Bioimpedance analysis for the study of body composition

Bioimpedance analysis, also known as bioelectrical impedance analysis or BIA, studies how the body resists the passage of a very low-intensity electrical current. Based on this measurement and data such as weight, height and age, the equipment uses equations to estimate various components of body composition. It can complement a nutritional assessment, contribute to the study of muscle mass and help to track changes in the body over time.

What is bioimpedance analysis and how does it assess body composition?

Tissues rich in water and electrolytes conduct electrical current more readily than adipose tissue. The device records the resistance to the flow of this current, known as electrical impedance, and uses the result to estimate various body composition components.

The device measures electrical properties; it does not directly observe fat, muscle or water. These variables are calculated using equations or algorithms, so the results may vary depending on the device used, the calculation model, the population for which it has been validated and the person’s clinical condition.

Most people do not feel the current passing through them.

What does bioimpedance analysis measure?

Bioimpedance analysis can estimate total body water, fat-free mass and fat mass. Some devices also provide estimates of muscle mass and the distribution of these components between the arms, legs and trunk.

Fat-free mass is not the same as muscle mass. It also includes water, organs, bones and other non-adipose tissues. Therefore, an increase in fat-free mass does not necessarily mean that muscle has been gained.

Some analysers display parameters such as visceral fat, bone mass, basal metabolic rate or metabolic age. These are estimates or calculations made by the device’s algorithm and are not equivalent to a direct anatomical measurement.

Certain devices also calculate the phase angle, a parameter derived from the electrical properties of tissues which can provide additional information in some clinical contexts. It does not, on its own, establish a diagnosis.

Types of bioimpedance and body composition equipment

Some devices take measurements via the feet, others use both hands and feet simultaneously, and some use adhesive electrodes whilst the person is lying down.

Some analysers operate at multiple frequencies or provide separate estimates for the arms, legs and torso. The path of the current, the frequencies and the algorithms used all influence the result.

For this reason, two different devices may give different readings for the same person. To assess progress, it is usually preferable to repeat the measurements using the same equipment.

At the Publio Cordón Memorial Hospital, InBody is used—a bioimpedance analyser for assessing body composition. InBody is the name of the equipment used; the medical technique is bioimpedance analysis.

What is bioimpedance analysis used for?

It can complement a nutritional assessment when it is useful to gain an approximate understanding of the amount and changes in fat mass, fat-free mass or body water.

It can also be used during a nutritional treatment, an exercise programme or a recovery process when monitoring changes in body composition provides relevant information.

Bioimpedance analysis, muscle mass and sarcopenia

In the study of sarcopenia – which is associated with a decline in muscle strength and in muscle mass or quality – bioimpedance analysis can be used to estimate muscle mass.

It does not confirm the diagnosis on its own. The assessment also includes muscle strength and, where appropriate, physical performance.

Bioimpedance analysis and obesity

In people with obesity, it can complement weight, body mass index and waist circumference. Body composition provides additional information, but does not on its own determine the severity of obesity, nor does it replace the assessment of metabolic risk and its potential complications.

At the Publio Cordón Memorial Hospital, the Obesity and Cardiometabolic Risk Unit incorporates advanced bioimpedance analysis into body composition analysis and integrates it with the assessment of metabolic profile, cardiovascular risk and the diagnosis of obesity and sarcopenic obesity.

Not everyone who wishes to lose weight or change their diet needs this test. Its usefulness depends on the clinical question and whether the information can contribute to a decision or to follow-up care.

How to prepare for a bioimpedance analysis

The exact preparation depends on the device and the protocol used. There is no universal guideline applicable to all equipment.

The aim is to carry out the measurement under comparable conditions and to minimise variations caused by recent changes in diet, fluid intake, exercise or fluid distribution.

The following is usually recommended:

• Follow the instructions provided regarding food and drink.

• Maintain your usual fluid intake, avoiding drinking unusually large amounts immediately beforehand.

• Avoid strenuous exercise in the hours leading up to the test, where the protocol so specifies.

• Empty your bladder if asked to do so.

• Carry out the tests, where possible, under similar conditions.

Before the test, you must provide information about any pacemakers, defibrillators or other implanted electronic devices. The team will check their compatibility with the analyser being used and with the manufacturers’ instructions.

You should also inform the team of any pregnancy or possibility of pregnancy, oedema, ascites, amputations, prostheses or recent significant changes in fluid intake. You must not stop taking diuretics or other medicines on your own initiative.

How to carry out a bioimpedance analysis using InBody

Before starting, details such as weight, height and age are verified, as the analyser can incorporate these into its calculations. Any circumstances that may influence the measurement or its interpretation are also checked.

With a bioimpedance analyser such as InBody, the person stands barefoot on the platform’s electrodes and maintains contact with other electrodes using their hands. During the measurement, they must remain still for a few seconds.

The device applies a very low-intensity electrical current and records the impedance. Based on this information, it generates various estimates of body composition.

Most people do not feel the current. The technique does not involve needles, radiation or contrast agents, and does not usually require any recovery time. The measurement is obtained quickly, but the automatic report generated by the device does not, in itself, constitute a clinical interpretation.

How to interpret the results of a bioimpedance analysis

The results should be understood as estimates calculated from an electrical measurement, not as an exact snapshot of the tissues.

Before interpreting them, the healthcare professional assesses whether the scan has been carried out correctly and whether there are any circumstances – particularly significant changes in hydration levels – that might influence the figures.

Fat mass and body fat percentage

The estimated amount and percentage of fat can be useful for observing trends over time.

They should not be regarded as an exact anatomical measurement nor used in isolation to determine clinical risk.

Fat-free mass and muscle mass

Fat-free mass includes muscle, but also water and other tissues. Therefore, an increase in this figure does not necessarily indicate muscle gain.

Some analysers calculate total muscle mass, skeletal muscle mass or height-adjusted indices. When used to study sarcopenia, these are combined with muscle strength and, where appropriate, physical performance.

Body water and hydration

Water has a significant influence on the measurement. Fluid fluctuations can alter estimates of fat-free mass and fat mass even if the tissues have not changed in the same proportion.

Some devices also estimate how water is distributed across different body compartments. This information can complement the assessment, but on its own it does not diagnose oedema, dehydration or heart, kidney or liver disease.

Comparison between different measurements

To assess trends, it is usually preferable to use the same device and carry out the measurements under similar conditions.

A change becomes more significant when it is consistent across subsequent check-ups and correlates with changes in weight, body measurements, strength, mobility or clinical status.

The result is then considered in conjunction with the reason for requesting the test. An isolated figure or graph does not, on its own, usually justify changes to diet, fluid intake, exercise or treatment.

Limitations, risks and precautions associated with bioimpedance analysis

Bioimpedance analysis relies on equations and algorithms developed for specific populations. Its ability to accurately represent body composition may be reduced when a person’s characteristics differ significantly from those used to validate these models.

Hydration is one of the main factors influencing the results. Oedema, ascites, dehydration or rapid changes in fluid balance can significantly alter the estimates.

The devices are also not directly interchangeable. Different electrodes, frequencies, electrical pathways and algorithms can produce different results.

Some figures included in the reports, such as metabolic age, visceral fat, bone mass or basal metabolic rate, are secondary calculations and should not be interpreted as anatomical measurements or diagnoses.

The result may also be affected by movement, poor contact with the electrodes, an unsuitable posture or incorrectly entered personal details.

The current used is very low in intensity and is not usually felt. The test does not use radiation nor does it require needles. Implanted electronic devices, such as pacemakers or defibrillators, must be checked before the measurement to ensure compatibility with the equipment used.

During pregnancy, there is no exposure to radiation, but physiological changes in body fluids and body composition may mean that some standard equations are less representative.

Bioimpedance, DXA and other body composition tests

The most appropriate technique depends on the information the medical team needs to obtain. Weight, body mass index and waist circumference provide different information and can complement a body composition analysis.

When assessing muscle function, dynamometry – which measures strength – may be used. Dr Susana Monereo includes both bioimpedance analysis and dynamometry amongst the techniques used in her clinical practice at Memorial.

Dual-energy X-ray absorptiometry (DXA) also allows body composition to be assessed using a different principle. Its results and those of bioimpedance analysis are not directly interchangeable.

CT and MRI scans can provide information on muscle and fat when carried out for a specific clinical indication.

If the aim is to determine resting energy expenditure, the specific technique is indirect calorimetry. The basal metabolic rate shown by many bioimpedance analysers is a calculated estimate.

Specialists in bioimpedance analysis and body composition

At the Publio Cordón Memorial Hospital, bioimpedance analysis forms part of the Endocrinology and Nutrition department, within the Metabolic Centre. The specialism includes the assessment of body composition as part of the approach to obesity and cardiometabolic risk.

Dr Susana Monereo Megías specifically includes bioimpedance analysis for the assessment of body composition amongst the techniques routinely used in her clinical practice, alongside dynamometry, nutritional ultrasound, liver elastography and indirect calorimetry.

Furthermore, she heads the Obesity and Cardiometabolic Risk Unit, where body composition analysis using advanced bioimpedance analysis is integrated with the patient’s medical history, weight trends, metabolic profile and cardiovascular risk assessment.

The care pathway begins with a clinical assessment to determine what information is required. The measurements provide supplementary data, which the specialist integrates with the rest of the assessment to guide follow-up or subsequent decisions.

Preguntas frecuentes

Are InBody and bioimpedance analysis the same thing?

Not exactly. Bioimpedance analysis is the technique used to estimate body composition. InBody is the analyser used to carry out this measurement. The results depend on the device, its algorithms and the conditions under which the test is carried out.

Do you have to go on an empty stomach?

It depends on the device and the protocol used. The preparation is designed to minimise variations caused by food, drink, exercise and recent changes in fluid intake. The instructions provided before the test must be followed.

Can I have this done if I have a pacemaker or an implantable defibrillator?

You must inform the staff before the measurement is taken. Whether the measurement can be carried out depends on the implanted device, the analyser used and the manufacturers’ instructions.

What does ‘visceral fat’, which appears in some reports, mean?

It is usually an estimate calculated by the device’s algorithm. It is not the same as directly visualising or measuring the fat within the abdomen using CT or MRI scans.

Can bone mass measurements be used to diagnose osteoporosis?

No. The ‘bone mass’ shown by some analysers is an estimate and is not a substitute for a bone densitometry scan.

What does ‘metabolic age’ mean?

This is a comparative calculation generated by certain devices. It does not constitute a medical diagnosis and its clinical usefulness is limited.

Why might the result change even though the weight is the same?

The distribution of body fluids or the estimates of fat mass and fat-free mass may have changed. The conditions under which the measurement was taken and the device used also have an influence.

Does a home scales give the same results as a clinical analyser?

Not necessarily. Devices may use different electrodes, frequencies and algorithms, so their readings are not directly comparable. To assess progress, it is usually more useful to use the same equipment and similar conditions.

Medical information for general guidance

This factsheet aims to help you understand what bioimpedance analysis measures, which body components it estimates, how it is carried out, and why factors such as hydration and the device used influence the interpretation of its results.

The Publio Cordón Memorial Hospital promotes accurate, understandable and useful medical information and carries out research and scientific outreach aimed at generating knowledge, furthering medical progress and helping patients and their families to better understand healthcare processes.

The indication for the test, preparation for it and the interpretation of the results must be tailored to each individual’s clinical situation and the device used. The figures obtained should not be used in isolation to diagnose a condition or to alter diet, hydration, exercise or treatments without professional assessment.