Weight loss versus fat loss: important difference or pedantic point?


Weight loss, fat loss, who cares? They are essentially the same thing…right? While it might seem pedantic at first, the difference between the two terms is quite important, despite the fact that most people (myself included) tend to use the terms interchangeably—we most likely mean fat loss. If this does not make immediate sense, fear not! My plan today is to illuminate why the idea of weight loss, by itself, is not the entire story, and that fat loss is arguably the more important metric by which we should judge a “weight loss” diet. In doing so, I also aim to show you what the body is comprised of (i.e. body composition); a handful of different ways we can measure it; and, somewhat counterintuitively, why weight by itself actually is an important metric and how you can use it intelligently to achieve your dieting goals. What will become evident, as will be the case in most if not all of my posts, is that things are a little more complicated than they appear.    

First: a simple example

To begin to illustrate a point in the simplest way possible, envision the following scenario: You wake up in the morning, minimal clothing, stumble to the bathroom and weigh yourself on your scale. Let’s just say you weigh 150 lbs. (A weight chosen to equally offend everyone.) You then proceed to “decaffeinate” (as my former PhD advisor would say), as well as take part in the process of egestion. (You guessed it, the exact opposite of ingestion.) You then weigh yourself again: 147.5 lbs. Within minutes no less! Now, obviously you didn’t lose 2.5 lbs. of body fat. If only it were that easy…  

What we just illustrated in that ever-so-relatable example is the concept of body weight and the beginnings of the fact that your body’s weight is made up of more things than just fat. Which brings us to the concept of body composition.   

Body composition: what you are (quite literally) made of 

The absolute “Gold Standard” for evaluating body composition is, well…dissection. Obviously, this is not practical for evaluating the composition of the body in living people, but historically, these types of analyses have been done, and the body compartments have been categorized and inventoried.   

From the most superficial level of analysis, we can categorize your body into two compartments: fat and then everything else. This is literally called a two-compartment model, and many of the techniques used to estimate body composition in living humans are based on this fundamental concept [1].  

Thus, you are made up of fat mass (FM) and everything that is, by definition, not. Everything that is not fat is lumped together and termed fat-free mass (FFM). (How clever.) I should note that you will often hear people use the term lean body mass (LBM) interchangeably with FFM. For the purposes of this piece, I will consider the two equivalent, although, technically, they are not. (We have our pedantic plates full enough as it is!)

To place the two-compartment model in mathematical terms, your total body weight (TBW) is as follows:


Simple enough. Well, sort of. In reality, the FFM portion can be delineated even further into everything that it is comprised of—i.e. body protein that makes up skeletal muscle and your organs; minerals that make up your bone; glycogen (the storage form of carbohydrate); and, as already illuminated at the beginning of this piece, body water. Additionally you have weight that comes from residual, undigested food in the GI tract (i.e. feces), but this is not a part of your body, per se. Thus, our simple two-compartment model can be expanded to incorporate more compartments that are better defined:

TBW = Fat + Protein + Bone + Water + Glycogen

And there you have it, a full breakdown of all the components of your body that contribute your body’s weight. For the average “Reference Man” this amounts to ~15% body fat, ~45% muscle mass, ~15% bone, and ~25% other (i.e. water and glycogen for the most part); and for the average “Reference Woman”, body composition amounts to roughly ~27% body fat, ~36% muscle, ~12% bone, and ~25% other [2]. Obviously, there is wide variation in these averages, but men tend to be bigger and have more lean tissue whereas women are on average smaller and have proportionally more fat tissue.   

Returning to our initial distinction between weight loss and fat loss, it now it obvious that a simple weight change on the scale can indicate a change in any one of these body compartments. Clearly, when people engage in a weight loss diet, their goal isn’t to lose body water—and certainly not muscle mass, organ weight or bone mineral density, despite all compartments decreasing during an energy deficit. Rather, the goal is, implicitly, to lose body fat. That is what people want.   

Why the difference is critical

Having gone through the ins and outs of body composition, this brings us to the crux of why the distinction is so important (if it was not obvious already), as well as why the distinction is completely useless for most people.

First, why it is important.

For most people, and much of what I discuss on this site, it has to do with claims about diet ‘X’ and how advantageous it is for “weight loss.” What you really want to know (as beautifully illuminated by yours truly) is how effectively it allows you to lose body fat (and maintain LBM for that matter). Objectively, on a scientific level, this is all we care about when those claims are made—that is, unless we are looking at some other aspect of the diet independent of effects on body weight/composition.

To illustrate, take the claim that low-carbohydrate diets lead to greater weight loss compared to higher-carb weight loss diet in the initial phase of the diet. On the surface, this claim is absolutely true. Weight is certainly lost to a greater degree compared to diets higher in carbohydrate and lower in fat (assuming a similar caloric deficit). Now, understanding the difference between body weight and body composition, you now hold the tools to critically evaluate this claim.

Indeed, when we look at what contributes to the initial rapid weight-loss seen during a low-carb diet, it becomes painfully obvious that a lot of the initial losses are due to depletion of glycogen stores and the associated water weight that accompanies those stores [3]. For every 1 g of glycogen you store, you have another 3-4 g of water associated with those glycogen molecules. Thus, if we assume even moderate glycogen storage levels (300-400 g) at the onset of the diet, losing those stores over the course of 1-2 weeks and the associated water can produce a meaningless (i.e. non-fat) weight loss of 3+ lbs. (And much higher losses have been reported.)

So, while the scale moved, and you likely felt really good about your dieting efforts, the truth is that any additional weight loss advantage conferred by the low-carb diet is explained by the extra losses in glycogen and water.

A quick disclaimer: Before the low-carbers get themselves all twisted up (obviously not like a pretzel!), this isn’t to say that low-carbohydrate diets aren’t useful for losing body fat (that is not the point I am trying to make), because they most certainly can lead to meaningful fat loss—assuming a sufficient caloric deficit. My point is that they are no more effectivefor fat loss [4, 5]. The initial extra weight loss advantage lay in the glycogen/water losses. (Note: I have every intention to cover this diet, fully, in future installments.)

Having now established why the distinction is critical, let’s go through how we can actually measure body composition before moving on to how changes in body weight can be a more important metric for the average dieter.

Methods for body composition analysis [1, 6]

My intention here is to cover, cursorily, a handful of body composition methods and their strengths and weaknesses. What will become painfully obvious is that (almost certainly) you will not have access to these methods. Thus, for the vast majority of people, the most valuable and least accurate method for tracking dieting progress is, well…the scale. First, let us look at how body composition is measured.

Body mass index (BMI)

Starting with perhaps the most widely known method: BMI. Contrary to popular belief, BMI is not a measure of body composition. It is simply the ratio between a persons’ weight (measured in kg) to their height (measured in m2). As such, it is used to assess an appropriate weight for a given height and therefore only merely indicates body fatness, and it correlates pretty well with disease risk (see categories of BMI directly below):

·       BMI < 18.5, underweight (<16 suggesting an disordered eating)

·       BMI 18.5 to 24.9, normal weight, healthy/low health risks

·       BMI 25 to 29.9, overweight, associated with increased risk of disease

·       BMI 30 to 34.9, obese (grade 1), associated with further increased risk of disease

·       BMI 35 to 39.9, obese (grade 2), associated with higher risk of disease

·       BMI ≥ 40, morbidly obese (grade 3), highest risk of disease

That being said, BMI does correspond fairly well with actual body fatness at the population level, for the average person (see figure directly below) [7]. Which is why this method is used often in large-scale studies where you can’t directly measure hundreds or thousands of subjects.

Conversely, BMI is terrible for athletes, the elderly, children (who need to couple it to growth charts), and (to be complete) pregnant women. To take the case of the athlete, it becomes obvious that their BMI is not indicative of excess fat despite having a BMI that would classify them as ‘obese’ (see figure below). This is because muscle is denser than fat, so for any given body weight, if you don’t take into account the composition of that weight, you won’t get an accurate representation of health and disease risk. 

(Same BMI, vastly different body compositions)

Thus, BMI is a crude estimate to determine how much body fat an individual carries. While BMI does tend to correspond to body fat levels and health risk at the population level for the average person, it has significant limitations for estimating true body composition in athletes and the other special populations noted above. For the average person not exercising intensively, a decrease in BMI will likely reflect beneficial changes in weight. Once you start transitioning to the realm of exercise/the athlete where changes in muscle mass become more pronounced, the measure becomes useless.    

Waist circumference (WC) and Waist-to-hip ratio (WHR)

Here is another example of a low-tech and non-body composition analytical technique. Just as it sounds, it is simply a circumference measurement of the waist and therefore offers some insight into regional fat deposition. By itself, however, it tells you nothing about the overall composition of the body. There are prediction equations that can estimate body fat levels by plugging in your WC. I would not put much stock in this method given the vast number of assumptions placed on the model. However, if it is the best you have, as long as you understand the limitations, it can be a (very, very) rough estimate of body fat levels to add some context to where you are starting.     

WHR is simply comparing WC (presumed to be narrower) to your hip’s circumference (typically is wider). This has been shown to be a good indicator of disease risk the closer the ratio closes in on one—i.e. your WC becomes more similar to your hip circumference, which assumes more fat storage around the abdomen. Not good.    

Obvious limitations of these methods are changes in water balance (e.g. edema that can increase water accumulation in interstitial layer of the waist), as well as user error, such as pulling the tape too tightly or measuring slightly different locations upon repeated measurements. Nonetheless, if you are suing WC to track progress, larger changes in the tape over time will indicate positive (i.e. fat loss) results.  

Skinfold calipers

Ah, our first technique to actually measure body composition. Commonly taught in any exercise science department, here, skinfold thickness at various sites (typically three to five sites) is measured and the values are plugged into a prediction equation that estimates body fatness. Because skin is roughly 0.5-2 mm thick [8], the majority of the skinfold is presumed to be fat. The other assumption with this method is that fat stored right below the skin (i.e. subcutaneous fat) has a direct relationship to total body fat levels. This is quite misleading since fat is stored in a number of places that are not right below the skin (e.g. within muscle and within the abdominal cavity/viscera, etc.). Thus, there are some obvious limitations if you are interested in fat stores that are not subcutaneous in nature (such as the metabolically dangerous visceral fat).  

Other limitations are user error wherein the measurer doesn’t standardize their measurement sites well or accidentally pinches some muscle tissue in the fold. Measurement also become more difficult the more body fat someone has. Regarding the technology itself, calipers can range from relatively inexpensive (and therefore less accurate) to quite expensive and therefore more likely to give true measurements. 

Overall, this method can be quite useful assuming the individual doing the measuring is well- trained and is consistent with their measurements over time. (For those interested, I am certified in skinfold measures via the International Society for the Advancement of Kinanthropometry [ISAK] and can attest to the rigor and training of this certification. I highly recommend for those who work closely with athletes/clients.)

Underwater weighing (UWW) and Air-displacement plethysmography (ADP)

Getting to some of the more sophisticated body composition measurement techniques, the next two are similar in their underlying principles. Both UWW and ADP (known more commonly as the BodPod method) measure body density— by way of water displacement or air displacement. Because the displaced water (or air) is equivalent to the volume of the person submerged (or contained in the BodPod), if you know their body mass, you can calculate body density. It is a little more complicated than what I just described, but for the most part, this is fundamental underlying principle.

Once you know someone’s body density, you can then estimate body fatness based on more assumptions about the density of FM and FFM. Ultimately, both UWW and BodPod can be quite accurate measures of body composition, although, it must be acknowledged that, like all measures of body composition, there are a number of assumptions built into the model that may not hold true for every single person (e.g. is the density of FFM truly the same in all of your lean tissues? How about your lean tissues versus someone else’s? You get my point.) Additionally, unless you have special access to these methods, you aren’t using these to track your weight loss progress.

Dual energy X-ray absorptiometry (DEXA)

Here, a person lies down on a scanning table and is subjected to low-dose X-rays. (Thus, a small amount of radiation is present: ~1-10% of a typical chest X-ray [6].) Based on this scan, total fat mass, bone-free LBM, and bone mineral density can all be calculated. As such, this method is an all-in-one three-compartment model. Because more body compartments are measured, there are fewer assumptions built into the model, leading many to consider this method today’s “Gold Standard” for body composition assessment.  

It is relatively common, but typically reserved for research settings. If you are at risk for bone loss, you have a greater chance of getting approved for a DEXA scan. Thus, typically only older individuals get these scans.

Another advantage of this method is its ability to give regional fat and lean mass calculations (e.g. trunk versus limbs); thus, you can see how your body’s composition changes in multiple sites during your weight loss program. (Very cool!) 

It is likely, however, that this will not be your method of choice for tracking progress.

Bioelectric impedance analysis (BIA)

The final method that I want to touch upon (and there are many others that we could talk about) is BIA. For this method, a subject has electrodes applied to their hands and feet and small current is sent through their body. This allows body fatness to be estimated based on your body’s conductivity. (Lean tissue is a good conductor [low resistance], whereas fat mass is a good insulator [high resistance].)  

The perspicacious reader can already intuit the limitations of this method. If acute changes in body water and hydration status are not tightly controlled, measurements could be impacted significantly. For women this could be huge given large shifts in body water due to the menstrual cycle. Thus, a high degree of standardization should be employed if possible. Further, most prediction equations and their underlying assumptions that are built into this method are not relevant for older individuals, so caution must be used for those who are on the older side.   

Many of you may even have experience with this method and not even know it. Some bathroom scales are also BIA units. The major limitation here (outside of standardization and whatever prediction equation is loaded into the scale) is that the current forms a circuit for the lower body only. Thus, the body fat percentage it spits back is only indicative of lower body fatness. In the end, I don’t put much stock in these BIA scales. You probably shouldn’t either. Reader be warned.

Having gone through a host of methods, many of which are just not practical or relevant for the average dieter, this now brings us to the utility of using body weight as measured by your average scale to track your progress. It is likely the best option you have, and if employed accurately, it can be quite informative and helpful.    

When body weight is a useful metric

As already stated, unless you are a researcher or have colleagues/co-workers in the fitness industry who have access to the methods outlined above, you are likely left with body weight as your primary metric—the common scale being your tool of choice. While it obviously has its limitations, it can certainly be used to track your progress if understood and implemented skillfully. I will also note, for better tracking and diet implementation, additional measures are also encouraged—e.g. waist circumference and/or prudentially taken pictures. More on those in a second.

From a simple, surface level perspective, if body weight goes down, that is a good thing. The question is, is it meaningful weight loss? Unfortunately, without the sophisticated analytical methods above, we just do not really know. This is where systematic monitoring and deep understanding of changes in body weight is critical.

Let us put this into perspective: You initiate a weight loss diet. Let us assume you have come to me for coaching and forwent the fad diet books. (You are smart after all!) We inventory your food intake (as best we can—this is a whole other can of worms I’ll likely get around to at some point), and notice you’ve been weight stable for the past couple of weeks. Your macronutrient distribution comes out something like the following: 45% carbohydrate, 25% protein, 30% fat. Given that your protein intake is likely adequate (maybe we bump it up slightly depending on your end goals), what we have left to take from and induce a caloric deficit are carbs and fat. Thus, as we start dropping carbs, some glycogen will be lost as you move through the diet. Therefore, the scale will reveal some glycogen/water-weight losses—this is unavoidable. But we acknowledge this, and as long as the scale number is moving down, and your diet and exercise are consistent, we assume progress. The second the scale stops showing progress, we reduce calories from carb and/or fat again. (Wash, rinse, repeat.) Should you have a tape measurer, all the better! We can then keep an eye on changes in waist circumference that will also contribute to our tracking data points.  

Now, what if the scale does not move initially? Two things are likely: 1) we underestimated your true food/caloric intake and the deficit we calculated wasn’t as large as we thought, or, 2) something happened that caused you to deviate from the plan, and you ate a little more than expected that week (life happens). No problem, in either case it is an easy fix. If it is the latter, we simply restart the diet as planned and see what happens at our next check in. If it is the former, we just take away some more calories (from carb and/or fat; we NEVER touch protein) and proceed from there. Easy peasy! If the scale moves down, we are right on track!    

A notable exception

Before moving on to the topic of weighing frequency, I want to touch upon one notable exception to everything stated above, that being individuals who carry significant amounts of body fat and who have never exercised very consistently before. In a situation such as this, it is not uncommon for these individuals to not only lose body fat but to also gain a decent portion of muscle mass. Thus, losing 3 lbs. of fat and gaining 3 lbs. of muscle will appear on the scale as if no “progress” was made. In reality, body composition greatly improved. In this case, other objective measures, such as strength/fatigue resistance in the gym, would be a more appropriate metric to judge progress. Also, because muscle is denser than fat (i.e. same weight takes up less physical space), your clothing will likely start to feel less tight (more baggy?) and the waist measurement will likely show improvements as well. Excellent! Moving on…    

How often should I weigh myself?

Weigh-in frequency is likely going to be made on a person-by-person basis (at least it should be). That being said, we can make some rough generalizations that will hold true for most situations. First, multiple weigh-ins throughout the day are a no-no. Nothing meaningful is happening here in terms of fat loss, so we can eliminate this option right off the bat. Thus, we are left with variations of weighing in throughout the week. And no matter how frequent the weigh-in occurs, one thing should be held constant no matter what. That being a standardization of the weigh-in. The most common example of standardization is weighing yourself first thing in the morning, in minimal clothing (none is preferable), after voiding (remember our bathroom example at the very beginning?). This eliminates (quite literally) a few things. First, you get rid of any urine and residual food (i.e. feces) that contribute to non-meaningful weight—this could amount to a couple pounds in some instances. Second, because you haven’t eaten anything in many hours (9-10 h perhaps) you don’t have any undigested food in your GI system that could also contribute to non-meaningful weight.

What about later in the day? Weighing yourself in the evening and/or in full clothing (I see this often at the gym) just complicates things, especially if the weigh-in is post-workout and sweat rate is high. Also, if you’re trying to compare a morning weight to evening weight with clothing on across the week to make any meaningful inferences, you’re likely doing more harm than good for your sanity. The name of the game is standardize your weigh-ins.

And now for the reasonable options…

Option #1: Weekly (low frequency)

This is probably the simplest weigh-in frequency model. Pick one day of the week and weigh yourself under the same standardized conditions every time, one week apart. Pros to this method (assuming high standardization) are that you are not focused myopically on daily fluctuations in weight (we’ll get to daily weigh-ins shortly), so you’re not likely to make rash decisions halfway through the week. Cons are that if you only have the one day to check in with your progress, you miss the opportunity to change things had you noticed the scale moving up significantly throughout the week. Similarly, on your weigh-in day the scale might be picking up an acute fluctuation due to shifts in water balance that is hard to make heads or tails of since it is your only measure. As always, the individual and their time line for results and personal “quirks” will dictate if this method is used.

Option #2: Bi-weekly (moderate frequency)

This one is pretty obvious: Pick two days during the week to weigh yourself. Likely a mid-way point and the end of the week. The advantages here are the ability to recognize changes moving in the wrong direction before the later check-in date. Cons are if the you/the client get “hung up” on checking-in frequently. Thus, it might be better to just stick to the weekly option above.

Option #3: Daily (high frequency)

While this method has some wonderful pros (especially for the coach, assuming you have one or are one), this method has the assumed drawback of incessantly checking weight and getting hung up on (what appears to be) slow progress—meaningful changes take time! While that may certainly be true, there is some evidence that that is not the case (at least not to a significant degree) [9], and that consistent self-monitoring of weight actually promotes weight loss by focusing behavioral efforts to the diet [10, 11].

For the coach, a pro is that you can take the daily numbers and get a weekly average that, over time, you can compare to previous weekly averages and see larger trends in weight loss progress. Additionally, daily weigh-ins allow you to notice single-day fluctuations and account for those changes that would be otherwise be missed with the weekly option. Again, consult wisely with yourself and/or your coach.  

What about progress pictures?

Ah, a favorite of many-a-coach. Visual changes are, after all, what many people aim to achieve (particularly bodybuilders and other physique athletes). While, on the surface, pictures seem like a no-brainer, it is also easy for them to deceive. (Think back to your old MySpace profile pics. Better yet, don’t.) Changes in lighting, positioning of the camera, slight changes in body positioning (be it conscious or not) can all affect what is shown/seen. This tends to be an issue with “progress pics” taken at short intervals (e.g. weekly). The longer the interval between pictures, assuming progress is taking place, the starker the visual changes will become. Taking pictures once a month or possibly even two months apart is most prudent. Additionally, having the pictures is a wonderful way of documenting your success. Should you be successful, there is a 99.9% chance you post that picture on Social Media. (Don’t fact check me on that one.)   

Odds and ends about weight loss

Before closing this out, I wanted to hit a couple miscellaneous items regarding significant weight loss. No matter what, you will lose weight from all body compartments. Yes, that means muscle and even bone mineral. A great way to stave off such effects are a well-structured resistance-training program (personal favorite of mine supported by the literature), coupled with an adequate, higher protein intake (>1.5 times the RDA, especially when exercising). Which is, in part, why both behaviors are highly recommended during weight loss diets [12].  


So…which term is more appropriate to use? Weight loss or fat loss? As you can see, it depends.

Almost everything related to diet and nutrition requires nuance—dare I say, context. I hope that I was able to provide that here today. Most important, I hope you gained some truly meaningful information out of this piece, and that you are well on your way to becoming a better consumer or provider of nutrition/diet information (however you might define that). Should you have any questions, comments, concerns or things to add, please, do not hesitate to e-mail me at my address below. Should your question(s) rise to the level of readership importance (I just made that up), I may take the opportunity to address it with everyone in this forum. Thus, you are not only benefitting yourself, but others as well in your quest for information. Until next time!

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