Techniques of Measurement of Body Composition Part I

Summary

The measurement of lean body and fat mass has developed with the increase in sports participation and the prescription of exercise. Quantification of body fat is also related to the treatment of obesity and to assess nutritional status. Different levels of evaluation have been proposed depending on expertise and need.

Body density estimation from skinfold measurements has the advantage of simplicity, low cost and reasonable validity with predictions to within 3 to 4% for 70% of the population. The choice of prediction equation will depend on whether a generalised equation or a population-specific equation is appropriate. In all cases tester reliability and standard error of estimation should be established. There is strong evidence that a quadratic equation should be applied and that measures of the lower limb should be included with circumference and diameter measures to strengthen the prediction. Methodological errors need to be reduced by careful training of experimenters. Cross validation of regression equation will strengthen their validity, particularly when fat loss is to be quantified. The popularity of skin/old assessment of body fat is enhanced by the use of nomograms to predict body fat, although some accuracy will be lost. Skinfold estimation of body fat will continue to be a useful guide to adiposity for epidemiological studies and for popular usage.

The hydrostatic weighing procedure to estimate body density is considered by many to be the criterion method. Under carefully controlled conditions with maximum subject compliance, it is highly reliable, particularly if residual volume can be determined accurately. The conversion of body density to percentage body fat is based on a number of assumptions which need to be considered with respect to the population being studied.

Simple methods for adiposity include various weight for height indices, for example, the body mass index of weight (kg) + height² (m) which is often used to define obesity. frame size for the prediction of ideal weight and visual estimation for predicting body fat.

Differences in the literature concerning fat cell size and number have their origins in variations of methodology. The choice of site for removal of tissue will influence the size of adipocytes, those obtained from deep sites are generally smaller than subcutaneous fat cells. If total cell number is determined from cell size, there will also be variability of cell number. The correlation between different methods of sizing cells is high for most techniques, but as the validity of the criterion method is not based on statistical evidence, the validity of the other methods cannot be readily accepted. When comparing the findings of different investigations, the methodology and type of analysis and sample site must be considered as they can exert a profound influence on the results. The controversy concerning changes in cell number and size during obesity will only be resolved when the limitations and differences in methodologies are fully resolved.

Measuring fat from fat-soluble gases may be used concurrently with other in vivo Measurements, e.g. body water or body potassium, to establish average proportions and biological variability off at free mass components. With such information it would be possible to evaluate fundamental assumptions, and increase the usefulness of more widely applied techniques of body fat measurement. The procedure is prolonged and tedious for most subjects, and cannot easily be applied to large populations. Its potential is related to the direct measurement of fat, but it has not developed into a widely-used method.

24-Hour urinary creatinine excretion is a widely used biochemical marker for body muscle mass estimation. After a creatine-controlled diet, the total 24-hour urine is analysed for creatinine content. However, no definite creatinine equivalence for human muscle has been established. An alternative approach for estimating muscle mass is to use plasma or serum creatinine values, although as yet these methods are not widely used.

3-Methylhistidine is another excretory product which is proportional to human muscle mass. Excellent correlations have been found with total body potassium and fat-free body mass. The method is essentially similar to the measurement of 24-hour urinary creatinine. although it does require the use of an amino acid analyser and dietary control is essential.

Total body water measurement can employ a range of dilution techniques. Deuterium oxide dilution is a common method and fulfils the methodological criteria.

Total body potassium is a commonly used criterion method for evaluating lean body mass. It involves the measurement of the naturally occurring ⁴⁰K using screened scintillation detectors.

Nuclear-based techniques are generally expensive, require a high level of operator training and are ethically questionable because of the use of radioactivity. The dosage is generally considered to be within an acceptable range and the specific information would be difficult to obtain by other techniques. The methods include neutron activation analysis. total body nitrogen, total body carbon, photon absorptiometry (single and dual), nuclear resonance scattering, and multiple isotope dilution. These methods are more likely to be restricted to direct clinical applications and will be used relatively rarely on healthy populations for body composition assessment.

Ultrasound can be used to measure the depth of tissue interfaces, typically fat and muscle. Two types are commonly used, the B scan system to provide grey scale images. and the A scan device which records depth from reflected echoes. The latter method is reliable, portable, and could be of particular value in measurements on the obese.

In body composition research computed tomography (CT) scanning, subject to the ethical considerations of radiography, is commonly used. A CT scanner produces a cross-sectional image of the distribution of x-ray attenuation. It can be used to validate existing anthropometric techniques and to provide an understanding of inter-relationships between metabolic activity and body composition. Disorders of fat distribution can be detected and may be significant in various clinical conditions. e.g. Cushing’s disease and diabetes. In prolonged weight reduction it may be useful to apply CT to establish the relative quantities of fat, muscle and bone. Similarly CT can detect the details of apparent weight increase with patients suffering from renal, cardiac or liver diseases which may have oedema masking muscle wastage. In sports medicine research the hypertrophy and atrophy of muscle after rehabilitation or immobilisation can be studied with precision by the CT method.

Nuclear magnetic resonance imaging requires a large bore magnet capable of accepting the human body. The images, received by a radio frequency coil, are dependent on the behaviour of hydrogen nuclei in the magnetic field. The time taken for the protons to realign is detected and the image constructed similar to computed tomography. The realignment characteristics relate to specific tissues both in health and disease. Various modifications such as chemical shift imaging have improved the quality of the image such that it could now be usefully employed in body composition assessment. The capital cost of an NMR unit means that opportunities for use will be limited. However, the absence of radioactivity makes this approach ethically advantageous.

The total body electrical conductivity method is a current induction approach in which the conductivity of the human body perturbs the electromagnetic.field such that fat and non-fat tissue can be evaluated. It appears to be both reliable and valid. Compared with total body water and total body potassium, total body electrical conductivity seemed to track short term changes in nitrogen balance more effectively. Its main disadvantage is that of capital cost, but it is safe, simple, convenient, rapid, non-invasive and requires low operator and recurrent expense. It undoubtedly distinguishes between fat and non-fat tissue. so may have considerable benefit in body composition measurement, particularly if a low-cost version can be produced. If this does nor occur, researchers are likely to pursue current injection techniques as a more,financially viable alternative.

Electrical impedance or impedance plethysmography is a current injection method of relating body conductivity to composition. Typically a 50 kHz. 800μA electrical current is passed between the hand and foot and the resistance is recorded. This value is used in a regression equation to determine lean body mass, although the measurement is more directly related to total body water. The procedure is reliable, and has been shown to be valid in comparison with hydrostatic weighing. The quoted standard errors of estimate range from 1.7 to 6.1%, but population specific regression equations may improve this aspect. Its speed of use, safety and portability may make this approach particularly useful for epidemiological studies.

Infrared interactance is a method of body composition based on the principles of light absorption, reflectance and near infrared spectroscopy. It developed from work on animal foodstuffs but has been used successfully on humans, although only to a limited extent.

A true cross-validation using a distinct population is necessary as is the requirement for intra- and inter-reliability statistics on the methodology. Confounding results could occur because of inaccurate positioning of the fibre optic probe. The pressure of applying the probe could also influence the degree of energy scatter and absorption. The method uses 5 sites, so the precision of site location would, like skinfold readings, provide a potential source of error. However, the method has virtue in being rapid, safe and noninvasive so may, with further experimentation, prove to be a useful addition to body composition analytical techniques.