Intermittent fasting: do the facts stack up?



Intermittent Fasting (IMF) has grown in popularity over the last 5 to 10 years and has reached something of a cult status, like paleo & low carb diets; anybody who is doing it wants to let everybody know they are doing it! Proponents of IMF claim that fasting is the solution to many of our modern-day ill’s, while part of its popularity is the idea that as long as you are going through periods of fasting then you have carte blanche to eat what you like.

Obviously, there is more to it than this. Unfortunately, there are celebrity personalities and nutritionists guilty of bringing IMF into the realms of faddy diets, but does this mean that we should disregard it all together?

In truth IMF in various ways has been around for thousands of years with many religious beliefs including some form of fasting in their teachings i.e. Ramadan. Not to mention daily and seasonal variations in feast and famine which would have occurred on a regular basis.

IMF explained

IMF can be defined as cycles of fasting and non-fasting, generally within a set time period.

All of us routinely practice IMF when we sleep. We start the fast with the last thing we eat at night and then break-fast with the first thing we eat in the morning. Purposefully engaging in IMF looks to expand that fasting window. Although there are many different ways of doing this there are broadly 2 categories:

  1. Alternate day fasting (ADF): This is where you alternate days of normal eating with days of reduced or zero eating, the 5:2 diet popularised this way of eating.
  2. Time restricted feeding (TRF): This practice is all about extending your usual overnight fast, again popularised by diets such as the 16:8, 20:4

So, what are the positives and negatives of IMF? Let’s start off with the not so good aspects.

IMF Downsides

Disordered eating: unfortunately, in spite of good intentions, this type of eating routine can encourage poor eating behaviours that make regulating food intake harder in the future. People with a “dieting” mentality can get fixated on the ‘rules’ which accompany a diet and any deviation from these rules can lead to a temporary lapse or full relapse. With our stressful lives and easy access to food, longer term ‘denial’ becomes progressively harder.

Social interruptions: IMF can make it difficult to go out and socialise, so if you are not careful you could find it alienating you from friends.

Hunger and lack of focus: extending a fast is going to leave you feeling hungry and if you have a mentally taxing job this could cause fuzzy headedness and a lack of focus.

IMF Upsides

Alignment of natural rhythms:  a lot of research is going into how when we eat and how long we fast for impacts our other circadian rhythms. Our sleep quality, gut function and overall energy intake can be modified for the better if we align our eating pattern with our other behaviours.

Easy to implement and cheap: Once you find a style of eating that suits your lifestyle it’s easy to adhere to. Low carb or continuous energy restriction is hard to stick with long term. IMF also doesn’t require you to buy lots of expensive foods or supplements which make it more accessible.

What does the research say about Alternate Day Fasting?

In one study by Johnson et al 2007, overweight adults with moderate asthma ate only 20% of their normal calorie intake on alternate days (5:2 style) Over 8 weeks those who adhered lost 8% of their initial body weight and saw a decrease in oxidative stress and improvement of asthma-related symptoms.

Harvie, M. N., Pegington, M., Mattson et al 2011, also found that intermittent and continuous energy restriction are equal in their ability to improve weight loss attempts and other health markers.

As it stands, much of the research we have into fasting is based on animal and rodent studies however there appears to be a modest improvement in human trials on weight loss and metabolic parameters such as cholesterol and blood glucose, Patterson and Sears 2017¹.

What does the research say about Time Restricted Fasting?

Moro et al, 2016 found that in men who are resistance trained (lift weights), an 8-hour eating window helped improve some health markers, reduce fat mass and preserve lean tissue.

Mice studies have found that restricting food intake to fit around the normal night time cycle can help resolve metabolic condition such as non-alcoholic fatty liver disease and diabetes, Longo and Panda 2016.


IMF is not a panacea and it does not have magical unicorn dust like properties. However, for those who may be struggling with energy levels and sleep or those who are suffering from lifestyle conditions it may, if the right routine can be found, provide an effective way to self-manage your condition.

Care must be taken not to let it evolve into a binge/deprivation cycle and as always, the right approach must be taken for each individual and if someone is unable to engage in IMF then that is fine, there will be another way forward for you.

Personally, I back the idea of a fasting window which fits around my sleep pattern. I aim to finish eating no later than 19:00 which gives plenty of time for my digestion to wind down before bed. I’ll then eat breakfast between 08:00 and 09:00 resulting in roughly a 14 hour fast. I find this gives me more energy when I wake up and is manageable enough to fit in with my lifestyle.


¹Ruth E. Patterson and Dorothy D. Sears (2017) Metabolic Effects of Intermittent Fasting Annual Review of Nutrition  37:1, 371-393

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Ian is a Registered Dietitian and online weight loss coach with a background in the NHS. A specialist in weight management, disordered eating and metabolic health.

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