Will Intermittent Fasting Help You Lose Weight?


*Learn what the science really says about intermittent fasting and weight loss.*

You’re curious.

You’ve heard how people are using intermittent fasting to lose massive amounts of fat, get shredded, and eat whatever they want.

You like what you hear, but you know that [anecdotes aren’t good evidence](https://evidencemag.com/health-research-podcast). You’re also skeptical after years of being told to eat small, frequent meals to control your appetite and blood sugar levels.

Maybe you’re tired of eating so often, so you dig deeper.

You notice more and more diet books that are based on or recommend intermittent fasting, like:

*[The 8-Hour Diet](https://www.amazon.com/The-8-Hour-Diet-Disappear-Watching/dp/1609615905)*

*[The 5:2 Diet](https://the5-2dietbook.com/)*

*[The Warrior Diet](https://www.warriordiet.com/)*

*[Man 2.0](https://evidencemag.com/engineering-alpha/)*

*[The Fast Diet](https://thefastdiet.co.uk/)*

*[The Fast-5 Diet](https://www.fast-5.com/)*

You see claims that intermittent fasting can help you “…watch the pounds disappear without watching what you eat!”(1) Others say that intermittent fasting will help you lose more fat and less muscle during a diet.

Yet you hesitate. You’re not sure which of these claims are backed by good science, and how many are magical thinking.

That’s what you’re going to learn in this article. Here’s a quick primer on intermittent fasting before we begin.

### What is Intermittent Fasting?

Intermittent fasting is a nutrient timing strategy where you don’t eat for a short period (~12-48 hours) followed by a shorter period where you’re allowed to eat (~8-24 hours). There are several different versions of intermittent fasting, but they all involve a period of caloric restriction and a “feeding window.”

Let’s look at a few of the more popular intermittent fasting protocols as examples.

### 7 Popular Approaches to Intermittent Fasting

Here are a few examples of popular intermittent fasting strategies.

**1. Alternate Day Fasting**

36 hour fast. 12 hour feeding window.

**2. [LeanGains](https://www.leangains.com/) by Martin Berkhan**

16 hour fast. 8 hour feeding window.

Martin has a more scientific and skeptical approach than most other intermittent fasting proponents. He also doesn’t discount the [importance of calories](https://evidencemag.com/why-calories-count) or macronutrients.

**3. *The 8-hour Diet* by David Zinczenko**

The same 16/8 timing schedule as LeanGains.

**4. *[Eat Stop Eat](https://www.eatstopeat.com/)* by Brad Pilon**

24 hour fast twice per week.

Similar to Berkhan, Pilon is more reserved in his claims and still understands that calories count.

**5. *The Fast-5 Diet* by Bert Herring**

19 hour fast. 5 hour feeding window.

**6. *The 5:2 Diet* by Kate Harrison**

Two days per week, men eat 600 calories and women eat 500. You eat normally the rest of the week. This is closer to intermittent caloric restriction than intermittent fasting.

**7. *[The Warrior Diet](https://www.amazon.com/gp/product/1583942009/ref=as_li_tl?ie=UTF8&camp=1789&creative=390957&creativeASIN=1583942009&linkCode=as2&tag=armlegsathl0b-20&linkId=UXFLAIIZYVWKZZTQ)* by Ori Hofmekler**

20 hour fast. 4 hour feeding window.

*The Warrior Diet* really isn’t intermittent fasting given that you’re allowed to eat [fruits and vegetables](https://evidencemag.com/healthy-diet) during the fasting window, but it’s close.

Now let’s see why these strategies are supposed to help you [lose weight](https://evidencemag.com/weight-loss-habits).

### 3 Reasons Why Intermittent Fasting is Supposed to Help You Lose Weight

Depending on who you ask, you’ll generally hear three primary claims about intermittent fasting for weight loss. In order of least plausible to most, intermittent fasting is supposed to…

1. Help you lose fat without a [caloric deficit](https://evidencemag.com/fat-loss-deficit).

2. Help you [lose more fat and less muscle](https://evidencemag.com/layne-norton-peaking) given the same caloric deficit.

3. Make it [easier to sustain a caloric deficit](https://evidencemag.com/hunger-control-podcast), and thus lose more fat.

Let’s look at each of these claims in turn.

### Is Intermittent Fasting the Secret to Losing Weight without Cutting Calories?


There is no evidence that intermittent fasting will help you lose weight without a caloric deficit.

Studies that use intermittent fasting or continuous calorie restriction show that people lose about the same amount of weight.(2-5)

In fact, controlled studies have shown that changing your meal frequency in any way — either eating more often or less — has no impact on energy expenditure or weight loss.(6-10)

People can and do lose weight with intermittent fasting, but they’re also in a caloric deficit. The amount of weight they lose is also [proportional to the size of their caloric deficit](https://evidencemag.com/calories-count/).(3-5,11-13)

There is a single study that found when subjects ate only one meal per day instead of three — without cutting calories — they lost 4.4 pounds of fat and gained almost 2 pounds of lean mass in 8 weeks.(14)

However, there are some [major problems](https://alanaragon.com/an-objective-look-at-intermittent-fasting.html) with this study that make these results meaningless. The most glaring limitation is that the researchers measured body composition with bioelectrical impedance, which is [notoriously inaccurate](https://weightology.net/weightologyweekly/?page_id=218)(15-17) — especially on fasted people.(18)

A previous study that used dual-energy X-ray absorptiometry (DXA) — a far more accurate method — found no difference in body composition after 14 days of alternate day fasting when calorie intake was held constant.(19)

At least three studies have found that resting [energy expenditure](https://evidencemag.com/calorie-needs) increases after about 36-72 hours of fasting.(20-22) However, the increase is minor, and unlikely to have a significant impact on weight loss. We also don’t know if these changes continue over time with repeated fasting, or if they’re offset by other changes associated with intermittent fasting.

No controlled studies have found people eating the same amount of calories lose more weight with intermittent fasting, so it’s unlikely the small boost in metabolic rate will help you lose fat.

There is a small amount of evidence that suggests some forms of intermittent fasting may hinder weight loss. Several studies have shown that an irregular meal schedule can reduce the thermic effect of food, increase energy intake, and cause a larger insulin response after eating (which indicates the people were more insulin resistant).(23-25)

Another study on alternate day fasting found a decrease in resting metabolic rate of 59 calories per day. The people still lost just as much weight as the group eating a normal meal schedule, but the researchers concluded that it may lead to weight gain if the study had been longer than two weeks.(26)

Most other studies have found no change in resting metabolic rate while fasting, so it’s unlikely the results from this study are meaningful, or even present in most cases.

The bottom line is that changing your meal frequency — including intermittent fasting — doesn’t affect how much weight you lose at the same calorie intake.

### Intermittent Fasting is Still Dieting

Many authors claim that intermittent fasting will “help you lose weight without dieting.” Intermittent fasting, by definition, is a modification of your eating structure, which means it’s a diet. Saying that you can lose weight without dieting implies calories don’t count, [which they do](https://evidencemag.com/why-calories-count).

There is no evidence intermittent fasting will help you lose weight without cutting calories, or lose more weight on the same caloric deficit.

However, you’re a smart dieter, and you know that weight loss isn’t the only thing that matters. You want to [make sure that the weight you lose comes from fat, not muscle](https://evidencemag.com/fat-loss-podcast/).

Let’s see if intermittent fasting will help you get shredded.

### Will Intermittent Fasting Help You Lose More Fat —  Rather than Muscle — on a Diet?

Not based on the current evidence.

A review in 2011 compared 5 published (and 2 unpublished) intermittent calorie restriction studies to 11 of those on daily calorie restriction.(2)

*Intermittent calorie restriction* isn’t the same as *intermittent fasting*, but it’s close. With the former, you’re allowed a small number of calories. With the latter, you’re generally not supposed to eat any. *Daily calorie restriction* is what most dieters do — eat slightly less every day.

The researchers found that people in the studies on intermittent calorie restriction tended to lose less muscle mass than those who cut calories every day. In general, those who used intermittent fasting or calorie restriction lost only 10% of their lean body mass. The people who used daily calorie restriction lost around 25% of their lean mass.

At first glance, this seems like firm proof that intermittent fasting helps you lose more fat and less muscle while dieting.

However, there are six reasons this is probably not the case.

1. Many of the studies were either completely, or largely uncontrolled. They also often relied on self-reported food intake. This opens the possibility that the subjects were consuming different amounts of protein and calories, which alone could explain the differences in body composition.

2. Most of the studies on daily calorie restriction used DXA or MRI to measure body composition, which are considered some of the most accurate tools. In contrast, the studies on intermittent calorie restriction and intermittent fasting generally used bioelectrical impedance (BIA). As you learned a moment ago, BIA is much less accurate, [especially when measuring changes over time](https://weightology.net/weightologyweekly/?page_id=218) and on fasted people.(15-18)

3. Only one of the studies (still unpublished) used exercise. It’s possible that any differences in body composition between intermittent fasting and daily calorie restriction would have disappeared if the studies had included exercise, especially [strength training](https://evidencemag.com/how-to-build-muscle-podcast).

4. The longest trials on daily calorie restriction were 24 weeks, whereas the longest trials on intermittent calorie restriction were only 12 weeks. Most were far shorter than that, where rapid shifts in water weight could have further confounded the results. Shorter studies can also amplify results of any kind.

5. Most of these studies were on overweight and obese people. It’s not clear if these findings apply to leaner individuals.

6. The researchers were comparing results between completely different studies. This means it’s possible that the differences in the percentage of muscle loss could have been due to differences in study design.

Based on these limitations, these findings aren’t strong proof that intermittent calorie restriction or intermittent fasting spares muscle mass. It’s more likely that the changes were due to differences in [protein intake](https://evidencemag.com/dieting-protein-needs) and measurement error, rather than some “special” repartitioning effect.

A few studies have directly compared intermittent calorie restriction and intermittent fasting with a linear decrease in calorie intake. They found no difference in the percentage of fat or muscle lost while dieting.(3,4)

In contrast to the idea that intermittent fasting spares muscle mass, two studies have found that eating the same number of calories more often spares more lean mass during a diet.(27,28) However, there are some [major problems](https://www.bodyrecomposition.com/research-review/meal-frequency-and-energy-balance-research-review.html) with both of these studies that render their results basically irrelevant.

At this point, there’s no good evidence intermittent fasting is more muscle sparing than a normal eating pattern during a diet.

However, intermittent fasting may help you lose weight for other reasons.

### Will Intermittent Fasting Help You Create a Larger, More Sustainable Caloric Deficit?

Maybe. It depends on your preferences.

Most studies have found that people are able to stick to intermittent fasting regimens in the short-term. They also tend to lose a decent amount of weight.(4,11-13,29-33)

The longest trial yet was six months. It found that overweight women who used intermittent fasting or a linear calorie decrease lost about the same amount of weight and were able to adhere to both diets equally well.(3) However, a higher percentage of people in the intermittent fasting group complained of [hunger](https://evidencemag.com/hunger-control-podcast), poor concentration, bad temper, and [preoccupation with food](https://evidencemag.com/weight-loss-food-thoughts).

There is some evidence that intermittent fasting, or any form of lower meal frequency, might help some people control their appetites better, though the data goes both ways.

Eating less than three meals per day generally increases hunger, but eating more than that doesn’t usually decrease it, either.(9) Of course, there’s no reason you can’t eat three meals per day *and* use intermittent fasting.

Two studies have found that people who ate three meals per day felt fuller throughout the day than those who ate six meals per day.(34,35)

Other studies have found no difference in appetite (36-38) or energy intake (2-4,11) between more or less frequent meals.

One study found that eating once per day increased hunger levels compared to eating three times per day, but that’s pretty extreme.(14)

Another study found that hunger did not decrease after 22 days of alternate day fasting.(32) On the other hand, a more recent study found that people became significantly less hungry during the “fast days” after about two weeks into eight weeks of alternate day fasting.(12)

Most data also indicates that even though people may not always stop being hungry while intermittent fasting, they usually don’t overcompensate when it’s time to eat.(3,4,7,11-14,29-31,39)

Several studies have found that alternate day fasting can increase hunger and irritability.(3,32) If you don’t like going long periods without food, or you’re tempted to [overeat](https://evidencemag.com/binge-eating-prevention) after fasting, it’s probably not a good long-term strategy.(39)

A few studies have found that eating *more often* helps people eat less at a later meal.(40,41) However, these studies were very short, the meals were low in protein, and the protocols didn’t really reflect how most dieters eat in the real world.

It’s possible that intermittent fasting, or reducing your meal frequency, could help prepare you for times when you need to “just say no” to extra food. From a psychological standpoint, not having to think about food, pack meals, or eat more than a few times per day can also be quite liberating.

How much you eat during a diet is important, but [so is how much you move](https://evidencemag.com/eat-less-move-more). You also need to consider how intermittent fasting might affect your activity levels and exercise performance.

A few studies have shown that intermittent fasting doesn’t seem to change how much people move throughout the day.(3,7,12-14,19,30)

However, most of these people in these studies were barely moving as it is. They were often only walking or burning a few hundred calories per day. It’s possible that if they had been more active, fasting could have had a negative impact. At this point we don’t know.(2)

In general, intermittent fasting doesn’t seem to affect how many calories you burn through daily activity.

Studies have shown that athletes often perform worse during Ramadan,(42,43) where Muslims don’t eat or drink during the day. Some of this might be due to dehydration, but other studies have shown [athletes perform worse after fasting despite being allowed to drink](https://anthonycolpo.com/intermittent-fasting-and-the-crazed-rantings-of-martin-berkhan/).(44-47)

People also tend to be less alert, cheerful, and wakeful during Ramadan.(48-50) Much of this is probably because of dehydration, lack of sleep (they have to eat more during the night), and the fact that they aren’t used to fasting, but it’s worth considering.

None of these studies during Ramadan were very well controlled, so they’re probably not a great reflection of how most people will respond to intermittent fasting. People often don’t eat less during Ramadan, and sometimes they even eat more.(39) However, this data does indicate that not everyone is going to be happy while fasting. Some athletes feel horrible while intermittent fasting, and others feel fine.(51)

There is a huge amount of data showing that exercise can help you lose weight and maintain muscle mass during a diet.(52-60) If intermittent fasting significantly compromises your ability to exercise, it’s probably not a great plan.

You have to find what works for you. If intermittent fasting or reducing your meal frequency helps you manage your hunger levels, doesn’t make you feel lethargic, and doesn’t interfere with your training, then go for it.

Tip: If you want to try intermittent fasting, it’s probably a good idea to train on the days or times when you eat. If you don’t mind fasted training, then train whenever you prefer.

### Will Intermittent Fasting Help You Lose Weight?

If it helps you create a caloric deficit, then yes.

Intermittent fasting, or changing your meal frequency in any way, is not going to help you lose fat without a caloric deficit. No deficit = no fat loss.

There is also no credible evidence that intermittent fasting will help you lose more fat and less muscle while dieting.

There *is* evidence that intermittent fasting, or reducing meal frequency, may help some people better control their appetites. If intermittent fasting helps you control your hunger, avoid obsessing about food, gives you more structure and confidence in your diet, and helps you maintain a caloric deficit, then it’s worth trying.

**Now here are a few questions for you:**

Do you think intermittent fasting has any “special” effect on body composition?

Have you tried intermittent fasting to lose weight?

Did it help or hurt?

What effect did it have on your appetite, mood, and ability to exercise?

Leave your thoughts in the comments section below.

> Did you enjoy this article? [Click here to check out my book, *Flexible Dieting](https://evidencemag.com/flexible-dieting-book)*. Want an even more in-depth education on how to lose weight, build muscle, and get stronger and healthier? [Join Evidence Mag Elite](https://evidencemag.com/elite) and get member’s-only reports and interviews.

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  1. ceekaye on December 8, 2015 at 9:38 am

    For 8 weeks i ate a plant-based, SOS-free diet (no Salt, Oil or Sugar). I consumed at most 1300 calories a day…i lost 8 pounds the first 3 weeks, and the scale didn’t budge for 6 weeks following. That is until i adopted a couple of strategies: I eat between 12 and 6 pm daily, and i my intake of food is mainly unchanged. I favor eating beans and greens….and i follow Ray Cronise’s advice about adapting my body to cooler temperatures to let my body’s BAT do some work (mild cold stress has been shown to increase the metabolism by as much as 17%,) When I started these two things on November 17th I weighed 171 pounds. it’s a little over 2 weeks later and today I weigh 160 pounds. I’ve lost 11 pounds inside of 3 weeks. I’d say that many of those conclusions are missing some controlling aspect, because it is certainly working like crazy for me!! You may want to keep an eye out for the latest research papers on the subject of metabolism human over-nutrition. The peer-reviewed paper is entitled “Metabolic Winter Hypothesis”. (Raymond J. Cronise, BS,1 David A. Sinclair, PhD, and Andrew A. Bremer, MD, PhD4) Meanwhile check out this blog (www.thermogenex.com).

  2. Kinsen Siu on January 7, 2018 at 1:15 pm

    I’m doing a full water fast right now and have been able to exercise at a reduced rate (both volume and intensity). It’s interesting to me that I lost the most weight when I sat around doing nothing when I was semi snowed in. Very anecdotal. Having said that I do think that IF is a tool like any tool in one’s toolbox. Perhaps for people who have no issues with keeping their diet adherence dialed in then IF might seem gimmicky. For those who struggle with that and being diligent with their food journal due to work, family, life circumstances, etc then IF might be a relatively simple way to reduce calories. Another anecdotal perspective is a friend of mine who is a busy managing director. He fasted for 2 days a week with 600 calories and ate “regularly” the other 5 days. What he noted was that on the other five days his appetite appeared to be decreased and it was a simple matter for him to just eat less. I think an underrated part of a fast is the mental portion depending on how mindful you are i.e you’re thinking about your new found power over food rather than obsessing over it, I do think you can come out of it with a mental edge over some of your food habits that you know are bad but haven’t gotten a handle over.

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