Assessment of Body Composition in Bodybuilders

There are frequently posts on regarding how to measure body fat percentage (%BF). Bodybuilders get too hung up on what their body fat should be. Whilst the figure has its uses there are problems:

Firstly, measuring %BF accurately and precisely is extremely difficult; there are a number of methods which will be discussed below.

Secondly, bodybuilding is a sport of aesthetics and how much we weigh, how fat we are, etc are completely irrelevant. In my experience as a bodybuilder it is very easy to become overly obsessed with these figures.

In my opinion it’s far better to go by what the mirror says, how you feel and comments by your trusted bodybuilding friends.

Body Mass Index (BMI) or Quetelet Index

BMI is an index measure of body fat often referred to by doctors and life assurance companies as an assessment of disease risk. It is a figure which gives an indication of your supposed weight for your height calculated by tables (there are many on the Internet) or by the equation:

BMI = weight in kilograms
          height in metres2

Thus BMI is measured in kg/m2

In 1981 Professor Garrow who proposed grades of obesity to be:

Underweight <19
Normal 19-24.9
Grade 1 (overweight) 25-29.9
Grade 2 (obese) 30-39.9
Grade 3 (morbidly obese) >40

There are a number of flaws with the BMI measurement. To start with, BMI only indicates the degree of being over- or underweight, and does not reveal what the underlying problems are. It also makes no account for the distribution of obesity; indeed, where fat is distributed on the body has been linked to increased risk of certain diseases. BMI is more reliable for assessing disease risk (such as cardiovascular diseases and some cancers) when used in conjunction with waist-hip ratio (WHR).

WHR = waist circumference (cm)
          hip circumference (cm)

WHR <1 – gynoid deposition of fat (pear shaped) and reduced risk of diseases WHR >1 – android deposition of fat (apple shaped) and increased risk of diseases

Furthermore, BMI does not take into account skeletal size, amount of body water and, most notably for the bodybuilder, muscle mass. Using the BMI scale in bodybuilding puts most of us as ‘overweight’ and in many cases ‘obese’, even if we have very low body fat! When a degree of objectivity is used for the ‘normal’ person BMI can be useful, but for the bodybuilder this measurement has no use.

Methods of Measuring Body Fat

There are a number of methods which can be used to measure %BF. I will discuss most of these with their merits and disadvantages. In evaluating the use of each method accuracy, precision and relationship need to be assessed. Accuracy is how true the measurement is, evaluated by comparing with other methods. Precision is how reproducible the result is and the frequency of getting the same answer. Relationship is the correlation of two or more of the methods.

Body Dissection

Cutting up your own body is an extremely accurate way of measuring percentage and distribution of body fat. But is not very practical really, is it?!

Under Water Weighing

One of the more accurate methods, but not very convenient as it needs a pool and measuring equipment. The subject is weighed in air, and then in water and the difference is the upthrust displacement by water. There are complex calculations from this, which do carry some assumptions including the density of fat being consistent.

Magnetic Resonance Imaging (MRI)

Many hospitals have MRI scanners which are extremely expensive. This is possibly the most accurate and useful method of measuring percentage, distribution and actual amounts of all the main body composites. Unfortunately every time a subject is measured it costs a fortune. This is not very practical.

Total Body Water (TBW)

This is a laboratory method where the subject is given deuterium oxide (or a similar substance which will distribute through all tissues and is not metabolised). TBW can then be measured and from this fat free mass (FFM) and %BF. There are potential sources of error, it’s expensive and again it is not very practical.

Total Body Potassium (TBK)

A gamma counter measures the amount of a type of potassium which is assumed to be a proportion of FFM. Again there are assumptions, the equations are complex and it’s impractical and costly.

Dual Energy X-ray Absorptiometry (DEXA)

A device emits X-rays at two different energy levels and a detector measures the attenuation of the energy through the body. From this bone density and soft tissue can be calculated. Another use of this method is for showing distribution of fat on the body. This is a very accurate and reproducible method, has good relationship correlation with other methods and is frequently used in hospitals. It is however impractical for bodybuilders and expensive.

Bioelectrical Impedance

This method relies on the assumption that fat is a relatively poor conductor of electricity and FFM (includes water) is a good conductor. An electric current is passed through the body and, with equations, FFM and fat can be calculated. In practice the machines used to measure %BF this way are either scales, finger held devices or electrodes to the feet and hands.

This method is possibly the favourite method of gyms as devices are cheap and easy to use. They are also ‘fun’ and clients like to be measured by an electronic gadget. Accuracy and precision are both poor in bioelectrical impedance. There are too many assumptions and factors which can skew results; if hydration status is slightly off from normal (basal) level the reading will be incorrect. In practice this method would be accurate and precise if the subject is measured first thing after waking, in good health, has not eaten or drank anything for 12 hours and has consumed no caffeine or alcohol for 24 hours. Unfortunately gym staff measure subjects often after a hard sweaty workout.

Near Infra Red Irradiation (NIRI)

This is another convenient and non-invasive ‘gym method’. These cheap and easy to use devices are strapped to the arm and measure body fat through infrared light as different tissues absorb this light in different extents. The main limitation is the assumption that the local tissue is representative of the whole body fat, which is obviously unlikely on the upper arm. There is also significant observer error.

Skin Fold Thickness (SFT)

This is where a set of callipers is used to measure the skin folds at various sites on the body, most commonly triceps, biceps, subscapular (below the tip of the scapular) and suprailiac (above the hip bone). The 4 sites are summed and using logarithms there is a linear relationship with total body density, age and gender specific. There are reference levels based on Durnin and Wormsley (1973) equations. In some cases other sites are used, though the four are the most universally recognised. Durnin and Wormsley correlated the levels with under water weighing.

This is a convenient and only moderately invasive method of measuring %BF. Callipers are cheap and easily available. One limitation is that the assessor needs to be experienced, as the exact location of the skin fold measurement needs to be consistent. For best results three readings should be taken at each site and the mean (average) of the three used.

Whilst callipers are not extremely accurate for the actual figure of %BF, the figure is precise and useful to show change in %BF over a period of time. However, precision has only been shown to be good if the same assessor is used.

Other limitations are in people with extremely low body fat, abnormal hydration status or the severely obese, there are assumptions in that the 4 sites are a good indicator of total body fat, observer training is necessary, measurements respond slowly (re-assessment cannot occur frequently).

SFT is easy, quick, convenient, widely used, precise and cheap. Thus it is possibly the most useful measure of %BF for the bodybuilder.

Assessing Body Composition

I hope this article has provided a basic overview of assessing body composition. I hope you will appreciate that the mirror and how you feel are the best assessments of gains in bodybuilding. However, taking your weight and using this figure with SFT assessment of %BF can give a black and white indication of progress.

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James Collier

James first started bodybuilding as a teenager back in the 1980s and obtained his degree in Nutrition and Dietetics from the University of Surrey back in 1995. After qualifying he worked as a clinical Dietitian for the NHS in various UK hospitals.

Having competed several times during the 1990s, his passion now lies in helping other bodybuilders, strength and fitness trainees reach their goals.

He is a Registered Nutritionist and a full member of The Nutrition Society in the UK. James is also co-founder and developer of Huel, nutritionally complete food.

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