The Skinny on Rapid Weight Loss Diets (Part 1)

The Lay of the Land:

~1,700 words (6-10 minute reading time)

Take-away Points:

  • Even if one were to completely stop eating for a week, most people just don’t expend enough energy to burn through more than 3-6 pounds of pure fat per week (again, this is with NO food and assuming that ONLY fat is burned, which won’t actually be the case, because…)
  • Weight loss and fat loss are NOT completely synonymous (caloric deficits tend to lead to the loss of both fat AND lean mass, although there are ways we can tilt the balance) 
  • Metabolizing a small amount of lean mass (e.g. muscle) for energy translates to a relatively large decrease in body weight (~8 times more than that from an energy-equivalent amount of fat loss) due to lower energy density and the accompanying loss of water.
  • In short, the quick changes in scale weight that one might observe during a rapid weight loss diet might seem encouraging, but it is quite likely that a good portion of the weight lost is lean tissue (and accompanying water), which in turn likely means that a good portion of the weight will quickly be regained once a more normal caloric intake is resumed.

We’ve all seen them: the lose X pounds in Y days “plans”.

Usually, the premise is more or less: “You need to starve yourself for Y days if you are really serious about losing those X pounds, because weight loss isn’t supposed to be easy, right?! Oh, and definitely make sure you buy our product (or eat such and such magical superfood) too, because that’s, like, TOTES super important…”

Well, I’ve totes seen too many of these magical weight loss promises (the most recent one being a circulating piece on how to lose 10 pounds in 3 days). More importantly, I’ve seen enough people hopping on board with them that I felt like I needed to write this piece to point a few things out.

As an aside, I’m pretty hesitant to talk about weight loss a lot of times because I get pretty discouraged with the body shaming that has become so ingrained in our culture and what it can do to peoples’ senses of self-worth. That being said, I also clearly don’t feel that trimming fat because you want to improve your general health or because you happen to enjoy sculpting your body into a certain aesthetic mode is a “bad” thing.

Without delving too deeply into what is probably an issue for another time, the reason I am writing this pretty much boils down to this: if someone is dead set on losing weight (for whatever reason), I at least want to do what I can to make sure their efforts are not misguided or ineffective.

So, let’s get straight to the dire point.


I Feel Like I’m Taking Crazy Pills!

Just to clarify, let’s draw an arbitrary line for the context of this article and define rapid weight loss as a loss of anything more than 3 or 4 pounds per week.

Now, to immediately point out the problem with the premise of rapid weight loss diets, let’s make a few (over)simplifications and assume that the average human’s total daily energy expenditure is about 2,000 Calories and that you shed one pound of fat for every 3,500 Calories-worth of energy deficit that you create (one pound of human adipose actually contains closer to 3,750 Calories, and again, there are several potentially glaring issues with these blanket assumptions but I’ll get to these points in time if you’ll just bear with me).

Never mind for now that we could never actually count on all of the weight lost being purely from fat (if only…Ha!).

Moving on with these rough assumptions though, let’s say you turn the diet power up to 11 and just stop eating for a week. Literally no food for a whole week. In this perfect (and unrealistic) system, we would expect the dieter to lose 1 pound of fat every 1.75 days, or 4 pounds per week.

(3,500 Calories/pound) / (2,000 Calories expended/day) = 1.75 days/pound = 4 pounds/week

So, even if everything worked as seamlessly as in the above example (which it tends not to), most people’s bodies would simply not consume enough energy to burn more than what is contained in maybe 3-6 pounds (and 6 pounds is being pretty generous) of adipose tissue (fat) in a week, and that’s if they weren’t eating a damn thing the entire time.

Please, let that sink in for a moment.


But… Weight Loss Magic and Stuff

“If what you’re saying is true,” someone might ask next, “then how did I (or my friend, or that guy on Instagram, or whatever) manage to lose 10 pounds in a week, smart guy?”

This is an important question. How could someone lose more weight in a week than their body could be reasonably expected to burn from fat?

While there are several possible contributing factors (like the addition of exercise and the accompanying elevation in one’s total daily energy expenditure, for instance), the simplest and most likely solution is—as I alluded to above—that at least some of the weight lost was not from fat.

As it turns out, “weight loss” and “fat loss” are not completely synonymous terms. This is something that a lot of people overlook.

We have to keep in mind that other tissues (namely muscle) can also be catabolized (broken down) during caloric deficits, and with the loss of these tissues comes an even greater loss of water weight (this effect is less dramatic with the loss of adipose tissue, which tends to contain a lot less water than lean tissues such as muscle).

In fact, losing one pound of protein from muscle tissue (the catabolism of which would provide only ~1,850 Calories as opposed to the ~3,750 Calories provided by a pound of human adipose) can be expected to lead to the concomitant loss of around 3 pounds of water weight, on average (meaning a total loss of about 4 pounds).

In other words, metabolizing lean mass to provide energy can lead to a reduction in scale weight that will be about 8 times greater than what you would see if you filled that caloric deficit by burning up fat instead.

(1 lb. protein ≈ 1,850 Cal., and each lb. of protein lost is accompanied by 3 lbs. water, so 1,850 Cal. of protein burned ≈ 4 lbs. of weight loss; multiply that by two and you have 8 lbs. lost for 3,700 Cal. expended, which is about how much energy is stored in 1 lb. of human fat tissue)

Before anyone starts thinking this might sound like a positive thing (hey, faster weight loss, right?!), in case I haven’t been clear, losing lean mass is rarely desirable for general health, athletic performance, or the aesthetic that people are typically after when they decide to lose weight.

While these numbers can all fluctuate somewhat depending on ion balances, glycogen stores, muscle quality/size, and a plethora of other things, the point is that caloric deficits can lead to the loss of both fat and lean body mass, and that lean body mass loss (as well as changes in water retention that can occur as a result thereof or for a variety of other reasons) can account for much more drastic changes in total body weight than fat loss will be able to over a short time scale like a few days or weeks.

In short, while rapid weight loss plans might take some pounds off of your scale weight, they are NOT magical fat loss solutions, and a good deal of the weight you lose will very possibly be lean tissue and the water contained therein. This in turn means that you will likely regain a good deal of the lost weight as soon as you resume a more normal caloric intake and your body gains access to the resources to undo the damage.

There. That’s pretty much the punch line of this post. With the possible exception of severe, extenuating medical circumstances in which such drastic measures may become more prudent, rapid weight loss diets pretty much suck (at least for anything other than managing to transiently change the number reported by that troublesome little scale, but again, these changes do not translate to pure fat loss here).

So What’s the Alternative?

It’s not exactly exciting, but the alternative is pretty much just going into any weight loss (or rather, fat loss) endeavor with the understanding that it is not an exercise in instant gratification. Making meaningful changes to one’s body—be it shedding body fat or building muscle and strength—takes time.

It’s not a sexy answer, and it’s not what most people want to hear, but there it is: radical changes in body composition are not achieved overnight (or over 3 nights, or 5, or whatever).

At present, there is no “weird trick”, shortcut, or magic diet to change this. Sensationalism, be damned.

So, until someone actually does develop a pill stuffed full of fat-consuming nanobots or a wicked unnecessary gene therapy protocol for pushing you effortlessly towards being super lean no matter what, it seems that we are stuck with the boring, old-fashioned practice of working intelligently for what we want.

How quaint.

Still, the question remains: how fast can one reasonably expect to lose fat (as opposed to just losing weight, as discussed above)?

This is going to vary both from person to person and within the ebb and flow of an individual’s day-to-day life (as well as month-to-month, year-to-year, and so on). For instance, extremely obese individuals will typically be able to shed fat more quickly without negative consequences than those that are fairly lean or only slightly “overweight”. Basically, the less fat you have to start with, the harder it is to lose fat without also losing some lean mass.

Of course, it’s also important to keep in mind that there are several things one can do to promote the loss of fat and the preservation of lean mass during caloric deficits (as I’ve discussed here before). The crib notes are basically that resistance training, a high protein intake, a small caloric deficit (as opposed to a large one), and ample, quality sleep are all your good friends when trying to lose fat the “smart” way.

All of that being said, a decent place for most people to start seems to be dropping around a pound a week, or up to 1% of their total body weight per week. If you want a more detailed guide on how to accomplish this, read this and this!

In Summary

Losing 10 pounds of scale weight in 3 days? Doable, but not exactly meaningful (almost all of it will likely come right back as soon as you nourish and hydrate your body).

Losing 10 pounds of fat over the course of, say, 3 months? It doesn’t sound nearly as sexy, but this is the way to meaningful, sustainable changes in body composition.

If you don’t believe me, please, by all means, go try these things out and examine the outcome for yourself! And no, I don’t mean that spitefully. Experimenting is a great way to learn as long as you are honest with yourself about the results you observe. Just try to keep in mind that it is extremely easy to allow cognitive biases to twist and skew the truth to more closely resemble one’s expectations when it comes to these things (the same goes for most other things in life as well, really).

Just be sure to remember that weight loss and fat loss are not completely synonymous.

Next Time

Given that the number one piece of feedback I receive is that my articles are “freaking long, man”, I’m going to wait until my next post to delve into some of the nitty-gritty stuff I wanted to cover about the assumptions that I made at the start of this article (a 2,000 Cal. average energy expenditure and a 3,500 Cal. deficit equaling a pound of weight loss).

If you don’t want to miss it, sign up for the newsletter (and b-b-b-bonus, receive 3 FREE training programs)!!

As always, thanks for reading! If you enjoyed the work then please, For the Love of Lifting, share it around! Let me know in the comments below or on Facebook, etc. if you have questions or qualms concerning my take on rapid weight loss diets, or if there are other topics you’d like to hear more about!

Follow me for updates and other shenanigans: