The 24 Hour Fast: Quickest Way To Lose That Stubborn Fat?

Fasting has become an ever increasingly popular method of dieting and weight loss.

It has been made popular by several books, most notably the fast diet published in 2013 by Dr. Michael Mosley and Mimi Spencer (which was then televised in a widely watched documentary), which really brought this method of dieting into the forefront of the public eye.

Fasting in of itself is simply a prolonged period in which we do not consume any food and or liquid.

There are a variety of fasting protocols ranging from intermittent fasting (that is, fasting for most of the day and having a small window of time in which you can eat) to Ramadan (which is a form of fasting with religious connotations).

Related: BistroMD: Best Weight Loss Meal Prep Plan Review

What Is A 24 Hour Fast?

24 hour fast

Quite simply a twenty-four fast is a day in which no food is consumed. It is advised however that individuals consume water, and in most cases black coffee and or green tea is also allowed.

Benefits of A 24 Hour Fast

Fasting has several associated benefits. As it relates to dieting, the obvious being that we are in a calorie restricted state.

If we reduce the time and/ or days that we can eat by proxy.

We should then be eating a quantity of food that would put us in a calorie deficit and therefore initiate weight loss (bearing in mind that we don’t overeat or binge during the times and days that we are not fasting).

Calories are simply the energy we get from food.

Read More: Alternate Day Fasting – What Makes It Different, And How To Effectively Follow It

If we consume less calories than the amount, we require to maintain our weight we are in a calorie deficit or a state that should allow for weight loss.

Fasting may also promote the loss of body fat through the process of lipolysis (or fat breakdown). Lipolysis results in ketogenesis.

fasting benefits

This sounds complex, but essentially ketogenesis is the production of certain fatty based substrates which we can use for energy in the place of carbohydrate and other nutrients.

Ketogenesis occurs in the presence of low blood sugar levels and so therefore may be advised for someone who is fasting to maximize this fat loss protocol by combining their fast with a ketogenic diet on their non-fast days.

What are the other major benefits of fasting?

  • A possible simplistic tool for weight loss
  • Increased fat breakdown
  • Reduced inflammation
  • Reduced calorie
  • Reduce blood pressure
  • Increased insulin sensitivity and has been associated with improving if not reversing the metabolic syndrome in animal models. The challenge is, that the obese human population, the group we typically see metabolic syndrome in, may find protocols like fasting difficult to maintain.
  • Increased anabolism (building stuff up). When we fast for periods of time our body becomes more sensitive to the next source of nutrients it takes in (like exercise). We can exploit this by targeting a high protein intake in addition to a serving of carbohydrate if the goal is to improve lean body mass and or recover from exercise.
  • Improved cognitive function
  • There is also suggestion that fasting may induce less neuronal dysfunction and degeneration, and studies have shown fewer clinical symptoms in animal models of Alzheimer’s disease (AD), Parkinson’s disease (PD) and Huntington’s disease (HD).

Read Also: An Athlete’s Guide to Intermittent Fasting vs. Multiple Smaller Meals

Is There Any Negatives To Fasting?

  • Increased hunger (we produce less leptin, the hormone responsible for telling us we are satiated), means it can be mentally quite challenging
  • Reduced blood sugar can lead to fatigue, irritability, reduced exercise performance and a host of other detriments, if the individual has not yet adapted to producing energy through ketosis.
  • Reduced sleep quality because of the hunger response
  • Metabolic adaptation – that is a reduction in total daily energy expenditure – if the fasting is too prolonged, too frequent or chronic (which it shouldn’t be).
  • The brain prefers to run on carbohydrate and not products of ketosis. It may make fasting more challenging, from a mental perspective, and could be why many people are unsuccessful with fasting protocols.

Mechanisms derived from the 2014 paper by Longo and Mattson.

How often should you fast?

fit men

Twenty-four-hour fasts should only be performed at a maximum of three times in a weekly cycle.

Typical protocols follow anywhere from one to three days a week of 24 hour fasting (without consecutive days).

These protocols have been used in studies and have been associated with reducing the occurrence of; breaking the fast and or binging, reducing the risk of dehydration and or constipation and mitigating any issues metabolic adaptation.

The more experienced an individual becomes the more days you could, in theory, fast for in a week (up to three) and avoid the non-desirable issues previously mentioned that may occur when fasting.

Read Also: Intermittent Fasting – The End All, Be All Guide For Beginners

Fasting Food List

What can I consume during a fast?

  • Plenty of water
  • Carbonated beverages (zero kcal alternatives)
  • Coffee (taken black without milk)
  • Green tea
  • Diluting juice (zero kcal reduced sugar options)
  • Zero kcal/Very Low calorie per serving BCAAs and or amino acid drinks could be a fantastic option for preserving lean muscle mass during a fast.

How To Fast Safely

fast safely

Aim to progress into a 24 hour fast if this is what you’d like to try. Much like exercise, you don’t jump in at the hardest point and hope to be successful straight off the bat.

We ease ourselves in and steadily progress overtime as our body adapts.

Progressing from an intermittent fasting protocol to a full 24 hour fast may have the most successful outcome (when it comes to undergoing and adhering fully to a 24 hour fast for the first time).

Additionally, plan your meals for when you break the fast to avoid over eating/ binging.

Be mindful to consume enough water and other liquids (zero kcal variety) throughout your fast to remain hydrated as this may cause you to suffer from headaches as well as some other nasty issues linked to dehydration.

Additionally, it is important to be mindful that fasting can cause constipation, so ensure you are consuming enough fibre on your non-fasting days to mitigate this risk.

Fasting can be a useful tool but it is our energy balance (the calories we consume vs. That which we expend) that will always be the defining factor in weight loss.

Also, if you are diabetic or have poor glycaemic control, personally, we wouldn’t recommend fasting.

Read Also: 20/4 Intermittent Fasting Diet – The Keto Approach

We also wouldn’t recommend fasting for pregnant women, those with history of eating disorders and or those who are underweight.

As always, speak to your physician/doctor before you attempt any form of dietary protocol. Get blood tests and whatever else done prior to make sure you can perform this dietary protocol safely!

24 Hour Fasting Results


This year (Harris et al., 2018), a systematic review and meta-analysis (which is simply a study in which researchers collect all the available data on a certain subject, analyse it, and present a summary based on the data) was carried out on twenty-four hour fasting periods.

The study included data from alternate day fasting, fasting for two days, and up to four days per week research.

Their meta-analyses showed that intermittent energy restriction was more effective than no treatment for weight loss and equally as effective in comparison to continuous energy restriction.

Additionally, when we look at more clinical markers of health, a 2015 review by Tinsley & La Bounty found that “Alternate-day fasting trials of 3 to 12 weeks in duration appear to be effective at reducing … body fat (≈3-5.5 kg), total cholesterol (≈10%-21%), and triglycerides (≈14%-42%) in normal-weight, overweight, and obese humans.

Whole-day fasting trials lasting 12 to 24 weeks also reduce …body fat, and favourably improve blood lipids (≈5%-20% reduction in total cholesterol and ≈17%-50% reduction in triglycerides).”

This is important in a clinical sense as higher body fat, blood triglycerides and cholesterol are associated with increased risk of heart disease and metabolic syndrome.

The effect of fasting on exercise performance is incredibly varied and depends on a huge range of factors; type of exercise performed, duration of exercise, level of competition, the individual’s mood (as influenced by the fast), their current fed state (based on what they ate prior to engaging in a fast) and many other factors.

It is known however that nutritional intake around exercise is important in promoting the adaptive response in tissues to an exercise stimulus and can directly affect performance.

Fasting may be an effective tool therefore for weight management and or weight / fat loss but may not be an ideal tool for an individual who’s exercise and or sporting performance takes precedent over being a certain weight/body fat percentage.


Will I Get Hungry Fasting?

This is highly interdependent (aka it depends on the person) and depends on personal perception of “hunger”, but leptin will decrease during a fast.

There is evidence to show however that if we consume liquid (especially in large volume and or carbonated alternatives) which “fills our stomach” this can trigger the release of leptin and could be an effective way to counteract the hunger.

Additionally, caffeine is also effective at reducing perceived ratings of hunger and could also be a useful tool for dieters to use.

Will I have Energy To Workout While Fasting?

This depends on a whole host of factors. Typically, we do see a decrease in performance however again this can be addressed by utilizing caffeine and or even using a BCAA supplement (although caffeine will most likely have the greatest effect on maintaining/improving performance).

Can I take Supplements While Fasting?

take supplements

Yes. Fasting refers to the absence of energy intake, not total ingestion of nutrient, mineral, and hydration sources.

A very basic protocol to follow would be to have a daily; well-rounded multivitamin, zinc supplement (avoid zinc oxide supplements as they are poorly absorbed), vitamin D supplement and possibly then look to add a source of caffeine and BCAAs/Amino acids (both of which should come from zero to very low-calorie sources).

Does Fasting Cause Muscle Loss?

It depends on the length of the fast, the type of training you’re doing (if any), whether you are consuming protein to preserve lean tissue (such as the BCAAs we mentioned).

However, it shouldn’t be that much of an issue if you are intermittent fasting. 24-hour fasts may be a different story and it may be advantageous to perform very low intensity cardio or have your exercise rest days on these days.

Will I Go Into Starvation Mode While Fasting?


Starvation mode refers to the whole concept of metabolic adaptation and the answer would be; not to a significant enough level to which it wouldn’t be very quickly fixed.

This only depends on the type of fasting you’re doing also.

The longer you don’t eat/ the more restrictive you are/ the less energy which is available the more your body will adapt and down regulate caloric expenditure.

This shouldn’t be an issue with 24-hour fasts, and only a worry for more prolonged periods of fasting and or chronic severely reduced energy availability.


Fasting is a relatively safe, effective tool for weight/fat loss for the average individual/ gym goer/ athlete.

It should be used in conjunction with energy balance protocols to provide the best outcomes and pairing with a ketogenic diet may provide further benefit (if sustainable and feasible).

Reference studies for the results section