What is the Best Diet to Lose Weight Fast?

People ask us all the time, “what is the best diet to lose weight fast?”.  To answer this question we first need to clarify 1) how quickly is fast to you? and 2) how much weight are you looking to lose? 


It would be great if you could lose a lot of weight in a matter of days but that isn’t mathematically possible (even with liposuction or surgery).  And furthermore, any weight that is lost in a day or two is going to be primarily water loss, not body fat.  Losing body fat takes weeks and months.  But before we discuss the best diet to lose weight (body fat) fast, let’s first discuss why it isn’t possible to lose body fat in just a few days, no matter how hard you exercise or how little you eat.



The vast majority of weight you lose on a diet in a day or two is going to be water weight.  I am going to make a very important point and if you take nothing else away from this article you should know that calories are very easy to consume but very difficult to burn.


Calories are Very Easy to Consume but Very Difficult to Burn


For example, you can eat hundreds and even thousands of calories in a matter of minutes but burning those same calories will take you hours and in some cases even days.  On the other hand, anyone who has consistently weighed themselves on a day-to-day basis knows that it is fairly easy to lose water weight through dehydration.  To illustrate this point, in Table 1 you can see how much weight you would lose by eating nothing all day long (0 Calories/Day) or by incurring a mild 2% Dehydration.  No matter how much you weigh (Lean, Class I, Class II, or Class III) it is far easier to lose water weight than it is body fat.  If you need to lose weight in one day, dehydration is the way to go.

Table 1. Subject Characteristics for Lean and Obese Subjects.  It is far easier to lose body weight through dehydration (i.e. 2% Dehydration) in one day than it is to lose fat by consuming 0 calories for the entire day.  Notice that the number of calories burned/day is called your Total Daily Energy Expenditure (TDEE) and increases with body mass.  The larger your body mass, the higher your TDEE and the easier it is to lose weight.  Data from Delany, JP; 2013 INT J OBES.


Physique athletes (i.e. bodybuilders and ballerinas) and those competing in weight class sports (i.e. wrestling, boxing, ultimate fighting) have made cutting weight into an art and a science.  Some strategies they employ include not eating or drinking anything, taking small sips of water and spitting it out, spitting into a cup, taking diuretics, exercising in a rubber suit or other heavy clothing, and exercising in hot and humid conditions (if available).  However, these techniques are not recommended for the general public, nor are they safe.  You can only safely lose 2-3% of your total body weight in a day.  And that’s it, there’s no doubling up and losing 2-3% today and 2-3% tomorrow as you would be dead in a matter of days.


On a side note I (Todd) once lost 7% of my body weight while running out of water 15 miles from home on a long road bike ride (I had already exhausted two water bottles and a camelback).  I should have stopped and called someone but I figured I was tough and could make it back into town.  I could only pedal with a fraction of the force I was accustomed to producing and when I got back into town I literally staggered into the nearest gas station.  My vision was blurry and it felt like I had drank 12 beers on an empty stomach.  After slamming a 32 ounce Powerade and laying on the cold, shaded concrete for twenty minutes, I rode the rest of the way home.  In hindsight, this was very stupid.  Please don’t do this.



One Week

The capacity to lose weight in one week is also going to be heavily reliant on losing water weight and not body fat.  Table 2 shows you that for Lean, Class I Obese, Class II Obese, and Class III Obese individuals that a 2% dehydration would result in a similar weight loss if you only consumed 1000, 800, 800, and 500 calories/day, respectively, for an entire week!  Losing water weight is much easier than losing body fat.


Table 2. Calculated Weight Loss per Week While Eating Between 0 and 2000 Calories/Day.  It is much easier to lose water weight in a short period of time than it is to lose body fat.


Weeks and Months

It is going to take weeks and months to lose body fat, so this is where your focus should be.  So what is the best diet to lose weight fast?  You’re not going to like this, but it depends.  There is no universally “best” diet and the cop out, easy answer that we can give you is that the best diet to lose weight fast is the diet that you can adhere to (stick to) over time but that isn’t very satisfying either.


When people ask us “what is the best diet to lose weight fast?” what they are really asking us is “what is the best fad diet?”  If you are familiar with our work, you’ll know that there actually isn’t an answer to that question, despite what many fad diet zealots will tell you.  The fad diet zealots have picked a proverbial horse in the race and they are going to ride that sucker till it dies, even if it is a loser (no pun intended).  Their livelihood depends on it and they have lied to themselves for so long that they are convinced that their diet is “the best” and no amount of data or evidence can convince them otherwise.  The fad diet zealot will tell you that in “their experience” a low carb or a low fat diet has been the most effective for their clients.  But the problem here is that “their experience” is not science and it is not data.  It is their opinion based on available recall and their underlying biases.


We’re here to tell you that The Science of Dieting has downloaded and categorized 1500+ scientific papers on 70+ diet types and meal timing strategies and found there to be very little difference between diet types and meal timing strategies in terms of short or long-term weight loss.  Whether you’re eating low-fat, low-carb or alternate day fasting all of these strategies can work for some, work great for others, and don’t work at all for most.


So instead of trying to answer an unanswerable question, we’d like to guide you through six common strategies to lose weight fast and to keep it off over the course of weeks, months, and even years.  To heighten the suspense, we’ll start off by talking about your worst option and working our way towards your best.


6.  Exercise:  Everyone knows (or at least they have been told) that exercise is a must if you are looking to lose weight.  But what people don’t tell you is that exercise, in and of itself, will lead to virtually NO weight loss (Table 3).  It is extremely difficult to burn calories but it is extremely easy to consume them.  Although overly simplistic, if you say that walking one mile = 100 calories burned and that 3500 calories = one pound of weight loss, that means you will have to walk 35 miles to burn one measly pound of body weight. 


     People that take up (or increase their amount of) exercise can expect to lose less than 2% of their body weight over the course of one year. Now don’t get us wrong, exercise is a key component to any weight management program and positively effects nearly every single system of the body, yet, if you’re looking to lose weight, your focus shouldn’t be entirely on exercise.  Exercise will increase your strength, overall fitness, and health but don’t expect it to help you lose weight.  Anecdotally, I’ve heard a number of people who have taken up marathon training actually gain weight rather than lose weight.


The vast majority of the weight that you lose from an exercise session is water weight.  We will need to write another post to give you a more thorough understanding of just how difficult it is to burn calories and lose body weight with exercise but I’ll leave you with an anecdote I like to share with my students.  On more than one occasion I have ridden 100 miles on my bike (this was back in the day).  I was no slouch and could maintain 20 miles per hour for 100 miles.  That means I was on my bike for five hours of pretty intense activity.  Guess how many calories I burned during that ride?  5,000.  Based on the 3500 calories = one pound of weight loss, I would have burned 1.43 pounds of body weight for five hours of really intense effort, effort that there is no way on earth I could achieve today!  The bottom line is exercise can be a great supplement/addition to any weight loss program but in and of itself will not lead to meaningful long term weight loss.  Exercise is for fitness not weight loss.  These two things are not the same.



5.  Caloric Restriction:  The total number of calories a person burns in one day (TDEE) is typically between 1800 – 3000 depending on a variety of factors including body size, lean mass, body surface area, gender, hormones, and age. 


     We define caloric restriction as eating fewer calories than your body needs to maintain your weight.  Caloric restriction can be of various levels, but we would define minor caloric restriction to be just below the number of calories you need, typically this means you consume somewhere in the range of 1200 – 1800 calories/day (Table 3).  With this level of restriction and depending on your body size/lean mass, you can expect to lose 1.5 – 3 pounds/week.  As a reminder, the larger you are as an individual, the greater your total daily energy expenditure (TDEE) is and therefore your calorie deficit will also be greater and the ability to lose weight will also be greater.  That’s a lot to keep track of but basically it means that the larger you are, the more calorie burning power you have and the easier it should be to lose body fat.  The vast majority of fad diets (i.e. Atkins, Paleo, Keto, Weight Watchers) will fall into the caloric restriction category.



4. Low Calorie Diets (LCDs):  In low calorie diets you further restrict your calorie intake to between 800 – 1200 calories/day (Table 3).  On a LCD, again depending upon body size/lean mass you could expect to lose between 2 – 4.5 pounds/week.


     LCDs take more care and planning than does minor caloric restriction.  There is a much greater weekly weight loss payoff, but there is also far less margin for error.  For that reason, meal replacements such as bars, shakes or tv dinners typically make up 1-2 meals/day and the remainder of your food is made up of “real food” that you’d make for yourself.


     The reason that meal replacements and tv dinners are so valuable during a low calorie diet is that they are low calorie, easily prepared, nutrient dense, and most importantly portion controlled to keep you at your low calorie level.



3.  Very Low Calorie Diets (VLCDs):  In very low calorie diets you will consume < 800 calories/day (Table 3).  Consuming so few calories requires physician supervision and are generally only available through medical centers.  In VLCDs, you can expect to lose 3 – 5 pounds per week, again depending upon body size and your associated metabolic rate.  In general, the larger the body, the greater the metabolic rate and subsequent weight loss.


     Due to their excessive restriction, VLCDs are typically only available as meal replacements and the duration of the VLCD is oftentimes limited to 8 weeks or less depending on how well the diet is tolerated before transitioning back into to a LCD.  It doesn’t matter how much willpower you have, eating so few calories is not a sustainable weight loss solution.  Your drive to eat will force you out of a VLCD at some point in time.



2.  Weight Loss Surgery:  Although one could say that weight loss surgery isn’t a type of diet, it most definitely controls the way in which you eat.  There are a number of different types of weight loss surgeries each with different levels of severity (i.e. gastric bypass, duodenal switch, sleeve, lap band).  The greater the severity – the amount of intestine/stomach that is bypassed, the greater the malabsorption, and the greater the weight loss.


     For the first two weeks after surgery no more than 400 calories are consumed.  From 2 – 8 weeks after surgery 500 calories/day are consumed, and after that it is recommended you consume no more than 1000 calories/day.  Your stomach (pouch) can only hold so much food/liquid at one time.  In essence weight loss surgery forces people to reduce the amount of food they eat.  There are a number of restrictions that we won’t get into in this article but let’s just say that weight loss surgery enables/forces people to eat a VLCD due to their new gastrointestinal anatomy.



1. Total Starvation: The number one way to lose weight fast is to undergo total starvation.  Eating zero calories will lead to faster weight loss than eating 500 calories and will be faster than eating more than 800 calories and exercising.  This may seem a little “tongue and cheek” to you but we wanted to run through all the biological options, including total starvation, which was much more commonplace during our evolution than what we see today.


When I ask people how long they think it is possible to go without eating most people will venture to guess 2-3 weeks max.  This is only true in people with extremely low body weights (i.e. anorexics).  The vast majority of us can go many weeks and months without eating and live to tell about it.  The longest documented record of going without food is 382 days.  Granted this individual started his journey at 420 pounds and took a daily multivitamin; however it goes to show that eating every day isn’t necessary.  Now, with that being said, I enjoy eating and I hope you do to.  Prolonged fasting was in vogue up to the 1960s as a weight loss treatment but I do not recommend going this route for safety reasons.


Table 3. Diet Types, Level of Restriction, and Expected Weight Loss/Week.  Every single diet type out there (even fad diets) fall within one of these categories: energy balance, caloric restriction, low calorie diets, very low calorie diets, depending upon the level of caloric restriction.  Note: Weight Loss/Week is based upon Class III Obesity.  For Lean, Class I, and Class II, please refer to Table 2.


People (and even experts) get caught in the trap of worrying about the ratio of macronutrients (carbs, fats, proteins), the timing of eating (morning, evening or every other day) or the quality of those foods (the level of processing) has on weight loss.  The number of calories you consume and burn is what matters.  Those other factors are less significant, second order considerations.  If you don’t have calories in/calories out under control, nothing else matters.  To me, the big question is, how much discomfort can you tolerate and how long can you tolerate it to maximize weight loss during a given period of time.


The big question is, how much discomfort can you tolerate and how long can you tolerate it?


An individual with a BMI of 46 (Class III Obesity from Table 1) could potentially lose between 2.5 and 6.5 pounds/week by eating 2000 or 0 calories/day respectively (Table 4).  If that same individual continued consuming between 0 and 2000/day for 8 weeks, he/she could expect to lose between 19.9 and 51.9 pounds.  From a purely mathematical, on paper, perspective if given the choice of losing 19.9 pounds or 51.9 pounds, you are surely, 100% of the time going to choose to lose 51.9 pounds.  But with that being said, you have to keep in mind how much discomfort you are willing to tolerate for how much reward (weight loss) you can gain.

Table 4. Potential Week-to-Week Cumulative Weight Loss in Class III Obese Individuals at Daily Calorie Levels Between 0 and 2000 Calories/Day.


If eating 800 calories/day means I can lose 39.1 pounds after 8 weeks and further reducing my calorie intake to 500 calories/day is only going to yield an additional 4.8 pounds of weight loss (43.9 – 39.1), I am going to choose to eat the additional 300 calories/day.  The additional discomfort of restricting my calorie intake to 500 calories/day just doesn’t seem worth it to me.



Other Things to Keep in Mind

The information we have shared with you today has a number of caveats that must be considered. 


1.      3500 calories does not necessarily equal one pound of body weight loss.  The general rule of thumb in the health & wellness industry is that 3500 calories equals one pound of body weight loss.  But this is only a rule of thumb.  To be honest, we really do not know how many calories equals one pound of body weight loss.  3500 calories gets us in the ballpark but it could be a few hundred less and it could be a few hundred more.  We utilized 3500 calories to make our weight loss estimations because there is nothing else to go off from and there are too many variables in the published scientific literature (i.e. age, gender, body fat, lean mass, hormones, physical activity level, diet) to be able to provide generalized recommendations.  With all that being said we stand behind our calculations as they are the best we can offer at this point in time.


2.     We assume perfect dietary adherence.  Our calculations are based upon the assumption that you are eating exactly the number of prescribed calories/day.  Not more, not less.  Life is not that straightforward and unless you are a robot, you will have days where you will eat more and days you will eat fewer calories.  If you are heavily restricting your calories (i.e. < 1200) chances are you will have plenty of days where you go over your calorie goals.


3.     As your body mass decreases with weight loss, so does your metabolic rate.  The larger you are, the more body mass your body has to maintain and take care of and the greater the number of calories you will require on a daily basis (see Table 1 to compare lean to Class, I, II, and III’s TDEE).  As you lose weight, your metabolic rate naturally decreases and so does the difference between the number of calories you consume and the number of calories you burn.  For example, in looking at Table 1, what would happen if you decreased your body mass from Class III to Class I?  Your TDEE is nearly 400 calories lower!  Even if you continued consuming the same number of calories, you are burning fewer calories because of your reduced body size (there is less tissue to maintain and maintaining tissue is metabolically expensive).  We did not take this into account in our modeling as it is far more complex than the scope of this article.


4.    The vast majority of the weight you lose on a diet will come within the first two months on that diet.  Although beyond the scope of this article, the vast majority of the weight you lose on a diet (upwards of 80 – 90%) tends to occur within the first two months on a diet.  Thereafter weight loss tends to decrease and by 6 months, you tend to start gaining some of the weight you lost back.  This is something to consider when choosing a weight loss program and we will talk more about this fact in another article (but as a plug, we cover this extensively in Todd’s 12 Steps to Healthy Eating).




What is the best diet to lose weight fast?  The one with the lowest number of calories that you can stick to for the longest period of time.  This might mean a low calorie diet for some, it might mean a very low calorie diet for others, and for some it might mean weight loss surgery.  The point is, there is not a best diet to lose weight and you have to weigh out the pain for the gain and at the same time be realistic about how much weight you can actually lose on any given diet based upon the number of calories you are likely to consume.  Losing weight is hard, no matter how you go about it.  But you need to be honest with yourself and realistic in your goals.  As we saw in table 4, even if you didn’t eat any calories for 8 weeks (total starvation), you are not going to lose more than 50 pounds.  Weight loss programs and testimonials that flaunt people that losing 80 – 100 pounds quickly are just not true.  It is mathematically impossible to lose that amount of weight, that fast.  This also doesn’t mean that you can’t lose 100 pounds but you have to keep in mind that it is going to take you a year or two to accomplish this.


We hope you enjoyed reading this article and have a better understanding of your options and what you’re up against.  If you’d like more information please contact us or reach out to us at Phoenix Fitness Denver and/or The Science of Dieting.