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By: Paul Hough (Lead Sport/Exercise Scientist)

One of my clients recently transformed his body, reducing his weight by 10.9 kg and body fat from 24.7% to 9.5% (figure 1). He achieved this dramatic result in just six months!  

Chart showing a weight reduction of 10.9 kg and body fat reduction from 24.7% to 9.5% in six months

Like many people, Tom routinely measured his weight and noticed he was putting weight on. However, he specifically wanted to reduce his body fat and knew that monitoring weight didn’t allow him to quantify fat loss. Therefore, Tom came to see me for a consultation where we discussed his diet/exercise habits and developed a plan to gradually reduce his body fat. The St Mary’s social media officer caught-up with Tom to ask him about his dramatic transformation.

In this article I will briefly discuss the difference between ‘body weight’ and ‘body composition’ and the purpose of measuring body composition. In part two I will discuss how Tom reduced his body fat using the ‘calorie counting’ method alongside a structured exercise programme.

Losing weight

People who want to reduce body fat intuitively focus on changes in their *body weight* (regular weigh-ins) to monitor their progress. The ‘losing weight’ approach assumes that all body weight is equal and any reduction in weight is a positive outcome, but this is not always the case.

*In science, ‘body weight’ is referred to as ‘body mass’ (e.g. the body mass index). The two measures are technically different, but this distinction is not usually important in practice.  

What is body composition?

Body composition can broadly be defined as the proportion of fat mass and fat-free mass (FFM). The amount and distribution of body fat and FFM are important for health and performance. In general, a healthy body composition is one that includes a lower proportion of body fat and a higher proportion of FFM. This ratio is particularly important for athletes.

Why is measuring body composition useful?

A reduction in body weight can provide a rough indication of fat loss. However, conventional weighing scales cannot detect what type (fat, muscle, water etc.) of mass has been lost or gained. For example, a decrease in muscle mass (and subsequent decrease in total body weight) could be perceived as a positive outcome. However, this is misguided as muscle serves important functions within the body and is crucial for health and longevity. Therefore, it is useful to monitor body composition alongside weight.

What clients come to the lab for a body composition assessment?

Most of my clients are athletes, such as cyclists, who need to stay relatively lean for their sport, as there are few sports where having extra body fat is an advantage. I also work with clients who have specific (non-athletic) body composition goals. For example, some people want to lose body fat for an event (holiday, wedding etc.) whereas others are trying to increase muscle mass.

How can body composition be measured?

There are various techniques to measure (or more accurately ‘estimate’) body composition. Each method is based on assumptions regarding the proportions and properties of fat mass and FFM. The two most accessible/affordable methods are the skinfold (calliper) technique and bioelectrical impedance analysis (BIA). However, universities and medical facilities offer more sophisticated methods, such as Dual-Energy X-Ray Absorptiometry (DEXA) and Air Displacement Plethysmography (Bod Pod).

What is the ‘gold-standard’ method of body composition assessment?

The only direct method of measuring body composition is cadaver dissection, i.e. only the dead can be guaranteed a body composition assessment that is 100% accurate! As it isn’t possible to directly measure body fat in living humans, we use indirect methods that estimate body composition. Each assessment method has a degree of variability, which is largely dependent on: the equipment, tester, and environmental conditions. If these variables are controlled carefully, then the variability of the measurement is reduced. 

How is body fat estimated?

Most body fat methods are based on a two-compartment model, where the body is divided into two compartments:  fat and FFM.  Fat includes all fat within the body and FFM includes everything that isn’t fat, such as organs, muscle, bone and water.

Does an increase in FFM indicate more muscle?

Clients who want to increase muscle mass for performance or aesthetic reasons often focus on increasing FFM. However, when using methods that apply the two compartment model, it is important to note that a change in FFM does not always indicate an increase/decrease in muscle. For example, a change in hydration affects total body water, which is included within FFM. This consideration is particularly important when using bioelectrical impedance analysis. 

Bioelectrical impedance analysis (BIA)

Various commercially available weighing scales can estimate body fat using a principle called bioelectrical impedance analysis (BIA). BIA is based on how an electrical current is conducted through different body tissues. The BIA device sends a low (safe) electrical current through the body, which the user can’t feel. The current flows more easily through areas of the body that are composed mostly of water (such as muscle) than it does through fat. The analyser measures this ‘impedance’ and combines the data with other factors (height, weight, gender and age) to estimate body composition.  

When using BIA, it is common for body fat to ‘change’ from day-to-day and even within a day. This variability is usually caused by changes in hydration status, body temperature or taking a reading at a different time of day. For example, if a person is dehydrated, the amount of fat will likely be overestimated. We demonstrate this effect during a practical class, which involves measuring the body composition of a student using BIA. After the initial measurement the student drinks 1-2 pints of water and the measurement is repeated after 30 minutes. The record change in body fat in this class was a decrease from 15 to 10%. If only losing body fat was this easy! 


Skinfold calipers are used to measure subcutaneous body fat (fat under the skin). The tester uses specialised callipers to ‘pinch’ a double fold of skin and subcutaneous adipose tissue, which is then measured using the calliper in millimetres (mm). Several skinfold measures are taken around the body and are reported as a sum of (∑) skinfolds, which involves summing all the measurement sites. Using various equations, the skinfold data can be used to calculate body density and body fat percentage. As with all body composition methods, the skinfold technique involves a series of assumptions, such as adipose tissue compresses in a predictable manner and the thickness of skin is negligible.  

We typically use the skinfold method when working with athletes, as the method offers good repeatability and we can perform this when travelling. Furthermore, this method allows us to perform segmental analysis. I.e. identify the areas around the body where the subcutaneous fat has changed. Other methods, such as BIA, often produce a single figure (% body fat), so it is not possible to identify where the fat has increased/decreased.

The accuracy of skinfold measurements is strongly dependent on the competency and accuracy of the person taking the measurements. Therefore, practitioners should use standardised measurement sites and, ideally, undergo specialised training in anthropometric measurement. The gold standard for this training is provided through the International Society for the Advancement of Kinanthropometry (ISAK). This training is offered by my colleague, Dr Nicola Brown

How does the Bod Pod work?

For the majority of my body composition consultations I use The Bod Pod, which applies a principle called air displacement plethysmography to measure body volume and mass. These two variables are used to calculate body density. Body density is then included within an equation to provide an estimation fat mass and FFM.

Why do different methods produce different results?

Each body composition method applies a different principle to estimate fat mass and FFM. This means the actual measurement of body fat varies between different methods, often significantly. Therefore, it is important to use the same method to detect meaningful changes in body composition over time.

How can I Improve the accuracy of the measurement?

As discussed, all body composition methods have a degree of variability. To minimise the variability and maximise accuracy, it is essential to standardise the measurement as best you can:

  • Use the same equipment
  • Measure at the same time of day
  • Avoid measuring after exercise
  • Go to the toilet before you measure

Are there other ways I can monitor body composition?

If you do not have access to the methods above, measuring weight is a good option. However, as weight varies from day-to-day, I recommend measuring your weight daily (following the tips above) and taking a weekly average. It is also a good idea to measure circumferences, particularly *waist circumference* (WC). Monitoring changes in circumferences (girths) around the body provides a rough estimation where body fat is changing. Common sites to measure are: chest, upper arm, thigh and calf. However, if you are performing resistance training, your muscles could be increasing in size (hypertrophy). The circumference measurement won’t enable you to identify the type of tissue that’s changed, so it is also useful to take some photos to monitor your progress, as Tom did: 

*Regardless of body composition goals, measuring waist circumference is a very useful indicator of health, as excess fat around the waist is associated with various health issues. A waist circumference of 102 centimetres (40 inches) or more in men, or 88 centimetres (35 inches) or more in women is associated with an increased risk of type 2 diabetes, heart disease and high blood pressure.


  • A change in body weight provides a rough indication of changes in fat mass over time.
  • Body weight fluctuates daily and can be misleading. For example, an increase in weight does not always mean body fat has increased.
  • It is useful to monitor body composition, as this provides an indication of the type of mass you are losing or gaining.
  • There are many body composition assessment methods that estimate body fat. Each method has a degree of variability, which can be minimised by standardising the measurement protocol.
  • If you do not have access to body composition tools, it is useful to monitor your weight, waist/limb circumferences and take photos. It is important to standardise these measures as best you can.