The subjects of body weight and body composition can get a little muddied—even frustrating—when it comes to understanding why every time you step on the scale, the number seems to creep up, not down, despite regular exercise and diet changes. Logically, you know that a pound of muscle weighs the same as a pound of fat (a pound is a pound is a pound, after all), but if you’re adding muscle and losing fat, why might your weight increase over time?
The answer really comes down to tissue density, and the speed at which your body composition changes in response to diet and exercise. It’s not as “cut and dry” as every time you lose a pound of fat, you gain a pound of muscle. Fat doesn’t “change into” muscle, and muscle, likewise, doesn’t “change into” fat.
Muscle and fat are entirely different tissue types, and they grow and change independently based on lifestyle factors and an individual’s metabolism, hormones, and response to exercise. It’s entirely possible to increase muscle mass without losing fat, and it’s entirely possible to lose fat without gaining muscle mass. This is why it’s important to understand that your body composition is infinitely more important to monitor than weight itself—body composition provides information about your internal tissue makeup that the number on the scale doesn’t provide.
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Body composition versus body weight
Body composition is a way to measure and monitor your fat mass and your fat-free mass. Fat-free mass includes any tissue that isn’t fat—muscle, bone, water, blood, cells, organs, and so forth. While fat plays important roles in overall health, too much fat can be detrimental. Measuring body composition periodically allows you to track changes in fat mass and fat-free mass over time, which can be helpful when trying to determine if you’re successfully moving in the direction of your health and performance-related goals.
Weight, on the other hand, is a reflection of nothing more than how gravity acts on your body. It provides no information about fat content, water, muscle, or bone content, and therefore can’t accurately determine whether a person has potentially healthy or unhealthy levels of these tissues.
Likewise, as weight fluctuates, there’s no way to know exactly how the fluctuation is occurring. In other words, if you lose five pounds, you don’t know how much of that weight came from water, fat, muscle, or even bone. The same is true if you gain five pounds—weight alone won’t be able to tell you whether the change is due to increases in muscle, increases in fat, or some combination thereof. And for those looking to put on muscle without necessarily losing fat, this is an important consideration to track.
“When adding muscle with resistance training, if not doing so in a caloric deficit, you would expect to see increases in weight,” explains Lem Taylor, Ph.D., director of graduate studies at the University of Mary Hardin-Baylor in Belton, Texas. “This number on the scale is not going to go up rapidly, based on the simple fact that adding muscle mass is not a rapid process for the majority of people.” But monitoring body composition is a good idea to keep track of your fat mass. You can take a measurement every four to six weeks to keep an eye on things.
The downside: While standard scales are readily available in homes, gyms, and doctors offices, making it particularly easy to track changes in weight, accurately tracking body composition is slightly more challenging.
The most accurate tests to estimate body composition include lab-based tests like DXA scans, BodPods, and hydrostatic weighing tanks that are most frequently found in hospitals and university labs (although sometimes they’re available in gyms).
Taylor explains that these methods are all based on body density—either through assessing the amount of water or air displaced in a predetermined space in comparison to body mass, or through an advanced x-ray technology that can measure tissue density. Regardless of the testing type, in general, more density means lower body fat percentage.
There are also affordable, home-based body composition scales and handheld devices that offer a reasonably accurate reading based on bioelectrical impedance analysis (BIA). This technology sends a tiny electrical current through your body when you use the device, and measures how long it takes for it to travel from one foot to the other (or one hand to the other, depending on the type of device). This electrode moves more quickly through less-dense tissue, like fat, than it does through denser tissue like muscle or bone. Based on the speed of this impulse, the devices produce an estimate of fat mass and fat-free mass.
While the results of these at-home devices aren’t as precise as lab-based tests, they tend to be an effective way to track changes over time. In other words, if you’re using a BIA-enabled body composition bathroom scale and it estimates you have a body fat percentage of 24%, its accuracy might be off by as much as 5%. However, if you consistently use the same device to track body composition changes over time, you can feel confident that a 2% decrease in body fat percentage (so a change from 24% to 22%) accurately depicts your change in composition, even if your actual body fat percentage is higher or lower.
Another option for monitoring body fat percentage is via skinfold measurement. “You can track circumference measurements in key places to monitor what’s happening at specific regions. Areas like the biceps or thigh are likely only going to increase in size in the short term—about 4 to 8 weeks—when adding muscle,” Taylor adds.
All about tissue density and how that plays into muscle weighing more than fat
The big difference between a standard scale and a tool that tracks body composition is that the body composition devices or tests account for tissue density. So while a pound of fat weighs the same as a pound of muscle, the amount of space each pound of tissue takes up is significantly different.
“Weight versus density allows us to understand or establish the role of density in body composition to allow us to talk about fat versus fat-free mass. Weight is just a number on a scale with no reference/measurement of density, but our tissues consists of bone (the most dense), muscle (more dense), and fat tissue (least dense) that comprise most of our weight. Density gives us a better explanation of what our weight is,’” Taylor says.
There are a few different ways to picture this concept.
First, imagine for a second that you blow up a balloon with air and tie it off. You then fill up a second balloon with water until it’s the same size as the first balloon. Even though the two balloons are exactly the same size, the balloon filled with water will weigh more than the balloon filled with air. This is because water is more dense than air.
Likewise, muscle is more dense than fat. “The density of muscle is 1.06 grams per centimeter cubed and the density of fat is 0.9 grams per centimeter cubed,” says Julia Connolly, P.T., D.P.T., a physical therapist at Fusion Wellness and Physical Therapy in Los Angeles. This may not be as significant a difference as water and air, but it certainly makes a difference when looking at the physical size of a pound of fat or a pound of muscle.
So, if you look at two identical twins who are the exact same size and shape, but one has more muscle mass than the other (so their body compositions are different), the one with more muscle mass will weigh more than the one with more fat mass, despite their identical sizes.
To picture things another way: Imagine filling one pillowcase with a pound of feathers, and a second pillowcase with a pound of marbles. A pound of marbles is going to take up significantly less space in the pillowcase than a pound of feathers because marbles are more dense than feathers.
In this same manner, 10 pounds of muscle will take up less space than 10 pounds of fat—if you gain 10 pounds of fat, your physical size will increase more than if you added 10 pounds of muscle. In a situation where your weight remains unchanged, but your body composition is changing—say, gaining a pound of muscle while losing a pound of fat—your body will physically take up less space due to the increase in the denser muscle tissue.
While these changes and differences may not be readily obvious at the start of your exercise journey, they become significant with time. And they can be a motivating factor that goes beyond the scale.
Fat isn’t “bad”—it’s a matter of finding the right balance
Regardless of your current weight or body composition, or even your long-term goals, it’s important to understand that both fat and muscle play important roles in your body.
People tend to view fat negatively because excessive levels can lead to detrimental health outcomes. Connolly mentions cardiovascular changes, cognitive decline, insulin resistance, and inflammation, just to name a few of the effects excess adipose tissue (or fat tissue) can have on your health. But fat itself is an essential nutrient and your body requires it in order to function correctly.
“Fat gets a bad reputation due to the superficial values of society. Fat has many different functions within your anatomy that are essential for health. For example, the storage and release of energy, the protection of organs, insulation, maintaining female reproductive health and menstrual regulation, facilitating nerve impulse transmission, offering structure to the cells, and storing and using fat-soluble vitamins and nutrients,” Connolly explains. These fat-soluble vitamins include vitamins A, D, E, and K.
Certainly for women, the role fat plays in reproductive and menstrual regulation is vital. “When a woman’s body composition becomes too lean, she can lose her menstrual cycle, compromising her reproductive ability,” Taylor explains. This is one of the reasons why women, biologically, have higher fat stores—the female sex hormone, estrogen, encourages fat deposits in sex-specific areas in women, particularly the thighs, butt, hips, and breasts.
So even if you’re looking to lose fat mass as part of your health or fitness goals, you shouldn’t try to lose too much. “Genetically, the amount of fat an individual retains or gains varies significantly between genders and even individuals,” Connolly says.
As per the American Journal of Clinical Nutrition, women between the ages of 20 and 39 should aim for 21 to 32 body fat percentage, while men should aim for 8 to 19 percent body fat. Women between 40 and 59 should aim for between 23 and 33 body fat percentage, while their male cohort should aim for between 11 and 21 percent. And for women between 60 and 79 years old, a range from 24 and 35 percent is considered healthy, while men should shoot for between 13 and 24 percent.
“Elite athletes tend to have body fat percentages around 6 and 13 percent for men and 14 and 20 percent for women,” Connolly adds.
That said, different organizations offer different parameters and norms, so some slight variation from these ranges may still be okay.
Because of the roles fat plays in the body and the varying levels of healthy body fat, the issue of “how much is too much” fat is a little bit gray. There are sex, age, and genetic differences in metabolism and how a person physically responds to fat accumulation, as well as different benefits for different sports or lifestyles. “It’s hard to put a precise line on when the level of body fat becomes detrimental to health. There’s a large gray area between what percent is okay or not because of the complexity with which fat interacts with other body systems,” Connolly says.
For this reason, it’s important to have regular physicals and to monitor markers like blood pressure and blood sugar to get a picture of your overall health that goes deeper than body composition or appearances alone. “Above all, the best measure is a question to yourself, ‘How do you feel?’” says Connolly. Feeling energized, healthy, and strong are good signs that you’re on the right track.
Adding muscle mass is almost always beneficial
Whereas finding the right balance of body fat based on your age, sex, and performance goals may take a bit of trial and error (and a lot of consistency with diet and exercise), gaining muscle mass almost always provides health and performance benefits.
“Functionally, life gets ‘easier,’ [that means] moving, standing, lifting throughout the day—when muscle mass increases, it leads to more strength, better balance and stability, and overall better functional capacity. It’s pretty easy to see how adding muscle, or at least lifting weights to maintain muscle mass throughout a lifespan, has huge implications on quality of life and remaining independent,” Taylor explains.
Taylor is also quick to point out that muscles, as a metabolically active tissue, can help keep your metabolism revving all day, ultimately burning more calories at rest than less metabolically active tissues, like fat. In other words, muscle, by its very nature, requires more calories to maintain. That means when you have more muscle mass, you burn more calories to support muscle maintenance. Granted, the actual number of extra calories burned per day per pound of muscle is relatively small.
According to a study published in the American Journal of Human Biology, each pound of muscle you have burns somewhere between seven and 13 calories per day, depending on sex and individual differences in metabolic rates. A pound of fat, on the other hand, burns slightly less at an estimated five to 12 calories per day.
So if you gain a pound of muscle, your daily calorie burn could go up by two to eight calories per day—not a whole lot. But with ongoing changes in body composition, increasing muscle mass and decreasing fat mass, the little differences could help you maintain a healthy body composition. To put it in perspective: 10 pounds of added muscle mass could increase your daily resting energy expenditure by 20 to 80 calories. Over the course of a month? That’s 600 to 2,400 calories. Sure, not a huge gain, but that’s extra calories you’re burning without doing a thing.
Possibly even more importantly, muscle offers disease-fighting benefits. This is because muscle serves as a reservoir for amino acids—the building blocks of all cells, which means muscle can play a protective role in cellular turnover (providing necessary amino acids to help protect against disease) as a whole, when the body is presented with a stressor.
As Taylor points out, this is particularly true when it comes to a disease like type 2 diabetes. “More muscle, especially if you stay active and maintain muscle mass over time, means the body has more places to store glucose (in the form of muscle glycogen), which leads to a better ability to maintain blood glucose levels throughout the day.”
Stay consistent and monitor body composition changes over time
The thing to keep in mind when it comes to setting weight or body composition goals is that significant change takes time and consistency. And while you may have the general goal of “dropping a few pounds,” a better reflection of whether your body changes are positive may come down to whether you’re increasing muscle mass and gradually decreasing fat mass.
Staying consistent with an exercise regimen means choosing an activity you enjoy and will do regularly. “But the real challenge with body fat is staying consistent and disciplined with diet and strength-training intensity and volume,” Connolly says.
It also requires finding the right balance of calories, nutrients, and training. “Not to mention, it requires the consideration of confounding factors such as sleep, hormones, basal metabolic rate, fat loss plateaus, genetics, musculoskeletal pain or injury, mental health, time constraints, finances, and accessibility,” Connolly adds. “Trust the process and appreciate the more precious and enduring health benefits that are happening under the surface, unseen by the naked eye.”
Laura Williams, M.S., ACSM EP-C holds a master's degree in exercise and sport science and is a certified exercise physiologist through the American College of Sports Medicine. She also holds sports nutritionist, youth fitness, sports conditioning, and behavioral change specialist certifications through the American Council on Exercise. She has been writing on health, fitness, and wellness for 12 years, with bylines appearing online and in print for Men's Health, Healthline, Verywell Fit, The Healthy, Giddy, Thrillist, Men's Journal, Reader's Digest, and Runner's World. After losing her first husband to cancer in 2018, she moved to Costa Rica to use surfing, beach running, and horseback riding as part of her healing process. There, she met her current husband, had her son, and now splits time between Texas and Costa Rica.
