What’s Your 'Body Composition' All About? Methods Of Assessing Body Composition

How many of you have stood on the scales after Christmas with dread of what is going to flash back at you, but sort of accepts that the impending weight gain is undoubtedly because of festive indulgences? On the other hand, how many of you were disciplined over Christmas and the New Years, attended the gym most days as normal and hoped to have at least maintained your pre-Christmas weight?

Well a few customers have hit up our social media channels perplexed as to why they GAINED some festive weight despite staying pretty darn disciplined the whole time…how could this be? Well quite simply you might not have been quite as disciplined as you thought you were, or alternatively, you absolutely were as disciplined as you thought, and actually ended up changing your body composition…so basically you will have changed the type of tissue (muscle or fat) that your body is predominantly made up of.

Different bodily tissue types weigh more than others, a classic example is muscle and fat whereby a pound of fat takes up far more space than a pound of fat. The extra space required to store 10 pounds of fat compared to 10 pounds of muscle is vast, and this shows on the outside when you stand an overweight person next to a muscled athlete. So if you’ve noticed that despite your discipline and hard work that you in fact put on some weight, then chances are that you did lose fat, but gained some lean, hard earned muscle in its place!

So how do you go about proving this theory, well, a good start is to take waist, mid-upper arm and mid-upper thigh circumferences in order to gauge fat loss. Should you maintain weight, yet your waist size has decreased then you almost certainly burnt fat and swapped it with muscle…well done you! This is one way to decipher this, but should you wish to know your exact body composition i.e. what proportion of your body is muscle and fat, or fat mass, fat free mass and lean body mass (bones, muscles, organs, and connective tissue), then there are several ways of measuring this.

Methods of assessing body composition

All body composition assessment techniques are estimations, they are never exact, in fact the only way to do this is to assess a cadaver…better known as a dead body. Consequently, should you want to measure body fat, you would need to factor the potential error that comes with such assessments. Even the best methods of assessment come with a 2-3% error range. Here is a list of some of the main methods of assessing body composition:

Hydrostatic Weighing (underwater)

This is a process that requires being fully lowered into a water-filled tank for approx 10 secs having expired all of the air out of your lungs. Our weight on land is different to our weight in water (mass stays the same of course), and so the difference between these two figures is used to estimate body density. Experts can use this figure to estimate body composition based on the density reading, as dense readings imply dense bodies, and dense bodies mean more mass. This technique is regarded as the ‘Gold standard’ way to measure body composition…similar to how Optimum Nutrition 100% Gold Standard Whey is deemed to be one of the best proteins on the market (shameless supplement plug I know ha)!

Air displacement (Bod Pod)

This method of assessment also uses whole body measurement, only there is no water involved on this occasion…this time it’s air. You sit in an enclosed egg shaped capsule known as a Bod Pod for approx 1 min, during this time the system assesses the amount of air that is displaced by your body. The amount of air displaced by the physical space your body occupies is then used to estimate the density and thus composition of your body.

Dual- Energy X-Ray Absorptiometry

You ladies and gentleman might know this as a DEXA scan. According to Eberle, (2014) the DEXA has quickly become the new Gold standard for estimating body composition because of its precision, accuracy and consistency. You are required to lie still for 10-20mins whilst a whole body scanner emits 2 low-dose X-rays, one scanning bone and one scanning soft tissue. The absorption of X-rays is recorded and used to establish the composition of the body, in particular the bones (bones absorb photons from X-rays). The machine and practitioner can then give relative estimations of fat and lean tissue.

Bioelectrical Impedence Analysis (BIA)

Probably the most common method of assessing body composition out there. You’ll probably see numerous fitness instructors or personal trainers walking around with hand held varieties of BIA machines, or maybe the stationary units in the assessment rooms, these are very popular because of their cost effectiveness and reasonable estimation capacity. They work by holding and/or standing on metal pads (electrodes) that pass a low voltage electric current through your body. Some BIA units also attach electrodes to your hand and ankle (usually on the left side). The electric impulse is better conducted through water and electrolytes, and considering your lean muscle contains most of this, so the more muscle you have the faster the impulse passes around your body…the time is then factored to estimate your body composition.

Skinfold Calipers

A handheld method that is actually one of the more reliable techniques. The practitioner uses hand held callipers that open and close to gently pinch fat tissue directly under the skin (subcutaneous fat). The main sites measured are the abdomen, back of the arm, thigh, hip and back of the shoulder. The practitioner should be careful not to pinch lean tissue too as this can skew the readings, consequently human can often play a part in this method meaning you should try to source a qualified and skilled practitioner. Body composition is calculated by applying the skin fold measurements into a set formula that will give an estimation of fat in relation to lean tissue. There is a 3-5% error range in this technique, so the more skilled the practitioner the better!

Body composition analysis techniques are used in many walks of life, but mainly among athletes and by health practitioners. Different body compositions suit different goals, so it would be counter intuitive for a marathon runner to have a high body fat percentage because this literally means they have to transport more weight from start to finish…NOT conducive to economical running! So the ideal body composition varies from person to person and from sport to sport. Scales serve as a reasonable gauge of weight loss over time, but if you hit a plateau or want a truer representation of your progress then do try to use a body composition assessment technique.


Eberle. S, G. (2014). Endurance Sports Nutrition. 3rd Ed. Fuel your body for optimal performance. Methods of assessing body composition. IL: Human Kinetics.



About the Author

Job Role Qualified Dietitian and Sports Nutritionist Qualifications BSc (Hons) Sports Science | BSc (Hons) Dietetics Tom has always participated in sport both recreationally and competitively which led to an unquenchable thirst for information on anything health, nutrition and fitness. After leaving school Tom went on to play for a football academy during which time he studied Sport and Exercise Science. From here he went on to study a BSc (Hons) Sport Science at UEA followed by his second BSc (Hons) degree, this time at the University of Hertfordshire studying Dietetics. Tom has worked in the fitness, educational and clinical nutrition industry starting out at David Lloyd Health and Leisure Clubs. He then went on to work as a Dietitian (RD) in the NHS, during which time he conducted clinics for healthy eating, weight loss and weight gain, as well as specialised consultations on Diabetes, IBS and Coeliac disease to name a few. He has vast amounts of experience at devising diet plans and supplement regimens, as well as working in the community with schools and competitive athletes. As Head Nutritionist and Supplement expert at Discount Supplements Tom is here to provide current and evidence based health and nutrition information to help you reach your health and fitness goals!
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