Body composition is a crucial aspect of metabolic health. It shows us the distribution of lean mass and fat mass in our bodies, as well as why the location of fat tissue matters for your metabolism.
Unfavorable body composition, like high fat mass and low lean mass, is strongly associated with significant chronic health conditions, like heart disease, stroke, hypertension, Type 2 diabetes (T2D), certain cancers, sleep disorders, and even death [1-5].
Learn everything you need to know about your body composition, including how to measure it, its relationship to metabolic health, and what you can do to change it.
What is body composition?
Body composition refers to the full spectrum of your body makeup.
At its broadest definition, body composition is your total body mass or weight — but it goes beyond this to include your specific makeup of various tissue types . Body composition is defined by the relative amounts of fat tissue and lean tissue in the body. Lean mass includes bone, muscle, organs, and water, while fat is found in adipose tissue.
Body composition is a better predictor of individual metabolic health than body weight or BMI. Body weight is determined by measuring your total mass (muscle, fat, bone, and tissue), whereas Body Mass Index (BMI) is a score based on your height and weight .
Both body weight and BMI can be used to classify your weight status, but they don’t give a full picture of body composition. For example, you can have a normal BMI and still face risks associated with excess body fat, or be metabolically unhealthy.
How does body composition differ from weight and BMI?
Because your BMI is calculated as your weight indexed to your height, it’s a somewhat better indicator of healthy weight because it takes your stature into account.
BMI is most useful when comparing the weights of people of all different heights or when used to understand health trends on a population scale. It's less useful as a tool for understanding the weight and health of one single person.
While BMI may be a more comprehensive metric than weight, the current BMI categories may not be universally applicable to all populations .
For example, those with high levels of abdominal fat (i.e., greater waist circumference) are at increased risk for cardiovascular disease and other metabolic diseases even if they have a normal weight or low BMI.
Additionally, a high body weight or BMI may not reflect the full picture with different types of body compositions . Athletic individuals who have high muscle mass and/or low percentage of body fat (think, Olympic athletes or bodybuilders) may be exceptionally metabolically healthy but have a seemingly “unhealthy” BMI or body weight .
This is why, on an individual level, body composition is a more effective measure for understanding personal metabolic health.
How is body composition measured?
While body weight and BMI are relatively straightforward and easy to measure, there are several ways to test body composition . From comparison pictures and skinfold calipers to bioelectrical impedance analysis (BIA) and DEXA scans, these measurements can better indicate muscle tissue and fat tissue distribution, which paints a clearer picture of metabolic health status.
1. Comparison pictures
The easiest way to see body composition is by just looking at your body! Visually, we can see our fat and muscle distribution to give a rough idea of body composition.
Comparison pictures are an easy way to track visual progress over time — just take a picture when you start your health journey and continue taking pictures once a month from there, keeping lighting, body stance, and other variables as consistent as possible. That said, while entirely free and easy, pictures alone don’t allow for actual measurements or a deeper meaning.
2. Skinfold calipers
Skinfold calipers measure the thickness of the fat layer directly under the skin to estimate body fat . Clinicians commonly use this measurement as a quick and easy way to find body fat percentages.
This technique has limited use as only tissue under the skin is being measured — visceral fat (fat around organs and within the abdomen) is not measured . However, skinfold thickness can be helpful in monitoring changes over time, provides a data point that photos don’t, and is cheap compared to more advanced methods.
3. Bioelectrical impedance analysis
Bioelectrical impedance analysis (BIA) indirectly measures body fat by sending a very weak electrical current through the body and calculating the resistance to electrical flow . The current flows easily through tissue with high water content, such as muscle, but flows slower through tissues with low water content, such as fat.
By using BIA, you can calculate both lean mass and fat mass in the body. BIA can be measured with a full-body or handheld device but can vary in cost and reliability depending on the machine’s quality.
BIA is used broadly — many gyms have small or handheld versions that can give a rough estimate of body fat percentage, while healthcare clinics may use more advanced or larger machines to give more accurate measurements.
Dual energy X-ray absorptiometry (DXA or DEXA) uses low-energy X-rays to directly assess body composition and can measure the density of tissues in the body .
Although it’s most used for calculating bone density, DEXA scans can also be used as an accurate method to calculate muscle, bone, fat distribution, and body fat percentage. Compared to other methods, DEXA can be expensive and inaccessible as it’s done using a large DEXA scan machine, in a facility with trained technicians.
For those reasons, DEXA is typically only used in health research studies, to diagnose and assess the progression of diseases, and to monitor the body composition of professional athletes. However, DEXA is becoming widely recognized as a useful tool for measuring and tracking body composition and is gaining popularity and use .
5. Body fat percentage
Contrary to popular belief, not all body fat is bad. While excess fat tissue can be harmful, our bodies require fat for basic metabolic functions. Fat tissue is made up of cells called adipocytes, which are hormonally active cells. The greater the number of fat cells or the larger the fat cells are, then the higher the body fat percentage.
Where fat is located in the body matters . This is where body composition comes in – BMI doesn’t consider the distribution of fat throughout an individual’s body and cannot accurately provide information on different kinds of fat deposits.
How does body composition affect metabolic health?
Fat can distribute in a variety of different patterns on a given person’s body, which has implications for what your metabolic response may be to different foods, activities, or stressors.
Scientists have also found that a higher fat deposition in your midsection, or a large waistline, is strongly associated with worse health outcomes . This is because having a lot of belly fat may be more inflammatory (a large waistline is actually one of the markers of metabolic syndrome). A higher percentage of belly fat is also associated with an increased risk for metabolic syndrome, T2D, and death .
This is especially true in people with excess belly fat and is caused by increased free fatty acids and triglycerides in the blood, leading to insulin resistance .
This is an issue for several reasons. When you have insulin resistance, your body stops responding properly to insulin, making it harder for your cells to absorb glucose from the bloodstream.
This can lead to higher levels of circulating glucose and insulin in your blood — which promotes fat storage and triggers a vicious cycle that makes it hard to manage your weight.
High levels of glucose in your bloodstream over a long period can also have severe implications for your heart health, leading to decreased elasticity of your blood vessels (making it harder for blood to pass through) and increasing your risk of heart disease.
That said, while having more fat mass than muscle mass increases your risk of insulin resistance, having increased muscle mass has a number of benefits, including improved insulin sensitivity, lipid profile, and blood pressure control as well as reduced mortality risk.
This is because muscles need glucose for energy, and the more muscle mass you have, the more glucose your muscles will absorb from your bloodstream. On top of that, muscle is more metabolically active than fat (i.e., it needs more energy than other types of tissue) and burns more calories even at rest.
So what does this mean?
While all exercises are great for metabolic health and insulin resistance, building muscle — via resistance training workouts like lifting weights or bodyweight exercises like burpees and squats — is the most important area of focus if you’re trying to improve your body composition .
Put simply, having more muscle mass and less fat mass can improve your metabolic health — and paying attention to body composition is more important than simply seeing the number on a scale go down. Keeping your blood sugar levels stable by monitoring them with a CGM and adjusting your diet and lifestyle in response to your body’s own data is a key part of managing your weight and improving your body composition.
How do you change your body composition?
You can’t tell whether someone is healthy by looking at them — skinny doesn’t necessarily equal metabolically healthy. But a healthier body composition can lead to better metabolism, and the first area of focus (apart from a healthy diet) is focusing on building lean mass, or muscle mass, to help you manage your weight.
1. Resistance training
Resistance training is key to improving body composition. It’s been shown to increase muscle mass and strength, regardless of whether the study participants were metabolically healthy, prediabetic, or had T2D .
Resistance training itself, even without dietary restrictions, can also lead to improved glycemic control, decreases in fat mass (mostly visceral fat tissue), and better cardiovascular health outcomes. Even just one hour per week of resistance training (spread out between 2 sessions) can help improve insulin sensitivity and build muscle mass.
2. Moderate-intensity exercise
Moderate-intensity exercise can improve glucose tolerance and overall metabolic health, even without drastic changes in weight . Aim to get 2.5-5 hours per week of moderate-intensity exercise, switching between aerobic exercises like walking (which can help regulate glucose levels), HIIT-style training that involves burpees, yoga, squats, and more. See our full list of exercises to improve insulin sensitivity.
3. Losing weight
Losing weight may change body composition, but this weight may not be all fat mass – you may also be losing muscle mass. You can improve your metabolic health and maintain muscle mass by having a diet that is overall high in protein, fiber, and fat, but lower in refined carbs . These types of foods help give you stable blood sugar. Sardines, avocados, lentils, and berries are especially great for improving insulin sensitivity.
4. Meal timing
Time-restricted eating or caloric restriction, with or without exercise, can not only lead to decreased weight and a more favorable body composition, but may also help regulate glucose tolerance [25, 26].
Understanding your body composition in detail is crucial to better implementing a healthy lifestyle and food choices that benefit your personal physiology.
- Body composition is a better gauge of metabolic health than body weight or BMI, as it takes tissue distribution into account.
- You can use measurements such as comparison pictures, skinfold calipers, bioelectrical impedance analysis (BIA), and DEXA scans to assess your body composition.
- Fat distribution affects metabolic health. Knowing your body composition and response to different foods can help determine the best lifestyle habits for you.
- Changing body composition through diet and exercise can help regulate blood sugar and insulin levels, promoting better metabolic health. Focus on building muscle via consistent resistance training (at least 1 hour per week) to make the biggest impact on your body composition.