Fasting is not a new concept and has been around for thousands of years but what exactly is Intermittent Fasting? Fasting involves alternating food absence with periods of normal food consumption.

We break a ‘fast’ every day when we first eat – breakfast even if not at breakfast time.

In the ‘diet trend’ sense, intermittent fasting usually refers to something like a 16:8 regime where you fast for 16 hours and eat within an 8-hour window. For example, finishing eating at 8pm and beginning again at 12pm the next day. This is technically called time-restricted feeding but is often referred to as intermittent fasting. It does not have to be associated with a calorie deficit, although this may occur by default with eating reduced to a specific window of time.

Alternative intermittent fasting regimes include the 5:2 diet or alternate day feeding.

These are more traditionally associated with weight loss. The 5:2 regime involves following a strict calorie restriction of around 500-600 calories for 2 days out of 7. Although this is not technically fasting obviously, the same metabolic responses can occur, more on this later.

There are also some who will fast for extended periods of time, or only eat one meal per day.

Like all things nutrition, there are some who like to take things to the extreme and it is a little ironic that the art of eating nothing can seemingly create as much controversy as all the other ideal nutrient combination debates.

Is intermittent fasting good for weight loss?

Like all diets, intermittent fasting is capable of putting the body into a calorie deficit, which in the short term creates weight loss.

In a review of 11 RCTs comparing IF to calorie restricted dieting, 9 out of the 11 studies showed no clinically significant difference between groups in terms of weight or fat loss. So, from a weight loss perspective, intermittent fasting does not offer anything special.

As always, we have to look at the bigger picture of the sustainability of strict diet regimes, built on calorie deficits and consider their general failure in the long-term points to a much more sophisticated physiology that will adapt to return us to equilibrium.

What are the side effects of intermittent fasting?

Here results are very individualised. We know some people are very prone to changes in their blood glucose levels and may notice and suffer from hanger, low mood and lack of concentration quite easily.

Some, however, report increased mental clarity and mood enhancement.

There are so many confounding factors, what you are asking of your body that day, stress levels, sleep quality, menstrual cycle etc.

Fasting will increase cortisol levels (stress hormone) which could be a negative depending on levels of stress already in someone’s life and fasting will slow metabolic rate. However, practiced fasters do seem to adapt their food intake to account for this and do not appear to overeat in their eating window. Another example of the intricacies with which the body adjusts things at a subconscious level.

Who should not do Intermittent Fasting?

It would not be recommended:

For those with diabetes on medications that can cause blood glucose levels to drop
In pregnancy or breast feeding
For those with a history of eating disorders or disordered eating.
For those with specific medical conditions.

Unfortunately, wellness influencers like Ms Paltrow are partly responsible for making fasting seem healthful and popular – blissfully unaware of the harm they cause. In giving people permission to adopt intermittent fasting, without proper assessment, it can become another way to control calorie intake and obsess about food and ultimately mask some pretty deep-rooted complex issues.

What happens in the body when we go into a fasted state?

When in a state of fasting, the body is trying to preserve glucose (as without access to this we die) so it uses fat stores as a fuel instead. There is something to be said for this as it seems to improve our efficiency at ‘clearing’ a meal when not in the fasted state. If we fast for over 24 -36 hrs this is when we can start to create harm.

When in a fasted state we also see increased levels of autophagy (eating of our own cells).

It’s the body’s way of regeneration, cleaning up old cells by consuming them internally and regenerating newer and healthier cells – like a really good spring clean. It happens all the time but when we’re in a prolonged fasted state, cells activate pathways that increase defences against oxidative and metabolic stress and systems that remove and repair damaged molecules.

This is what has excited researchers the most as it appears this could have many benefits for human health including improved blood sugar regulation, increased resistance to stress and decreased inflammation. And it is true to say that in the modern world, with our access to food at every turn, it is quite rare this metabolic switch would be activated unless we are deliberately withholding food for extended periods of time.

So, what are the proven health benefits to intermittent fasting?

Animal studies (rats/mice) are still the most prevalent in answering this question, so we need to proceed with caution. We don’t have tails and there are many other differences between us and them.

Human studies need to be stronger before we can be clear on what this means for us all in day-to-day life, but we are seeing the following:

Potential improvements in blood pressure and blood lipids but more randomised controlled trials are needed.
Impact on insulin sensitivity improvements, irrespective of weight loss – we are certainly more sensitive to insulin in the morning, so irrespective of whether you want to go the whole hog of fasting, adjusting your meals so that most of your calories are consumed prior to 3pm seems to offer some advantages. We have more efficiency in the morning to ‘clear’ the fat from a meal and we can also see appetite responses are different in the morning to the evening when participants were given the same meal.
Better endurance and balance and coordination – only shown in mice.
Suppressed inflammation in the brain – which links to neurological disease and reduced risk of these conditions e.g. Parkinsons or Alzheimer’s – only in mice.
Improved memory – only in mice.

What exactly is Intermittent Fasting

My top tips and take homes

There are benefits for sure but we’re a little way off knowing for certain how it impacts different individuals. We need A LOT more human evidence before this can become mainstream and I can say that because I’m not trying to secure a book deal.

If you have a family history of diabetes, heart disease and metabolic syndrome eating earlier may help to reduce your risk factors.

Adhering to a 16:8 regime is hard work and may potentially not fit in with the rest of your life very well. We don’t have the evidence yet to see whether doing it partially is also of benefit.

Overall, the evidence is not strong enough to recommend doing it, if fundamentally it’s not going to work with your life and it’s going to make you more miserable and obsessed with food.

Arguable we function better when we’re matching our fuel intake to our energy needs i.e. fuelling our mornings when we’re busier and eating less in the evenings in front of the TV. The traditional eating between 12pm and 8pm window might suit our modern lifestyles but is potentially at odds with when we would benefit most from the fuel for both our physical and mental performance.

We don’t have any evidence that intermittent fasting helps us live longer, although it may indirectly help with this through reduction of risk factors.

However, the quality of your diet, your other health behaviours e.g. exercise and drinking in moderation, your relationship with food, not yoyo dieting and eating in tune with your hunger and fullness which are unique to you. These are all far more worthy of your attention than the clock.

Start there, and if you feel there’s room in your life (and your head) to want to try intermittent fasting as well, then give it a go if it appeals.