Fat vs. Muscle and How They Impact Your Body Composition

Understanding the Differences: Fat vs. Muscle and How They Impact Your Body Composition

Fat and muscle are integral tissues in the body. Each plays a different role in overall health and appearance, each serving different functions. However, while people generally understand that these tissues are different, there are many misconceptions surrounding how they affect body composition. Given that each brings something unique to the table, it’s important to understand those differences and how they impact your body composition and fitness goals.

What is Muscle?

Muscle is a soft tissue comprised of thousands of elastic fibers. These fibers are bundled tightly together, all working in unison to perform specific tasks.

Some of those muscles are called smooth muscles, which are automatically controlled by the nervous system. These muscles, also known as involuntary muscles, serve specific roles in the body, such as digestion and circulation. You can’t consciously control involuntary muscles—these functions happen all on their own under the instructions of the nervous system.

Skeletal muscles, called voluntary muscles, are controllable. These muscles are found throughout the body and connect to bones and tendons. You use these muscles, such as the glutes, traps, hamstrings, and other voluntary muscles, to do various tasks, such as sitting up, balancing, and moving.

What is Fat?

Adipose tissue, better known as body fat, is a connective tissue located throughout the body. It’s made up of fat cells called adipocytes, which are responsible for storing lipid droplets (fat). There are three primary types of body fat: subcutaneous, visceral, and bone marrow adipose tissue.

Subcutaneous fat is the fatty tissue lying underneath your skin. It’s the deepest layer of your skin, sitting beneath the dermis and epidermis. This layer serves various functions, including regulating body temperature, protecting bones and muscles, and storing energy.

Visceral fat, commonly called “toxic” fat, is the fatty tissue that surrounds your abdominal organs. It wraps around various important organs, such as your intestines, stomach, and liver. This fat can lead to serious health complications in large amounts. However, it does serve multiple functions, including energy provision and organ protection.

Bone marrow adipose tissue (BMAT), situated inside non-hematopoietic bone marrow, is a metabolic organ and secretory tissue. It accounts for approximately 10% of total body fat and serves many important immune, paracrine, and endocrine roles.

Does Muscle Weigh More Than Fat?

You might’ve heard people say that muscle weighs more than fat. However, this isn’t true. A pound of fat weighs the same as a pound of muscle. So, what do people actually mean when they say that? Usually, they’re trying to convey that a set volume of muscle weighs more than that same volume of fat.

See, muscle has a density of approximately 1.06 kilograms per liter, whereas adipose tissue (fat) has a density of about 0.9 kilograms per liter. So, if you were to take a handful of each fat and muscle, the fistful of fat would weigh less than the handful of muscle.

Muscle vs. Fat: In the Body

Since muscle is about 20% denser than fat, it looks considerably different in the body. A pound of muscle takes up about 20% less space on a person than a pound of fat. So, to get a better idea of how that might look on a person, let’s compare two 160-pound individuals.

Person A works out frequently, combining weightlifting with a few cardio sessions per week. Person B rarely works out, although they may occasionally go on walks. While they weigh the same, Person A might look substantially smaller than Person B.

Person A’s body composition consists of more muscle than Person B's, giving them a slimmer, more toned physique.

Of course, there’s more to this scenario. Genetics, workout routines, nutrition, and other factors contribute to the bigger picture. That said, this gives us a basic idea of how the same weight can look dramatically different based on body composition.

Body Composition and BMI: An Inaccurate Pair

These differences between fat and muscle are, in part, what make body mass index (BMI) a terrible measure of body composition. The two people in our example might have the same weight and height, giving them the same output BMI.

For example, let’s say that they’re both 5 feet 5 inches tall and weigh 160 pounds. If we plug those numbers into a BMI calculator, we’ll discover that both individuals have a body mass index of 26.6, which is considered overweight.

While BMI can be a helpful tool in certain groups, it doesn’t account for varied body composition—all it looks at are weight and height. So, while Person A might be in excellent shape and Person B might be average, the formula still thinks they have the same BMI.

It doesn’t account for muscle mass, bone density, overall body composition, or even unique racial or sex differences. So, for some individuals, especially those with more muscle mass, BMI is an inaccurate metric.

Muscle vs. Fat: Progress in the Gym

For many folks, the goal of exercising and eating healthy is to lose fat and gain muscle. However, as you move through your fitness journey, it can sometimes be hard to tell which kind of progress you’re making.

You might see a dramatic drop on the scale in the first month or two, only to plateau in months two or three. You might even start to see the scale climb back up. So, what are you losing and gaining in those months?

Early Drop

When you lose weight rapidly, you’ll likely lose muscle and fat. This is normal for many people. The big drop is primarily due to water loss. The slashed calorie consumption leaves your body to turn to glycogen stores for energy, a carbohydrate found in the muscles and liver.

Glycogen consists partly of water, so when your body burns it for energy, it releases that water. You expel that water, thus experiencing a dramatic weight loss alongside muscle and some fat.

So, in those early days, when you shed pounds like water off a duck’s back, there’s a good chance you’re losing muscle, too. And that’s okay—it’s normal.

Progress Plateau

Many people run into a roadblock during their fitness journeys: the dreaded plateau. It’s a normal part of the process, too. Remember how you lose muscle in that big drop in the early stages? Muscle helps maintain a specific calorie burn (metabolism), so when you lose muscle, your metabolism declines.

That decline causes you to naturally burn fewer calories than you did at the beginning of your fitness journey. It can cause you to reach a balance between the number of calories you’re consuming and the number you burn. While that calorie deficit may have worked for you initially, it might not work anymore because you'll plateau when you reach this point.

So, to continue your weight loss journey, you’ll need to boost your activity level to start burning more calories or reduce the number of calories you eat.

Weight Gain

If your goal is to shed fat and gain weight in muscle, you might start to put weight back on as you progress in your fitness journey. You might not feel like you look heavier, but the scale might say otherwise. This, too, is normal.

Since muscle is denser than fat, you may be heavier than you look. Muscle appears leaner and more toned than the same amount of fat, so if the scale is climbing despite your consistency in your workout and nutrition plans, it might be muscle. If that’s the case, you might feel stronger and may even see noticeable changes in the sizes of your muscles!

Of course, everyone is different, so the answer to whether you’re losing or gaining fat or muscle can vary for each person. If you’re unsure whether your fitness plan is helping you achieve your goals, chat with a personal trainer near you.

Muscle vs. Fat: In a Nutshell

The differences between muscle and fat go far beyond mere weight on a scale. Each type of tissue brings something different to the table, serving unique purposes. Muscle is lean, dense tissue, while fat is loose tissue that takes up more space. Both play important roles in regular bodily function, but too much fat can cause issues.

So, it’s all about balance—finding what works for you with diet, exercise, and lifestyle habits that promote overall well-being. Everyone is different, after all, so what works for you might not work for the next person. And, if you’re not sure what works for you, it might be a good idea to chat with a knowledgeable personal trainer near you. They can help you iron out the details and craft a strategy designed to help you look and feel your best.

Frequently Asked Questions

How Much More Does Muscle Weigh Than Fat?

Muscle doesn’t weigh more than fat—one pound of muscle weighs the same amount as one pound of fat. However, muscle is denser than fat, so the same volumes of fat and muscle weigh different amounts.

Why Do I Have a High BMI But Am Not Fat?

If you’re an athlete, weightlifter, or someone with a significant amount of muscle mass, there’s a good chance you’ll have a high BMI. Muscle tissue is dense and doesn’t take up nearly as much space as fat, so you might have a high BMI, even though you’re not fat. If that sounds like you, BMI alone isn’t an accurate measure of your health or fitness.

Will Your Body Burn Muscle or Fat First?

The answer to this question can vary based on multiple factors, including diet, exercise intensity, and workout duration. In general, you’ll burn fat first, but there’s a good chance you’ll lose muscle, too. When you’re losing weight, especially a significant amount of weight, you may lose a portion of muscle mass alongside fat.

Does Fat Turn Into Muscle?

When you shed fat and build muscle, that fat doesn’t magically turn into muscle tissue. These are two completely different types of tissue made up of different cells, so they don’t turn into each other. Instead, the fat cells shrink in size while the muscle tissue grows.

What Type of Exercise is Best for Losing Fat and Building Muscle?

Various types of training, such as resistance training and cardio, have their roles in fat loss and muscle building. Certain types can be more effective for certain goals. For example, resistance and weight training are better for building lean muscle than cardio. However, what works best for you will depend on the specifics of your body and goals, so if you’re not sure, chat with a personal trainer near you.