You started seeing them pop up a few weeks ago.  It was probably slow at first - maybe just one Facebook advertisment or one little post on Pinterest:

But now that Memorial Day and the end of the school year are getting closer and closer, you're reminded that your body is just so not ready for the beach more often than Hallmark reminded you to buy a card for Mother's Day:

But let's get real here for a moment.

Yeah, it's nice to think that you can look like that model in 21 days, and the juxtaposition of her abs next to "21 Days" is 100% to make you feel like that's what you'll get...even though both you and the advertiser know you know better.

Unfortunately, deep down, your emotional brain doesn't give a damn what your logical brain has to say on the matter.  It wants to be sexually appealing and comfortable NOW dammit!

But as you've read in my Bullshit Detector ebook, you know it's my mission to give you the information to better arm your logical brain.  So let's ask the only actually relevant question about 21 day weight loss challenges:

How much can you really change in 21 days?

Is 21 days enough time to make any actual visual change?  Can you look better in your swimsuit in a mere three weeks?

Anecdotes won't help us here.  When you're trying to logically judge whether you should believe in a certain idea or purchase a product, the last place you want to get your information is from the person promoting it.

Think of it this way, have you ever applied to a job, and they ask you the question "what's your biggest weakness?"  Your answer and your parent's or former employer's answers are probably quite a bit different:

• You: My biggest weakness is that I'm a perfectionist. (Nice 'humble-brag' answer, good job!)
• Your Mom: Her biggest weakness is that she takes 3 hours to do her hair in the morning and always makes us late, she always makes sure to use her essential oil that grows hair.
• Former Employer: Her biggest weakness is that it takes her 3 weeks to do a project that should only take 3 days.

See how the message sounds quite a bit different depending on the perspectives and motivations of the person telling it?  This is one of many reasons you can't just Google "21 Day Weight Loss Challenge Results" and expect to get an unbiased opinion.

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Where can you get an unbiased opinion?  Glad you asked!

Scientific Studies!!

So let's run the gamut of ways you could try to change your physique in 21 days by looking at studies on various diet plans, starting with the most drastic:

Complete Fasts

What better way to understand how much you can actually change yourself in 21 days than at its most extreme - abstaining from all food for 3 weeks?

There aren't a ton of studies done on normal, healthy people fasting for extended periods of time...it's probably not always the easiest to squeak by the ethics committee.

But, you'll most certainly lose a ton of weight.  How much depends on what size you start out at, but it's going to be a combination of:

• Water
• Muscle Tissue
• Fat
• Lack of anything solid in your system (aka no poop. Yeah, it registers on the scale)

But if you're a healthy, ~120-130 pound,  21-25 year old female, you might lose about 17 pounds of a combination of the above substances. (1)

A few studies (2, 3) seem to indicate that you can expect about half of weight loss to be due to water.  (As well, you can reasonably expect to gain all the weight back later. (4) )

At the same time, you run the risk of significantly increasing your cortisol levels, becoming emotionally distraught, and fairly irritable...so you'll maybe look a little better in a bikini but you won't give a crap.  On top of that, can you imagine how miserable and bloated you'd feel after breaking your 3-week fast on some chili cheese fries?!

(To be fair, this study only had 5 subjects and most of them did just fine emotionally during the 3-week fast.  However, they were basically secluded during the research and not given options to consume food.  If you increased the sample size and tempted them with food at every turn - like the real world does - what would the outcome have been?) 

Alternate-Day Fasting

So let's say you want to do something more "reasonable", like say, only fast for 1 day at a time rather than 3 weeks all at once.

This is accomplished by people who do a particular version of "alternate-day fasting" where the idea is that you won't eat for 1 day, then you'll get to feast the next.  Most people don't eat quite double the amount of food on the feast day, so weight loss occurs.

In one study, patients participating in alternate-day fasting for 3 weeks:

• Lost about 2% of their original bodyweight. (5) (So, if you were 130 pounds, about 2.6 pounds)
• About 1.1% of that was fat.  (1.4 pounds for our 130 pound example)
• Around 0.9% was muscle and/or water. (1.2 pounds)

I know, I know after 17 pounds lost in complete fasting, 2.6 pounds sounds measly.  But keep in mind that you'll

• Have more energy for exercise and life in general
• Won't lose as much muscle
• Are still in a completely reasonable and good realm of weight loss.

...Unfortunately, some participants were still hangry and irritable on their fasting days.  Go figure! So this does not appear to be a diet that could be maintained for a significant period of time by yourself.

So how about you eat a little something everyday, but cut your intake by half?  That's like the best of both worlds, right?

Low Calorie Diets

Usually these kinds of interventions (along with their sibling, the VERY Low Calorie Diet) are reserved for the super-obese who need to lose about 50 pounds of pressure off their heart and lungs now, and are in a hospital setting.  But you need to look hot in a bikini.  So that's like, comparable in urgency, right?

Well, in an old but very well-known and interesting study, a group of men were put on a diet that involved 1/2 of their daily calories needs, and were also expected to walk 22 miles each week.  (6)

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This study is so well-known because of the harrowing pictures of the men after 6 months of this and their extreme emotional duress and depression during the process.  One man had to be eliminated from the experiment after just a few weeks for emotional instability.

Just how few calories were these men on to induce such dramatic weight loss and mental problems - surely something crazy like 600 per day, right?

It was ~1,600 calories per day.

To be fair, this "diet" extended over 6 months time, much longer than any 21 Day Weight Loss Challenge.  As well, the diet was mostly carbohydrates and basically no protein to prevent muscle wasting, described as:

...starvation diet reflecting that experienced in the war-torn areas of Europe, i.e., potatoes, turnips, rutabagas, dark bread, and macaroni.

Tasty.  Anyway, a summary of this experiment is:

• Participants ate a little less than half of their maintenance calories for 6 months.
• Diet was a high-carbohydrate, low-fat and low-protein diet.
• Participants walked 22 miles per week.
• Weight loss came out to an average of 1.5 pounds per week for 6 months.

...Doesn't this actually sound kinda familiar?

This diet sounds like an exaggeration of what many of us do to lose weight:

• Eat a low-calorie, low-fat, high-carb diet.
• Log in the miles via walking / running / cycling
• Do so for an extended period of time - possibly indefinitely

I'm not trying to say that you're going to end up looking like the man in the above picture - health problems and mental issues and all - by following the above regimen.  If it was up to you, your body wouldn't let you get to that point assuming you didn't have an eating disorder.

What I am trying to say is if you ever beat yourself up about "falling off the wagon" after "being so good" for 5 days...think about that in a new light.  Your body doesn't like running 3 miles a day on 1,200 calories of Special K with skim milk for weeks on end.  For most of us, your body will overpower your mind.  That's probably for the best.

Summary: So what can you accomplish in 21 Days?

Complete Fast: ~17 pounds if you're a young, normal-weight female.  Half of this will likely be water.  The rest will be a combination of muscle tissue and fat, depending on your activity.

Alternate-Day Fasting: ~2% of your bodyweight for healthy, normal-weight people, half of it being fat.

Low Calorie Diets (~50% of daily needs): The men in the study started at ~150 pounds and lost on average 1.5 pounds per week over 6 months.

If you eat a similarly high-carb diet, you can probably expect this or a little bit more over 21 Days as weight loss is typically greater during the first few weeks.

If you eat a low-carb diet of similar calorie levels, you can expect to lose more due to losses in water weight.  Note that this won't affect your appearance much.

The online phentermine tablet sales report has demonstrated that there are people who have gone through the diet pill experience and that you can control the weight gain to get better results to reach your goal.

Overrall: In all studies, subjects gained back all, or close to all, their weight back afterwards.  Some even ended up heavier. Across the board were reports of irritability and distracting hunger, though this wasn't the case for all participants.

Moral

Look, I know the quick fix is always tempting, especially when you want to look good for a particular event.  Marketers PREY on that.  Just read Harper's Jumpstart to Skinny book (essentially a 21-Day Diet) - he repeats over and over how much you know you want to look hot for the beach / reunion / wedding!

And I'll be real with you: it's not that you're likely to kill yourself on these diets. (Though there is seriously a reason why all the participants on these diets, especially complete starvation, were under constant medical surveillance.) They're short-term, and the negative physiological consequences tend to go away after a couple days of re-feeding.  (Though the mental ones can last much longer, which is my main concern) You'll probably even lose a fair amount of fat, an amount that you might even notice.

But now it's time for you to be real with me and with yourself: will it really make you that much happier to lose at most around 5-8 pounds of fat?  Do you want to exchange 3 weeks of stress and misery for maybe 1 good day at the beach, only to have to repeat the cycle every year because you'll likely gain the weight back from the crash diet?

Why not make this year the one that you do something permanent about your weight-loss goals by making small changes that add up over 365 days instead of huge changes that crumble after 21 days and leave you right where you started?

Maybe you have an iron will and you'll be really strict during your 21-Day calorie restricted diet, be really strict after, and keep the results.  But if that was the case...wouldn't it be likely you'd be at a weight you're happy with already?

If you made it this far, thanks for staying with me.  Leave a comment below, or share this with your weight-loss-hopeful friends and family if you enjoyed it!

P.S. While researching this article I came across this book on fasting.  I only read Chapter 4 about fasting for medical interventions, but it looks like a good resource on the use of fasting for various reasons throughout human history!

P.P.S. I realize that many of the studies I posted are old, or I only used 1 study where it would be better to use at least 3.  Many of them have small sample sizes...unfortunately finding 3-week long starvation or semi-starvation studies on healthy, non-obese individuals was really, really tough.  So I worked with what I could find.