Body Composition Measurements

Laboratory body composition measurements

Laboratory body composition methods refer to clinical tests and assessments that provide an accurate idea of the subject’s body composition. The most common laboratory methods include air displacement plethysmography, bioelectrical impedance analysis (BIA), dual energy X-ray absorptiometry (DXA), hydrostatic weighing, and ultrasound. They are often used in universities, athletic centers, and laboratories for research purposes and developing new concepts. 

While laboratory body composition measurements are the most accurate, they are also expensive and not easily accessible for the larger population. Interestingly, one of their most important aspects is establishing the accuracy of anthropometric methods, or field methods. This is because field methods are a lot faster and easier to do. 

Air displacement plethysmography

Air displacement plethysmography uses air pressure to measure body density. From here, body density can be used to estimate both fat and fat-free mass, making it one of the most accurate methods of measuring body composition. In fact, it is even used as a reference method for other body composition measurements because of its ease-of-use and high level of accuracy. 

Air displacement plethysmography requires a specially made plethysmograph (enclosed chamber) that measures the reduction in chamber volume when a subject is sitting in it. In a way, it is similar to hydrostatic weighing but uses air instead of water to assess body volume. Plethysmographs, such as the BOD POD, contain two chambers – a test chamber and a reference chamber that are connected via a diaphragm. The subject must also be in as little clothing as possible or in tight-fitting clothing with a swimming hat.

Air displacement plethysmography assessment process is divided into three stages; measuring mass, body volume, and thoracic gas volume (air contained in the lungs). First, the subject’s mass is calculated using a precise electronic scale integrated into the chamber. Second, the plethysmograph measures air volume changes during expansion and compression in the test chamber and compares it to the air pressure in the reference chamber. Third, the thoracic gas volume can be directly measured, predicted, entered manually, or retrieved from a previous test. 

Air displacement plethysmography is considered to be as accurate as hydrostatic weighing, but also quicker and easier to do. While BOD PODs are expensive for personal use, they can be found in multiple research laboratories, universities, as well as fitness and athletic centers. 

Bioelectrical impedance analysis

Bioelectrical impedance analysis (BIA) utilizes a weak electric current that flows through the body. Since body water is a great conductor, muscle tissue with higher water content conducts electricity better than fat tissue of lower water content. Thus, measuring the impedance, or resistance, of this electrical current estimates the percentages of fat and lean muscle mass. 

While bioelectrical impedance analysis is a non-invasive and low-cost alternative to measuring body composition, it is not always the most accurate method available. For example, it is sometimes affected by caffeine and alcohol use, as well as exercising right before the analysis. On top of that, female hormones like estrogen and progesterone also affect water retention during different times of the menstrual cycle, which can give different results.

Another thing to point out is that BIA estimates body composition by assuming that every subject has a 73% hydration of fat-free mass. However, there is relatively high variability between individuals in body water content (68-81% of fat-free mass), which is why these results may not be directly comparable between individuals. 

Dual energy X-ray absorptiometry

Dual energy X-ray absorptiometry, also known as DXA or DEXA), is an imaging test that utilizes two X-ray beams at different energy levels to measure bone mass and density. Once the soft tissue absorption is subtracted, the person’s bone mineral density (BMD) can be determined by calculating how each beam is absorbed by the bones.

Unlike regular X-rays, DXA scans use a very low dose of radiation, making them safer regular X-rays. They are also able to detect even the tiniest reductions in bone density, which is why they are usually used to diagnose and monitor bone-related health problems like osteoporosis.

Hydrostatic weighing

Hydrostatic weighing, also known as underwater weighing or hydrodensitometry, is a method of estimating body composition by fully submerging a subject underwater. 

Hydrostatic weighing starts with weighing the subject on land. Once their weight has been determined, the subject is then put on a special scale and submerged into a tank of water. The subject must also exhale all air from their lungs and stay still until the scale gets an accurate reading. 

Once this procedure is repeated three times and averaged, the subject’s fat percentage can be accurately calculated using the Archimedes’ principle. This means that the buoyant force of an object is equal to the weight of the fluid it displaces. Because muscle mass is denser than fat tissue, a person with a higher fat percentage will be more buoyant – and vice versa. 

Although modern technology has provided some new methods for assessing fat percentage and lean muscle mass, hydrostatic weighing is still considered the most accurate method of measuring body composition. However, it also takes time and requires very specific equipment, making it less than ideal for a larger population. 

Ultrasound

Ultrasound has been used since the mid-60s to measure body composition. Ultrasound refers to sound waves with higher frequencies than humans are able to hear. Our ears detect sounds between 20-20,000 Hz whereas ultrasound for diagnostic purposes usually operates at frequencies of 2-15 MHz. 

Ultrasonic imaging works by sending an ultrasound beam through the skin via a transducer. When the beam is in contact with bone, muscle, or fat tissue, it is partially reflected back to the transducer as an echo. The scanner then counts the time of each echo and calculates the distance between different tissues. This creates a two-dimensional picture of tissues and organs. 

During an ultrasound exam, a gel is applied to keep air pockets from forming between the skin and the transducer, because this could block the ultrasound waves from passing into the body. 

Despite being one of the oldest ways to assess body fat, ultrasound is far less popular than other methods to measure body composition. Regardless of some newer analysis methods, ultrasound is still a reliable, non-invasive way to measure subcutaneous fat (directly under the skin) and visceral fat (around the organs). Ultrasound can also be used to measure muscle thickness and examine injuries. 

Anthropometric body composition measurements

Anthropometric measurements, or field measurements, refer to a series of quantitative measurements of the muscle, bones, and fat tissue to analyze body composition. This includes height, weight, body mass index, body circumference (waist, hip, and limbs), as well as skinfold thickness. These results can be used to estimate health and physical fitness, as well as monitor the progress of different nutrition and training programs. 

One of the biggest benefits of anthropometric measurements is that the results can be used for a wide range of people from children all the way to the elderly population. Even elite athletes can benefit from these measurements to track athletic development. 

Body circumference measurements

Body circumference measurements use a simple non-stretch tape measure to evaluate body composition. Because these measurements assess the circumference of the body itself, subjects are usually measured while standing and wearing as little clothing as possible. This helps get an accurate reading of the subject’s body. While circumference measurements offer little knowledge of the proportion of fat and muscle mass, they can be used to track changes in the body and predict possible health risks.

Body circumference measurements can be used for multiple purposes;

• Predicting body fat: Calf, mid-thigh, waist, hip, chest, upper arm, and neck circumference measurements are generally used for predicting overall body fat.
• Estimating changes in muscle mass: Calf, thigh, and upper arm measurements are often used to assess changes in lean muscle mass. Athletes also use this method to track the effectiveness of a training/diet program.
• Predicting the risk of cardiovascular and metabolic diseases: Waist and hip circumference measurements are used to analyze abdominal girth, which is closely tied to metabolic diseases like diabetes. For most adults, healthy waist circumference is roughly 90cm (35in) for non-pregnant women and 100cm (40in) for men.

Body circumference measurements can also be combined to provide a more accurate representation of body composition. One of the most common combined methods is measuring the waist-to-hip ratio to determine how the subject’s body mass is distributed. On the other hand, waist-to-calf and waist-to-thigh ratios are used to measure age-related loss of muscle mass in combination with obesity (sarcopenic obesity).

Body mass index

Body mass index (BMI) refers to the relationship between the weight and height of an individual. It is measured by dividing a person’s weight in kilograms (kg) by the square of their height in meters (m). For most adults, a healthy body mass index is between 18.5 and 24.9.