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Does intermittent fasting really work?

31st July 19 - 7 minute read

Headshot of James Hudson

Performance Nutritionist at Nutrition in Sport

Our expert:

James Hudson, Performance Nutritionist at Nutrition in Sport. James played Premiership Rugby for Gloucester, as well as lining up for London Irish, Newcastle Falcons and England Saxons. He has a degree and masters in biochemistry from the University of Bath and has completed the International Olympic Committee’s Diploma in Sports Nutrition. He is currently working towards a PhD at Liverpool John Moores University.

What’s in this article…

  • What is intermittent fasting?
  • The science behind it
  • Is it good for weight loss?
  • The benefits of fasting
  • Ways to do intermittent fasting
  • Top tips for intermittent fasting
  • Potential side effects

For centuries, fasting has been seen as a spiritual or religious exercise. But in recent time, intermittent fasting has become a go-to dieting technique for those looking to lose weight without the fuss of having to follow complex diet plans.

To help you decide if intermittent fasting is something you want to try out, we’re delving into the facts, health benefits and potential side effects with the help of nutritionist James Hudson.

At a glance

  • Intermittent fasting involves not eating during set times
  • It’s thought to be easier than more complex diets
  • Various fasting methods set either time or calorie limits

What is intermittent fasting?

Intermittent fasting is when you go without eating for a number of hours – perhaps eight hours - or even a full day, then go back to eating like normal.

We all fast intermittently when we’re asleep. Incorporating this gap into a diet plan simply involves extending the natural, non-eating window. Most diet plans recommend non-consecutive days of fasting, which is where the intermittent element comes in.

But flexibility is part of the appeal. There’s a huge range of established fasting plans out there, in varying degrees of intensity. From whole days of fasting to restricted eating hours, there’s a good chance you can find something that works for you.

The science behind intermittent fasting

With intermittent fasting, there are several forces at work on weight loss:

  • Fewer calories – with no calorie intake during fasting, your body uses energy stored as fat reserves to function¹
  • Less insulin – this hormone tells your body to store unused energy from your food as fat in your cells. Intermittent fasting can help lower your body’s insulin levels, which means less fat storage.²
  • More norepinephrine – most commonly known for its role in ‘fight or flight’, this hormone aids weight loss by encouraging your body to release more stores of fat to be burned. During short bursts of fasting, greater volumes of norepinephrine are released.³


Need to know: Intermittent fasting works by reducing your calorie intake and also instigating hormonal changes.

Is intermittent fasting a good way to lose weight?

Some studies suggest intermittent fasting can work for some people looking to shed a few kilos. The powerful combination of calorie cutting and hormonal response can result in fat loss.

A strong argument for intermittent fasting is also sustainability. Studies suggest intermittent fasting not only as ‘an option for achieving weight loss’, but also maintaining long-term progress if maintained.


Need to know: Intermittent fasting can be a more a sustainable way to lose weight.


Does intermittent fasting have any other benefits?

There are a shed-load of benefits that intermittent fasting offers over other diet plans. One of the main ones is its simplicity.

James says: “Intermittent fasting is a great tool for limiting energy intake and helping people make dietary changes, without having to go into the detail that more complex diets involve.

“It’s a very user-friendly way to lose weight.


“More complex diets can be harder to stick to. So in a broad way, people who want to eat less are being suggest time limiting diets.”

You don’t have to overhaul your shopping list, painstakingly count calories or spend hours in the kitchen getting to grips with new recipes. With fasting, the challenge comes in not eating.

Need to know: Intermittent fasting benefits from its relative ease when compared to more complex diets.

Ways to do intermittent fasting

There are no set rules on how big a time window has to be count as ‘fasting’, but here are some popular plans:

  • The 16:8 diet. Eat during an eight-hour window of the day - like 8am-4pm or 10am-6pm - and fast for the other 16 hours
  • Alternate day fasting. As the name suggests, you fast every other day. Some people eat a reduced number of calories – around 500 – on their ‘fasting’ days.
  • The 5:2 diet. Eat 500 calories for two days of the week, and eat as normal on the other five. This creates an energy deficiency that usually can’t be regained in the five days, meaning your calorie intake is lower overall.
  • Eat stop eat. 24 hours of fasting, followed by at least one day of ‘normal’ eating – but sometimes it could be two days. The full day fast can be tough, so if you’re new to the exercise the 16:8 diet may be a suitable place to start

The main distinction between these popular plans is they either set time limits or calorie limits.

Need to know: There are various methods of intermittent fasting that involve either calorie limits and/or set time periods when no eating is allowed.

Top tips for intermittent fasting

Successfully fasting involves more than just not eating. To reap the benefits, you should:

  • Choose your fasting days carefully. “Don’t plan them when you’re busy at work or planning working out, as it can affect concentration or energy levels,” says James. “Or when you’re going to be in social situations when food is available, especially over weekends.”
  • Drink plenty of water. Dehydration is an all-too-common side effect, so make sure you drink more water than normal to replace the fluids you’d normally get from food.
  • Eat a healthy, balanced diet on non-fasting days. When you’re fasting, you lose out on the valuable nutrients in food, so make sure you’re enjoying a vitamin-rich, nutritional diet on the other days.
  • Adapt your exercise schedule. Gentle activities like yoga are the best match for fasting days, with more rigorous workouts saved for when you’re stocked full of energy.

Side effects of intermittent fasting

One of the most important things is that you feed your body with all the nutrients it needs. In short bursts, fasting can have benefits – but if you go without food for too long, you could experience some of the symptoms below.
If you’re experiencing these symptoms, it could be a sign that your fasting plan isn’t quite right:

  • Dizziness – this could be a sign of low blood sugar.
  • Headaches – often a sign of dehydration.
  • Low energy – if you’re constantly tired, your overall calorie intake may be too low.
  • Mood swings – irritability is a common side effect of inadequate calorie intake.
  • Muscle pains – you’re not getting the vitamins and minerals you need.
    It doesn’t have to be a case of stopping fasting – you might just need to revisit your calorie intake to make sure you’re getting all the energy your body needs to function properly.


If you’re on regular medication, pregnant, have suffered from an eating disorder or are currently being treated for a medical condition, it’s really important that you talk to your doctor before you start fasting.


In summary:

Far away from the ever-growing list of fad diets, there’s real substance to the benefits of intermittent fasting. If you’re looking for a new way to lose weight or boost your metabolism, it could be worth a try.

 

[1] https://www.healthline.com/nutrition/intermittent-fasting-guide#weight-loss

[2] https://www.health.harvard.edu/blog/intermittent-fasting-surprising-update-2018062914156

[3] https://www.healthline.com/nutrition/intermittent-fasting-metabolism#section2

[4] https://www.health.harvard.edu/blog/intermittent-fasting-surprising-update-2018062914156

[5] https://www.medicalnewstoday.com/articles/324882.php

[6] https://www.medicalnewstoday.com/articles/324882.php

[7] https://www.medicalnewstoday.com/articles/324882.php

[8] https://www.menshealth.com/health/a26052066/side-effects-of-intermittent-fasting/

[9] https://www.livestrong.com/article/439303-muscle-pains-during-fasting/

 

Headshot of James Hudson

Performance Nutritionist at Nutrition in Sport

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