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2.1.4.1 Skin fold thickness

A special calliper can be used to measure subcutaneous body fat. The skin fold thickness is measured by pinching the skin at a number of predefined points on

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the body, such as upper arm, trunk, and thighs (756). The recorded measurements are then compared with age-sex-specific charts.

2.1.4.2 Underwater Weighing

Underwater weighing is the gold standard method of directly measuring body fat. Participants are weighed in air and then weighed again after they are submerged in a specialized tank in a laboratory setting. This procedure is based on Archimedes Principle; where density of different body components (fat mass, lean mass) is compared with the density of water. Individuals with high fat mass weigh less inside the water compared to those with a high lean mass. In addition residual lung volume is also calculated in maximal inhalation and about 100 cc of air is estimated as air trapped in the intestines (757;758). All these values are then put in the following equation to calculate body density

Density of body= density of water * weight of body____________________

(Weight of body –weight of immersed body)- density of water*(residual lung vol + 100cc)

2.1.4.3 Whole body air displacement plethysmography (ADP)

Air displacement plethysmography (ADP) measures body composition using the same principles as underwater weighing except with air instead of water.

Subjects are asked to remove clothes and enter a sealed chamber that measures their body volume through the displacement of air in the chamber. The body volume is combined with body mass to determine body density. The ADP uses known equations to estimates the percentage of body fat and lean body mass by using the previously calculated body volume, mass and density. ADP is a very fast, simple and non-invasive technique and does not uses x-rays and so is preferred over DEXA and underwater weighing where available. In addition the acceptance of ADP is also high (100%) as compared to underwater weighing (69%), despite both giving almost identical results (759;760). ADP measurements are extremely reproducible, making them ideal for monitoring pharmaceutical therapy, nutritional or exercise intervention, or sports training. However ADP was not a feasible option for use in the RISC study due to the cost of the equipment for each centre in a multicentre study.

116 2.1.4.4 Bioelectrical impedance or bioimpedance

Bioelectrical Impedance Analysis (BIA) using a variety of proprietary devices is one of the most reliable and accessible methods of screening body fat and composition in clinical settings. As such it was measured in participants of the RISC study in all study centres using the Tanita bioimpedance balance (a

machine was purchased for each centre) (see Chapter 2, Section 2.1.6). In BIA, a person’s height, age, gender and weight and other physical characteristics such as body type, physical activity level, ethnicity, etc. are first entered. Subjects are asked to have only a light meal at least one hour before and to void their bladder before the start of the measurements. More sophisticated methods are also available: in these, the person is asked to lie down and electrodes are attached to various parts of the body. If in the standing position, the

measurements are done after a period of at least 10 min standing upright to minimize potential errors from acute shifts in fluid distribution. Weight is

calculated and recorded by the balance. A small and safe electric signal (usually a 50 kHz, 500 μA current) is passed thorough the body and the impedance or resistance to the signal is recorded as it travels through the water that is found in body tissues. Every measurement is taken in duplicate and averaged unless the difference in two measurements was greater than 10 Ohms. The basis of BIA is that the greater the amount of water in a person's body, the easier it is for the current to pass through it. The more muscle a person has, the more water their body contains and so offers less resistance or impedance. In contrast, more fat a person has; less water is present, hence resistance is higher. BIA is a safe

method and the electric signals passed cannot be felt at all either by an adult or a child (761;762).

Along with the body fat and body fat% BIA can be used for other measures including; body water %, muscle mass, physique rating, daily caloric intake (DCI), basal metabolic rate (BMR), metabolic age, visceral fat and bone mass (761-763). BIA has many advantages including being non-invasive, relatively inexpensive, portable, whilst not involving exposure to ionizing radiation and low between-observer variation. BIA can be done in healthy people or in chronic disease which have the validated BIA equations available. However, its use is not recommended in people at extremes of BMI ranges and in subjects with

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BIA is also the preferred method for monitoring changes in body composition (fat mass and fat free mass) with time if they have normal hydration and BMI

between 16-34 kg/m2. Studies suggest that BIA gives good results in individuals with stable water and electrolyte balance as abnormal hydration and

electrolytes facilitate the flow of electric current. It is also complex to develop algorithms and equations in these abnormal hydration conditions (761;762). It has been suggested by some studies to use segmental or multi-frequency BIA or bioelectrical impedance spectroscopy in conditions of abnormal hydration, but these techniques require further research (761;762). Abnormal hydration conditions include: oedema, ascites, patients undergoing dialysis, kidney disease, liver pathology, cardiac disease, large volume intravenous fluids, diuretic therapy, post major surgery, patient in intensive care and pregnancy. Hypothyroid patients due to increased skin thickness and patients with any orthopaedic implants may also register inaccurate results and should not undergo BIA.

2.1.4.5 Imaging Methods

These are the ideal methods and their main advantage is accuracy and detailed body composition. Dual-energy x-ray absorptiometry (DEXA) uses low dose x-ray to record fat distribution in the body. It also calculates bone mineral density and muscle mass. The DEXA technique is based on the attenuation properties of bone, lean and fat tissues at two different x-rays energies. It measures directly the lean and fat mass and bone mineral content. The bone mass added to lean mass constitutes the fat free mass. DEXA usually takes 5 minutes to complete (764;765). Isotope dilution is another method where participants drink isotope- labelled water and then their body fat is calculated by analysing for isotope levels. Computerized tomography (CT) and magnetic resonance imaging (MRI) are the two other frequently used measures and can directly measure body fat mass in different parts of the body. Some of the limitations of imaging methods include: price, use of ionizing radiation, time, and suitability for serial

measurements; these factors influence choice of method for large population studies. However they are mostly used in clinical settings or for validating other methods of body composition measurement. In general no agreed cut-offs are available for body fat percentage (BF%) but the routine values in men are; normal weight (<18%), overweight (18-25%) and obese (>25%). Similarly in

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women the BF% values are; normal weight <25%, overweight 25-32% and obese >32%.