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In the last year, half of all of the adults in the United States made an attempt to lose weight. It’s a desire for many of us, however, it can also be a complicated endeavor — made even more confusing if you don’t understand what actually makes up your body weight.  

If you’re someone who has weight loss and improved health goals, tracking your progress based solely on the numbers on a scale can often be misleading, confusing, and frustrating. Not to mention, it doesn’t always paint a clear picture.

Let’s take a look at what your body weight really means — and perhaps redefine how we measure success. 

What defines the number on the scale?

When a client tells me “My goal is to lose X pounds,” I always respond with the same question:

“Pounds of what?”

Technically, you could achieve weight loss on the scale from a limb amputation, a bout of food poisoning, or an appendix removal. But those scenarios are obviously not what clients are referring to.

Yes, the question is purposely cheeky. However, its objective is to create a pause and a dialogue about their actual goal and the true meaning behind their desired weight.

Let’s break down where body weight comes from:

Fat Mass vs. Lean Mass

Think of fat mass like pillows, and lean body mass like bricks. Fifteen pounds of either one would spit out the same number on the scale, but they look and feel very different.  

Fat mass refers to actual adipose tissue, found mostly under the skin and around internal organs. Lean mass refers to everything that is not fat, including skeletal muscle, fluids, organs, bones, and more.

In any weight-loss plan, both fat mass and lean mass are lost. However, the healthiest approach is to prioritize losses from fat while minimizing losses from lean mass.

Organs and Bones

Normal weights of various organs are estimated to be up to 3.6 pounds for the brain, 4.1 pounds for the liver, 3 pounds for the lungs, 0.8 pounds for the heart, and 7.7 pounds for the skin. Your skeletal system is estimated to account for about 15 percent of your total body weight.

In total — also factoring in your intestines, bladder, stomach, and more — your organ systems not only keep you alive and functioning, but also make up a good portion of your total body weight. 


Glycogen is the form in which carbohydrates are stored in your body, and are found predominantly in your liver and skeletal muscle. We have approximately 600 to 860 grams of glycogen stored, and each gram of glycogen is bound to three grams of water.

Together with associated water, glycogen stores can weigh up to 7 pounds or more, depending on your body size, nutrition choices, exercise habits, and muscle mass.

It’s important to note that when you first start a fat-loss program, the depletion of glycogen and water makes the initial drop in weight more pronounced. (This is a common reason why many see quick scale success when starting a ketogenic diet.) This initial loss of glycogen and water is often used as a marketing tool for “quick fixes” and other gimmicky promises.

Waste Material

On any given day, we have bowel movements forming and passing through our lower digestive tract. These are made up of fiber, undigested food, sloughed-off cells, and bacteria. Fiber pulls water into the gastrointestinal tract and is the main determinant of the weight of your stool.

Bowel movements can weigh a pound or so each, and it’s optimal to have one to three per day. Obviously, the less often you go, the more each bowel movement will weigh.

It’s common to see some weight variance of waste material with changes in fiber intake that can happen from shifts in diet or atypical food intake (such as on special occasions), or from traveling.


Your total body water is made up of water inside your cells (intracellular water, or ICW) and outside of your cells (extracellular water, or ECW). Two-thirds of the total is estimated to be inside, with the remaining one-third outside. If you’ve ever felt “puffy” or noticed your rings being tight, you’ve likely experienced excess extracellular water.

As muscle mass increases, intracellular water increases. With inflammation and fat gain, extracellular water can increase.

Your fluid status is impacted by your electrolyte balance, salt intake, and temperature. Excess retention can be dangerous with certain underlying medical conditions, such as ascites (in liver disease), high blood pressure, congestive heart failure, and kidney problems.

Fluids are extraordinarily heavy. In fact, when I was working in a hospital setting, I saw medical conditions that could cause a 10-, 20-, or 30-pound weight gain from fluids in just a couple of days. Those situations obviously require medical intervention, but I share that example to highlight that day-to-day fluid fluctuations can cause significant shifts to what the scale says.

Why does the composition of your weight matter?

Don’t judge a book by its cover.  

We’re conditioned to focus on weight loss as a success marker, when in fact, it’s only one thing to track of many. You could be a “normal” weight for your height (in terms of having a body mass index under 25) and be strong and lean, or you could be at that same weight and have low tone and metabolic issues.

The term “normal weight, metabolically obese” refers to the almost half of normal-weight adults who have a body mass index, or BMI, under 25, but have lab markers that put them at risk for cardiovascular disease and diabetes.

A sedentary lifestyle, fat around the midsection, and a low level of fitness conditioning can all increase the risk of normal-weight individuals having significant metabolic disturbances. In layman’s terms, this is often referred to as “skinny fat.”

Generally, those who say they want to “tone up” are really looking for body fat loss and muscle gain. However, many have misconceptions around building muscle. Looking bulky from muscle mass is extremely challenging to achieve — and it most certainly does not happen by accident. For most of us, increasing muscle adds shape, firmness, and tone to our physique.

Rather than tracking weight alone, tracking your body composition provides better information about fat stores versus muscle stores in the context of your total body weight.

Body composition is usually reported in body fat percent, or the percentage of total weight that is made up of fat. It compares that in proportion to your total body weight. Your body fat percentage can drop from a loss of fat mass, a gain of lean body mass (since the total percentage of weight from fat would decrease), or both.

For most who are looking to feel and function better, a drop in body fat percentage is a more accurate, meaningful indicator of progress than a drop in weight. They often coincide, but not always.

How can I assess my body composition?

Your body-fat percentage can be measured in various ways:

  • Skin calipers: A lot of clients call this the “pinch test,” where caliper measurements are taken from various skin folds on the body and used to calculate body-fat percentage. Most commonly, the measurement is from either three or seven sites.
  • Biometric impedance analysis: This is a widely available scan that uses currents passed through the body. Based on the electrical properties of various types of tissue, body composition estimates can be made through distinguishing lean body mass and fat mass. This is the methodology behind the InBody scale found at many Life Time locations.
  • DXA scan: This is a dual-energy X-ray absorptiometry test. As indicated by the name, X-rays are used to provide information on body composition.
  • Other advanced measures: Often done in a lab or medical setting, hydrostatic weighing, measurement of gas displacement from breathing in a chamber, or other methods can also be used.

Each technique varies in its margin of error, accessibility, and cost. The first two — skin calipers and biometric impedance analysis — are most commonly used for general purposes.

What’s the most practical approach to track progress?

The most practical approach will always be the one that you can use most consistently. While each method of testing body composition has its accuracy limitations, relative changes tracked through a consistent method provides information as to how you’re progressing.

The body-fat percentage results obtained from one methodology should not be plotted against another to track progress. For example, the results of a DXA scan and biometric impendence analysis will vary and should not be tracked against each other.

Whichever method you choose, be sure to test each time under similar conditions. This means testing around the same time of day, with the same recent nutrition and fluid intake, exercise habits, amount and type of clothing, and bowel habits.

For those using the InBody machine — a biometric impedance analyzer found in many Life Time clubs — it’s ideal to use the machine first thing in the morning, prior to any training and after you’ve fasted from food and liquid for three hours, preferably after a bowel movement.

When it comes to overall progress, it’s helpful to also use additional inputs to assess your changes over time, such as taking photos, noticing how your clothing fits, regularly performing benchmark workouts, taking circumference measurements (waist, hip, thigh, etc.), tracking subjective symptoms (e.g. are you feeling better?), and having blood work performed through lab testing.

Success can be defined in many ways, and body composition is just one of them.

How can I optimize my body composition?

When it comes to losing fat and maintaining or gaining muscle, there’s no magic solution. However, there are core tenants that should serve as the crux of any program if your goal is to change your body composition.

Studies have shown that higher protein intakes and strength training can both help optimize fat loss while preserving lean body mass.

One weight-loss study looked at a group of people who ate 1.6 grams per kilogram of protein (which would be about 145 grams of protein for someone who weighs 200 pounds) and exercised. They lost an average of 17.7 pounds (with only 0.9 pounds from lean mass), while another group in the study who ate 0.8 grams per kilogram of protein (or about 73 grams for a 200-pound person) only lost 12.1 pounds — with almost half of that loss coming from lean mass.

A separate study on those with type 2 diabetes demonstrated similar results. The researchers found that:

“Participation in [resistance training] produced greater weight and fat loss and increases in muscular strength compared with energy restriction alone. Additionally, replacement of some carbohydrate for protein further magnified these effects, resulting in greatest reductions in weight, fat mass, [waist circumference], and insulin.”

In other words, instead of just focusing on cutting calories to get results, it was more beneficial to also swap out some carbohydrates for protein and add in weight training. Other research has shown that the more protein someone consumes during a weight-loss approach, the more likely they are to maintain their lean tissue.

While it takes more than just optimizing your protein intake and being physically active (especially focusing on strength training) to be successful long-term, these two components are absolute table stakes.

Why does my weight fluctuate so much day-to-day?

Not only is it normal, but it’s expected to see a couple of pounds of variance in your weight from day-to-day.

If you’re using scale weight as one way to track your progress, be sure you’re using trends and averages over time. Your average weekly weight taken first thing in the morning after a bowel movement over six to eight weeks is way more meaningful than comparing today’s weight to yesterday’s weight. In fact, comparing day-to-day fluctuations to determine whether or not your program is working is incredibly misleading.

For example, a tough workout (causing micro-tears in your muscles) with proper replenishment (from protein and carbohydrate) might cause temporary, expected inflammation, fluid shifts, and an increase in glycogen and water storage in your muscles. This would make your weight higher the next day — but all for anticipated reasons that are necessary for your long-term success.

Using creatine —a well-researched and effective supplement option that can support athletic performance and power — can cause fluid to move into skeletal muscles. While this is a good thing (and can even support an aesthetically toned look), it often also makes scale weight increase.

Bowel patterns have a role in day-to-day weight fluctuations as well. For example, traveling can sometimes lead to constipation or diarrhea, causing more or less weight in the body, respectively. The bulk of your stool and amount of water in your digestive tract also varies (and therefore so does your weight) as your fiber intake changes.

Weight is also impacted by cortisol, our primary stress hormone. When we are under-slept or experiencing a stressful situation, infection, or injury, cortisol levels can go wonky. As a result, a hormone called aldosterone, which helps regulate sodium and fluid levels, can shift as well, which could cause daily weight swings.

Medications such as corticosteroids and anti-depressants can also impact the scale, as can the timing of a woman’s monthly cycle, with the most excess weight usually being on the first day of menstruation.

When does scale weight matter?

Using scale weight is not inherently bad or misinformed — but it’s only one piece of data in a larger puzzle.

Your total weight is significant and important in certain instances, especially if there is a noticeable and unintended change. If your weight changes rapidly, be sure to work with your doctor immediately to rule out any cardiac, blood pressure, liver, or kidney issues.

Having a higher overall body weight — even in terms of a healthy body composition — can also impact joint health, lead to a higher risk of sleep apnea, and alter certain measures of athletic performance, such as run speed.


Measuring your body weight (not to be confused with body composition measures) can be useful in context, but hopefully you can see how relying on the scale number alone as a marker of your progress is misleading.

If you want to lose weight, it’s best to spend some time first thinking about the true reasons why you want to make a change, then closely monitor the markers that align with your “why.”

For example, if you’re trying to minimize your risk of type 2 diabetes and are in a family where the diagnosis is rampant, monitoring your body composition and lab work related to blood-sugar control might be most meaningful.

Conversely, if you’re aiming to keep up with your kids and stay fit without aches, pains, or feeling winded, using body composition measures along with range of motion and mobility assessments, and subjective measures, such as energy levels, might make more sense.

No matter which markers you choose, use several markers of progress, focus on trends over time rather than isolated readings, and don’t let a single scale reading that might seem unfavorable send you reeling.

Keep the conversation going.

Leave a comment, ask a question, or see what others are talking about in the Life Time Health Facebook group.

Samantha McKinney, RD, CPT

Samantha McKinney has been a dietitian, trainer and coach for over 10 years. At first, her interests and experience were in a highly clinical setting in the medical field, which ended up laying a strong foundation for understanding metabolism as her true passion evolved: wellness and prevention. She hasn’t looked back since and has had the honor of supporting Life Time’s members and nutrition programs in various roles since 2011.

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