Body composition: what it is, how it is measured and what it can tell you

What is body composition?

Body composition divides weight into fat mass and fat-free mass. Fat performs necessary functions. Fat-free mass includes muscle, water, bone and other tissues [1]. That is why “fat-free mass” does not mean “muscle”. A change in water can alter the estimate without you having gained muscle.

How these components change over time also matters. To assess a measurement, we ask about your activity, diet and health. The percentage alone does not summarise strength, wellbeing or metabolic health. We look for an interpretation that is useful to you, without turning the figure into a grade for your body.

Why are weight and body mass index not enough?

Weight is the sum of all tissues. Two people with the same weight can have different proportions of fat and muscle. Body mass index relates weight to height, but it does not separate tissues either [1]. To understand its categories and limitations, read our guide to underweight, healthy weight, overweight and obesity.

Weight can vary with fluid levels, food that you are still digesting or the time of the measurement. That is why a single reading says little about a change in fat. If you exercise and the scales leave you puzzled, read why your weight may not go down when you exercise.

How are fat and fat-free mass measured?

Bioelectrical impedance passes a very weak electrical current through the body and measures the resistance of the tissues. The device calculates body water and then estimates fat and fat-free mass using equations [1]. It is quick and practical, but hydration, recent exercise and the model used can affect the estimate. It does not directly measure each muscle.

At Alimmenta’s in-person consultations, we measure body composition using bioelectrical impedance. We use the result as part of the assessment, alongside your history and goals. If we repeat the test, we try to keep the conditions comparable. A different decimal place may be due to the method itself.

Skinfolds use callipers to measure the thickness of the tissue beneath the skin at specific points. They can be used to track changes in subcutaneous fat. Estimating total body fat requires equations and a trained practitioner; the choice of site and the pressure applied introduce error [1]. They do not provide a direct picture of all internal fat.

Circumference measurements use a tape measure to measure body contours. They are useful for observing changes in size when taken at the same place each time. On their own, they do not distinguish between fat, muscle and water [1].

DEXA densitometry (dual-energy X-ray absorptiometry) uses low-dose X-rays and distinguishes fat, lean tissue and bone mineral content [2]. It provides more detail, although it also requires a careful protocol and has limitations in interpretation [2].

A measurement is useful when it answers a specific question and is interpreted in the context of your life.

How to prepare for a bioelectrical impedance measurement

Preparation helps make bioelectrical impedance measurements comparable. Always follow the protocol given to you by the clinic [1]. Stay normally hydrated, avoid vigorous exercise before the test, and empty your bladder and remove metal objects immediately beforehand [1]. Make a note if you have recently consumed alcohol or caffeine.

Also, do not interpret a reading taken after heavy sweating as though it were the same as one taken on an ordinary day. It is helpful to note the time, recent exercise and any circumstances that may affect fluid levels.

If you menstruate, also note the phase of your cycle when comparing results. During menstruation, one study observed a weight increase of about 0.5 kg, mainly due to water retention [3]. This change is not fat.

Before repeating the test, check the conditions recommended by your practitioner. If you have exercised that day, eaten at a different time or are retaining fluid, we can postpone the test or interpret it with caution. Measuring less frequently may be enough to answer the clinical question.

What body fat percentage is considered healthy?

There is no single healthy body fat percentage for everyone. Age, clinical context and the measurement method all affect how it is interpreted [1]. If you need your result assessed, discuss it at a consultation.

DEXA results should not be compared with readings from a home scale. Published reference values vary according to the population and measurement method [1]. If you want to understand your result, we first consider your age, health, activity and how it was measured.

How do diet and exercise change body composition?

If you want to lose fat, we work on an eating plan that you can sustain. We recommend a moderate energy deficit, without extreme restrictions. Eating enough protein during weight loss helps preserve muscle mass in adults with overweight or obesity [4]. The right amount depends on the clinical situation; we do not give one figure for everyone here.

Strength training on two or more days per week helps preserve fat-free mass. One review found that, when added to a weight-loss plan, it protects fat-free mass and increases the amount of fat lost [5].

Exercise is also adapted to your starting point and any medical limitations. Gradual progression matters more than pursuing a rapid change in the measurement.

In practice, we can start by including a source of protein in your main meals and following a manageable strength-training plan. Then we review your progress, hunger, energy and ability to train.

For the specific goal of gaining muscle, see our guide to eating to gain muscle mass. For a complete fat-loss plan, see the guide to losing weight without regaining it.

Body composition in athletes

In sport, measurement only makes sense if it helps inform a decision about nutrition, recovery or training. A consensus on sports health found no single percentage that guarantees a performance advantage [6]. Strength, energy availability and the specific sport also matter. Measuring too often can shift attention towards a number [6].

If you train to compete, we discuss your goals and schedule before choosing a method. We avoid turning body fat into an aesthetic requirement. The training, meals and recovery plan is covered in sports nutrition. If your goal involves weight reduction, it is best to coordinate this with the person overseeing your health and training.

Is a home bioelectrical impedance scale useful?

A home scale can give an indication of trends if you always use the same device under similar conditions. Its percentage is an estimate. Hydration affects the calculation, and devices use different equations [1]. That is why we would not compare its decimal readings with a DEXA scan as though they measured exactly the same thing.

A home bioelectrical impedance scale gives estimates that can vary. You do not need to measure yourself to look after your habits, strength or wellbeing. If measurements cause you distress, we can discuss this with you and with Psychology.

What interpretation mistakes do we see in consultations?

We see three misunderstandings that make results difficult to interpret. The first is calling all fat-free mass “muscle”. The second is comparing different devices. The third is attributing a change observed after intense training to fat. These are interpretation issues, not a lack of effort.

We also see that some people are given a percentage without knowing what to do with it. Before repeating the measurement, we agree on what decision it could help inform. Sometimes it is enough to review strength, nutrition and how you feel. At other times, the measurement can help us adjust the plan. In either case, the information should serve you.