentioned earlier, hunger comes in waves and will often subside after about 20-30 minutes. That’s just 2 or 3 videos!

2. Fatigue / Brain Fog / Lightheadedness / Hangry

5 Most Common Side Effects Of Intermittent Fasting

I’ve grouped these together as they are usually essentially caused by one of the following 2 things.

1.Low Blood Glucose Levels (Hypoglycemia)
2.Not being adapted to burning fat

Low blood glucose levels

It might seem logical that you should have low blood glucose levels when you’re fasting but hypoglycemia shouldn’t really happen.

Your body still needs glucose to function properly, and when you fast, or cut out carbohydrates, your blood glucose levels don’t drop to nothing. There always needs to be some glucose going round in your body.

When we’re not consuming glucose (or things that turn into glucose) our liver can convert other substances into glucose.

Your body should turn to alternative energy stores to provide that glucose as well as increasing the burning of fat and the production of ketones.

So why can we get hypoglycemia when fasting?

Remember what I said about hormonal responses being conditioned….

If your pancreas is used to kicking out a load of insulin every morning at breakfast time to cover the glucose in that bowl of cereal, and then one day you don’t give it that bowl of cereal….

The pancreas is still going to squirt out insulin and your blood glucose may go a bit low.

It shouldn’t happen…. But it does happen…

Hopefully this will underline the importance of people who are on medication to lower blood glucose should consult with their doctors before attempting any form of fasting, as it may result in profound hypoglycemia, which can be life threatening.

The same is true for blood pressure. Fasting usually lowers blood pressure, and can also be a cause of lightheadedness… – again, important to consult a doctor before fasting if you’re on bp meds, or have low blood pressure.

We don’t want you falling over now do we?

Not Fat Adapted

And on the other side of things, at this point in your fasting career you’re unlikely going to be  adapted to mobilising your own fat stores to compensate for the lack of food.

The fatigue, brain fog and lightheadedness can just be because your body isn’t used to mobilising significant amounts of energy from its fat stores yet. 

You’re not yet metabolically flexible enough to switch between mainly burning carbohydrates for fuel to burning fat for fuel.  That comes with time.

Other people can have difficulty with actually producing significant amount of ketones from their fat stores because of their genetics.

So what do you do?

Break Your Fast Early

Just remember there are no prizes here. If you’re really struggling with these symptoms… just break your fast early. You can just pick things up again a day or two later. 

It’s a marathon, not a sprint.

Avoid Intense Exercise

Very often I see people who are motivated with changing their lifestyle up – they try and change everything at once, and they start doing pretty intense exercise alongside intermittent fasting. 

Whilst I’m a big fan of fasted training, it isn’t necessarily a good idea early on in your fasting career, especially if you’re not used to exercising. Take things slowly and aim to change one habit at a time.

3. Changes In Bowel Habit

signs you're in ketosis

Now changes in bowel habits happen when you change up your diet in any way.

I’m sure many of you will have experienced this while you’ve travelled abroad or even to a different part of the same country.

Now these changes can and do go either way. 

Constipation, or diarrhoea.

You’re likely changing several things that will have an impact on your gut bacteria, and they are going to respond in their own way.

How can we minimise these symptoms?

Fluid & Fibre

It’s important to stay hydrated, and can help to have plenty of fibre at times when you are eating.

If you’re still struggling, your pharmacist can advise you on medication you can buy over the counter to ease these symptoms while you transition.


Whilst I’ve found no evidence that coffee actually is an appetite suppressant, many people who are fasting seem to use it as such.

If you’re getting diarrhoea, one of the reasons is that you might be drinking a bit too much coffee.

By the way I made a video all about fasting and coffee – you can check that out by clicking the link up here.

4. Heartburn

GORD / GERD / Acid Reflux

Remember that Cephalic Phase Response?

Your stomach will still produce acid in response to a meal that never comes. In some, this is enough to trigger heartburn.


Might be that you’re a bit sensitive to drinking coffee on an empty stomach.

See your pharmacist

Who might be able to advise you on over the counter remedies you might take when you experience these side effects of intermittent fasting.

5. Feeling Cold

side effects of intermittent fasting

This appears to be one of those side effects of intermittent fasting that you either get, or you don’t get! I don’t remember ever really experiencing this, but lots of people report this to me.

What this is not…

Don’t worry It’s not your metabolism shutting down – a common myth associated with fasting. Fasting actually has quite the opposite effect. 

The only study I can find about getting cold when fasting was done a couple of years ago in mice which links it to reduced levels of one of the chemicals in the brain called Orexin.

Now there is quite a lot of variability in levels of orexin in the brain of humans, so this may explain why some people get cold when fasting and others don’t.

Things that can help

1.Coffee – can help stimulate orexin production.

Or you could just wrap up warm!

And talking of wrapping up, that’s about it for the main symptoms that people get when they start intermittent fasting. There are others associated with longer fasts, and the symptoms that have been described here are sometimes a bit more pronounced with longer fasts.

I promised at the beginning I’d take you through some of the steps you can take to reduce or eliminate some of the side effects mentioned above.

Start Slowly to avoid the side effects of intermittent fasting

Now I’ve already mentioned this earlier so I won’t spend long on this one…..I see people go right out like a bull in a china shop

They start exercising

They start dieting

They start fasting

They stop drinking alcohol 


Habits are built over time.

Take it 1 step at a time.

Get Nourished

If you’re coming from a diet which has been relatively poor in nutrition, you might well be deficient in certain vitamins and minerals, thus likely you will experience the side effects of intermittent fasting.

Energy Dense / Nutritionally Poor foods

First start by getting nourished, if you start fasting before you do this you’re going to make things much harder for yourself.

Eat Real Food

Focus on unprocessed, nutrient-dense foods.

1.Fish – particularly salmon & sardines.
2.Eggs – the yolk is incredibly nutrient-rich and the white is a great source of protein.
3.Meat – and not just the muscle – I’d really like you to consider things like liver and other organ meats. A great place to start is by making your own pate – it is really fast to make and is a great way to get started with organ meats. There is a really simple but tasty recipe on my website for this.
4.Green Leafy Vegetables – there is a reason that kale is associated with healthy eating. It’s full of nutrition!

I could go on with this list but you can just google ‘nutrient dense foods’ if you want to find more!

If you want to take things a step further and really easy your transition into intermittent fasting.

Low Carb, Real Food

Low Carb Chicken Puttanesca
Low Carb Chicken Puttanesca

Why low carb?

Fasting when you’re high carb can be really difficult.

A low carb diet will basically put you in the same physiological state as fasting anyway

If you’re low carb anyway when you start fasting, you’re going to be adapted to burning fat as your main source of fuel rather than carbohydrates so you won’t have to switch all the time.

You’re not going to get those insulin spikes as you did… so you’re not really going to go hypo-glycemic.

Your body is going to be used to mobilising its fat stores in order to provide you energy. You won’t feel as hungry.

When you’re well-nourished and burning fat as your primary source of fuel you’ll often find that you will wake up in the morning and won’t feel hungry.

Now, I wasn’t around at the time, but I’m pretty sure that our caveman ancestors didn’t rush out of their caves and pour out a bowl of cereal.


I hope that’s given you a bit of an insight into both why people experience the side effects of intermittent fasting when they start fasting, how you can manage those symptoms if you do get them, and how to minimise or completely avoid getting them in the first place!