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How Fasting Aids Weight Loss: Breaking down the Pros and Cons

How Fasting Aids Weight Loss

Navigating the world of weight-loss can be pretty challenging. With so much conflicting nutritional advice available, it can be difficult to gauge what clean-eating path to follow in the quest to slim down. Currently, health nuts are waxing lyrical about intermittent fasting – an eating regimen that alternates between periods of normal eating with extreme calorie reduction. Proponents of this voguish trend believe it can zap fat faster than traditional diets. Let’s find out if the claims hold up.


Whole-day fasting (5:2 diet)

For two nonconsecutive days a week, you consume just 500 calories plus water. You can eat this in one sitting or spread out over the day. The other five days, you have the freedom to eat whatever you fancy, although, of course, we encourage you to load up on veggies, fruit, whole grains and lean protein. If you like the idea of ‘cheat days’ and don’t want to sacrifice that evening glass of vino, you’ll fare great here.

Time-restricted feeding

You fast for 16-20 hours every day, consuming nothing but zero-calorie tea and water. Then, you can indulge in whatever you want within a 4-8 hours window, such as 10 pm-6pm or 12pm-8pm. If you like routine and structure, this method could work wonders for you. But if you relish your early morning workouts and post-work socials – and compromise isn’t your strong suit – this probably isn’t for you.

Alternate day fasting

For the first 24-hours, you consume just 500 calories plus water – in one meal or spread over the course of the day. For the second 24-hours, you can feast on whatever you want, whenever you want. You repeat the cycle every two days. If you want the freedom to indulge often but like routine – one day on, one day off – this might be just the ticket.


You do lose weight

A raft of scientific research proposes whole-day and alternate-day fasting can reduce body fat, weight and waist circumference i. In a systematic review, all studies claimed fasting lead to weight-loss. However, many researchers noted participants tended to shed weight earlier in the study, compared to the final follow up. This is likely to be an indicator of high dropout rates and raises questions over whether fasting is a sustainable dietary plan (more on this later!) ii. A more recent meta-analysis concluded weekly fasting was just as effective at shedding pounds as continuous calorie restriction iii. It’s worth noting, though, most of these studies were short-term, so it’s difficult to ascertain if individuals were able to keep the weight off.

No calorie restriction

Yup, you read that correctly – no calorie restriction! You can still eat the same amount of daily calories and don’t have to deprive yourself of certain foods (though, obviously, we recommend following a well-balanced, whole-food diet where possible). But, in short: yes – you can still have your cake and eat it, too.

It’s pretty simple

If you thrive on routine and structure, this regimen could be a great fit for you. Depending on your daily schedule, it is very possible you could seamlessly fit a period of fasting in. For instance, individuals who miss breakfast and don’t eat after an early dinner unintentionally practice ‘time-restricted feeding’. See – not that hard, really.


Low energy, unproductivity, and HANGER

Unsurprisingly, stints of fasting can be accompanied by a slew of unpleasant side effects: hanger (the infamous hunger-anger-combination), flagging energy levels and brain fog. And this could be problematic if you work long hours, run a household or have a demanding social schedule. Gym-goers, in particular, you may struggle with mustering up motivation to workout. In a systematic review, researchers found that individuals who fasted experienced constipation, feeling cold, headaches, bad temper, lack of energy and poor concentration iv. After all, getting enough fuel throughout the day is food for your mood, productivity and spirits.

Interferes with the social aspect of eating

The cooking, sharing and eating of food are immensely social activities. Birthdays, special occasions, anniversaries, life milestones and any other celebrations generally orbit around the act of ‘breaking bread’ with the people you care about. Since fasting doesn’t sync with the traditional eating windows, it may hamper your social hangouts that usually involve food. No late-night candle-lit dinners. No special birthday dinners. No lunch meetings with your co-workers. No spontaneous mid-week suppers. Ask yourself this: are you really prepared to give all these up? Remember, your emotional wellbeing should take pride of place, too.


Periods of fasting can build a pressing hunger. And this voracious appetite may see you wolfing down every morsel of food in sight once you break your fast. Trouble is, you could mistake this window for ‘feasting’ and give yourself a green card to munch on any old grub, which isn’t really the point. While you don’t need to count calories per se, you still need to enter a caloric deficit to lose weight, which binging could well undo. If you want fasting to work, self-control is crucial. Though it can be tempting, you simply can’t reward yourself after a successful fast with copious amounts of unhealthy food.

Long-term results?

To date, there haven’t been any long-term studies assessing the sustainability of fasting, most probably because of high study dropout rates. Here’s some food for thought: in a 2016 review, 31% of participants dropped out of the investigation v. This statistic is cause for doubt and poses the question: is this fasting a sustainable way to lose weight and keep it off?

Bottom line

There’s no one-size-fits-all magic bullet cure for weight-loss. Sure – fasting can help you lose weight, but so can plenty of other methods. Ultimately, it comes down to personality and preference; some people love fasting, and some don’t. If you find yourself gorging after a fasting window, or feeling light-headed and shaky during a fast, then it may be fair to say this eating regimen isn’t for you. Be mindful of your own eating habits and choose an eating approach that’s most conducive to you.



  1. & Effects of intermittent fasting on body composition and clinical health markers in humans. Nutrition Reviews, 73(10), 661-674.

  2. , , & Weight-Loss Outcomes: A Systematic Review and Meta-Analysis of Intermittent Energy Restriction Trials Lasting a Minimum of 6 Months. Nutrients, 8(6), 354.

  3. , , , & Short-term intermittent energy restriction interventions for weight management: a systematic review and meta-analysis. Obesity Reviews, 19(1), 1-13.

  4. & , & Weight-Loss Outcomes: A Systematic Review and Meta-Analysis of Intermittent Energy Restriction Trials Lasting a Minimum of 6 Months. Nutrients, 8(6), 354.

  5. ScienceDaily., Daily fasting works for weight loss, finds report on 16:8 diet. Available online: [Accessed 21 Nov. 2018].


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Our Author - Keri Filtness


Keri Filtness has worked in the Nutrition Industry for 19 years. She is regularly called upon for her professional comments on health and nutrition related news. Her opinions have been featured by BBC3, Prima, Vitality, The Mirror, Woman’s Own and Cycling Weekly, amongst others. She has also worked one to one with journalists, analysing their diets and health concerns and recommending changes and additions, where appropriate.

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