Body Composition: What It Is and Why It Matters

Do you have a love-hate relationship with your bathroom scale? The bathroom scale may be a good tool for measuring weight, but it may not accurately reflect your true health. Body composition takes your weight measurement one step further, figuring out what percentage of your weight comes from fat, muscle, and bone.

The difference between fat mass and non-fat mass (muscle and bone) paints a more complete picture of your health and fitness. 

This article reviews the basics of your body composition, what it means, and what you can do about it.

What Is Body Composition?

Your body composition determines what percentage of your total body weight comes from fat, muscle, and bone. Knowing your body composition provides more detailed information about your health. Two people can weigh the same, but have very different wellness and fitness needs because of their body composition.

Why It’s Important

Body composition is important because it measures your overall health and fitness level in terms of your body fat percentage. Your bathroom scale can’t tell the difference between how much of your weight comes from fat and how much from muscle. But body composition measurements can.

Less fat and more muscle tends to point toward a better level of health.

What Is Body Fat?

Body fat includes all the stored fat in your body. There are two types of body fat:

• Subcutaneous fat: This is the layer of fat under your skin. It insulates and protects your body. 
• Visceral fat: This is the fat that surrounds and cushions your abdominal organs. 

In addition to insulating and protecting your body, fat provides energy, carries fat-soluble vitamins, makes certain hormones, and serves as a building-block for cell membranes.

You need a certain amount of body fat to perform these functions—this is known as essential fat. 

Body Fat Percentage

Body fat percentage is the percent of fat that makes up your total body weight. Many factors influence your body fat percentage, including sex, age, fitness level, and lifestyle.

The body fat percentage ranges come from the American Council on Exercise (ACE).

Athletes tend to have a lower body fat percentage than people who are physically fit because having less fat improves their athletic performance. However, when body fat percentages dip too low, athletic performance suffers and immune function declines.

On the flip side, a body fat percentage that’s too high is a risk factor for chronic illnesses like diabetes, high blood pressure, and heart disease.

Body Composition vs. Body Mass Index

Body composition and body mass index (BMI) are tools that assess body fatness. However, the methods used to measure body composition and BMI differ. Additionally, BMI may not provide accurate results in all situations.

Health professionals use the BMI to assess an individual's risk for disease based on their body fatness, or where they fall on the BMI table. But recent evidence suggests that the BMI is a very poor measure of body fatness and disease risk, and may not serve as the best—or only—metric for measuring body composition and health.

Body Mass Index

BMI is a tool that has been used by health professionals to assess body fatness and health. It’s a mathematical equation that compares your weight to your height.

• BMI = (weight in pounds)/(height squared) X 703

For example: (150 pounds)/(66 inches x 66 inches) X 703 = 24

This BMI table is from the Centers for Disease Control and Prevention (CDC).

• Less than 18.5 = underweight
• 18.5 to 24.9 = healthy weight
• 25.0 to 29.9 = overweight
• 30 or higher = obesity

You don’t need any special equipment to measure BMI, making it a quick tool to assess body fat and health.

However, the BMI isn’t perfect and may not accurately assess body fatness in all people. For example, a bodybuilder may have a BMI that puts them in the overweight category, but have a low body fat percentage.

How to Measure Body Composition

There are a number of ways to measure body composition. However, you need more than a bathroom scale and calculator to figure out what percentage of your weight comes from fat and what percentage comes from muscle. 

Bioelectrical Impedance

Bioelectrical impedance (BIA) is a tool that estimates your body composition by measuring your body’s resistance to a low-level electric current, or impedance. Muscles have a lower resistance to an electric current than fat. 

Skinfold Measurements

Skinfold measurements involve the use of special calipers that measure the skinfold—subcutaneous fat—on different parts of your body. Fitness trainers use skinfold measurements to assess body fat because it’s quick and convenient.

DEXA Scan

The DEXA scan, or dual-energy X-ray absorptiometry scan, uses a low-level X-ray to measure bone, muscle, and body fat.

Hydrostatic Weighing

Hydrostatic weighing involves full submersion in a water tank, using water displacement to measure body composition. Since fat floats and muscle sinks, a person with more lean body mass weighs more underwater.

Bod Pod

Bod pod is similar to hydrostatic weighing, but uses air displacement instead of water displacement to measure body composition. For this test, you sit in an egg-shaped chamber, which uses your body weight and volume to determine your body composition.

Factors to Consider

Body composition provides valuable information about body fat percentage. However, your body shape is unique to you and there are uncontrollable factors that affect your body composition including:

• Age: You lose muscle as you get older, which affects your body composition. However, age-related muscle loss is most often due to a decrease in physical activity.
• Genes: Your genes determine your body type and composition and your inherited body type may have more body fat that’s hard to lose.
• Hormones: Hormones also influence body composition. Testosterone is a male sex hormone that increases muscle mass.
• Sex: Due to a combination of genetics and hormones, women have more body fat than men.

Changing Your Body Composition

To change your body composition, you need the right balance of physical activity and nutrition to reach your goals. Slow and steady changes work best when you want to increase muscle and lose fat. It's important to remember that some factors will remain out of your control.

Before making any changes to your diet or workout routine, consult with your primary care provider or a registered dietitian for guidance. 

A Word From Verywell

Your body composition may help you better understand your current level of health and fitness. It can also serve as a measuring tool to monitor progress when starting a new fitness or wellness program.

When trying to change your physique to improve your fitness level, it’s important to implement a safe and effective workout routine and a balanced eating plan. Seek advice from a health care professional before making any major changes to your diet or exercise routine.