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The use of body measurements to assess nutritional status

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Lin Perry, PhD, MSc, RGN, RNT, is senior research fellow, St Bartholomew School of Nursing and Midwifery, City University.

Body measurements have a long history of use as nutritional indices. Some reflect previous nutritional conditions (for example, height), while others can reveal information about more recent status. Some measurements are able to distinguish between fat and fat-free mass, providing separate information about energy and muscle protein stores.

Height

This is most commonly used as a nutritional index with children, with height measurement two standard deviations below median height for age-defining stunted growth. It is also required to calculate body mass index (BMI). However, accurate measurement requires the subject to stand erect under a stadiometer, which can be problematic. Equipment may not be available, many hospital patients cannot stand and it is not clear how measurements should be interpreted in people who have experienced height loss, kyphosis or scoliosis. Recalled or reported height may also be used but these risk incurring wide margins of error.

Alternative methods provide ‘height’ measurements for BMI calculation. Both have error margins of around 4cm but may be useful and acceptable with frail people.

Demispan

This is measured with the arm outstretched straight, palm forwards, from the base of the middle/ ring fingers to the sternal notch using a non-stretch tape measure. A calculation (Box 1) provides a ‘height’ value (Bassey, 1986).

For women

Height in cm = (1.35 x demi-span cm) + 60.1

For men

Height in cm = (1.4 x demi-span cm) + 57.8

Box1. Calculating height from demispan

Length of forearm

This is measured with the arm bent up across the chest (preferably the left side), with the palm flat across the chest and fingers pointing to the opposite shoulder. The length from the point of the elbow (the olecranon) to the mid-point of the prominent bone of the wrist (the styloid process) is then measured using a tape measure.

Knee height

This may be measured either lying supine with the knee flexed at 90o using knee-height calipers (Chumlea et al, 1985) or in the seated position using a stadiometer (Haboubi et al, 1990). Calculations for height approximations (Box 2) and nomograms are available(Chumlea et al, 1985; Haboubi et al, 1990). Illustrations of these measurement approaches are available in the explanatory guide to the Malnutrition Universal Screening Tool (Elia et al, 2003).

For women

Height in cm = (1.83 x knee-height cm) – (0.24 x age years) + 84.88

For men

Height in cm = (2.02 x knee-height cm) – (0.04 x age years) + 64.19

Box2. Calculating height from knee-height

Weight

Although weight is extensively used as a nutritional measurement both for assessment and monitoring, its limitations are well-recognised. Accurate measurement requires regularly calibrated and maintained scales capable of accommodating bed-bound people. Clothing, fluid balance including diurnal or monthly cycles, oedema and dehydration, tumor-load and alterations of drug prescriptions are all influential. Weight by itself is difficult to interpret, although change over time may be more informative. Percentage weight change can be calculated (Box 3). Significant weight loss has been suggested as >5% loss in one month or >10% in six months, although 5–10% loss may indicate nutritional ‘risk’. Loss of >5% over one year has been linked with increased mortality (Wallace et al, 1995).

Percentage weight change

previous/ usual body weight – current weight x 100

previous/ usual body weight

(NB all measurements must be in the same scale, e.g. kg or lb).

Box3. Calculation of percentage weight change

Current weight can also be compared to usual weight, if this is known (Box 4), and tables of ideal or reference weights. However, while commonly used, many of these are dated and American in origin, and it is not clear how applicable they are for individual people in the UK.

Percentage of usual body weight =

current weight x 100

usual body weight

(NB all measurements must be in the same scale, e.g. kg or lb).

Box4. Calculation of percentage of usual body weight

Body mass index

Weight alone takes no account of frame size; this is achieved in combination with height as the body mass index (BMI). At least four versions of BMI are available but Quetelet’s Index, weight (kg) / height (m2), is most often used for adults and Ponderal’s Index, height (m)/ weight (kg^-3) for children (Lees et al, 1995). Based on mortality and morbidity tables, adults are often considered underweight and/or malnourished at less than 18.5kg/m2 although in older people a higher cut-off (possibly 23kg/m2) may be indicated, due to relative changes in body composition with ageing (Beck et al, 1998). Obesity is defined as values above 30kg/m2. BMI does not distinguish fat from muscle protein.

Skinfold thickness

There are standard procedures for skinfold thickness measurements (Lee and Nieman, 1996), which can be taken at a number of sites including the abdomen, thigh, chest and below the scapula, although the triceps is the most accessible. Measurements may be used to predict proportions of body fat using several sites and relying on assumptions about tissue compressibility and distribution (Lee and Nieman, 1996). More frequently, single-site measurements are used. These may be compared with tables of population percentile values, although the same limitations apply as weight tables, particularly with older people. Most often, repeated triceps skinfold measurements are used for medium to long-term monitoring of nutritional progress.

Calibrated callipers and a tape measure are required, with training and regular practice to achieve and maintain accuracy. Variations between raters still occur.

Arm circumference

Mid-arm circumference measurement reflects fat, muscle and bone. An equation allows calculation of muscle protein stores (Box 5). This produces results within +/- 8% of computerised tomogram (CT) measurements of arm muscle (Heymsfield et al, 1992) but may still over-estimate in obese people. Like skinfold thickness, it is not sensitive enough to indicate short-term changes.

Using standard procedures:

1. Measure triceps skinfold thickness (TSF).

2. Divide TSF by 10 to convert mm measurement to cm.

3. Measure mid arm circumference (MAC) in cm.

Calculate Arm Muscle Area (AMA):

AMA =MAC - (3.14 x TSF)]2

4 x 3.14

Apply correction:

For women, minus 6.5

For men, minus 10.

Box5. Calculation of muscle protein stores as corrected arm muscle area

Both arm circumference and triceps skinfold thickness can be taken with subjects supine or side-lying.

Other measurements

Other body measurement methods are more demanding and seldom used in clinical practice (for example, densitometry). Bio-electrical impedance, used to calculate body fat in many gyms, is heavily influenced by hydration so less useful in acutely ill patients. Altogether, anthropometric measurements may help with making nutritional judgements, but the circumstances of individual patients and the accuracy and repeatability of the measurements must be born in mind.

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