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Practice questions

'Is BMI an accurate measure of nutritional status in older people?'

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Nursing practice often involves undertaking procedures about which there is debate or uncertainty. In Practice Question we ask experts to determine how nurses should approach these situations

What is BMI?

BMI stands for body mass index, and is a measure of bodily mass in relation to frame size. There are a number of different equations for this, with Quetelet’s Index, weight (kg)/height (m2), most often used for adults. There are two parts to measuring BMI: weight and height.

Measuring weight

This requires an appropriate scale to do it. For older people, this might mean using a chair scale or a hoist either with a built in or external scale. Correct technique and positioning are important: check the scale is set to zero before starting and, for a chair scale, ensure the patient is sat squarely in the middle and into the back of the seat with feet on the foot rest, and sitting still. It is important that equipment is regularly maintained and calibrated.

For weight monitoring, it is important to weigh the patient at the same time of day, in roughly the same clothing, on the same scales. Weight does not differentiate between muscle and fat mass, which is relevant for older people who tend to replace muscle with fat as they become less active. Neither can it take account of changes in fluid balance, important in a group with high co-morbidities, including cardiac and renal failure. Plaster casts and amputations can make weight difficult to interpret, although values to allow for these are available. By itself it takes no account of frame size; this is achieved in combination with height.

Measuring height

For adults, this is best done with a wall-mounted stadiometer, with the person stood looking straight ahead with heels flat on the ground and back against the wall. If this is not possible there are alternative methods, of which ulnar length is possibly the easiest, as it only requires the patient to position their arm diagonally across their upper body. All these measurements need either to be converted to a ‘height’ using an equation or slotted into a table along with weight to produce a BMI result.

Nutritional status

BMI is accepted by the World Health Organisation (WHO) as a valid indicator of nutritional status (see tinyurl.com/WHO-BMI). The WHO supplies category ranges to classify BMI values (see Table 1). This classification was developed based on the effect of increasing weight in relation to death and diseases such as diabetes, hypertension and cardiovascular problems. In multiple studies and data setsBMI has consistently been a strong predictor of death both above and below an apparent optimum range of about 20-25.

A recent analysis of 57 studies showed a 5kg/m² higher BMI on average associated with about 30% higher overall mortality, comparable to the effects of cigarettes (Prospective Studies Collaboration, 2009). However, this study illustrates some of the difficulties associated with interpreting BMI values in relation to nutritional status for older people: data was predominantly collected from younger people (mean recruitment age was 46 years in this study) and with a predominant focus on effects of obesity, rather than low BMI.

Optimum values for BMI in older people have been a topic of debate for years, with no consensus agreement from the major international health organisations. Some studies have indicated that obesity in this group may have less of an effect on mortality than among younger people, and that minimum mortality occurs at a higher BMI in older people. For example, among a cohort of 2,628 Swedish 70 year olds, BMI ranges with the lowest 15 year mortality were 27-29 and 25-27 kg/m2 in non-smoking men and women respectively (Dey et al, 2001).

However, many studies in this area have difficulties:

  • Not all have been rigorously conducted – for example, they may not have controlled for smoking;
  • Shorter timeframes, because older people have a relatively shorter lifespan, make it more difficult to demonstrate the effects of obesity on mortality;
  • Those who may be genetically prone to complications resulting from obesity may die early, leaving a more hardy group;
  • Cohort effects, because different age groups growing up in different times and places experience different lifestyles and risk factors, and hence may respond differently in older age (Elia, 2001).

Conclusion

BMI has advantages as an assessment of nutritional status in that it is generally fairly quick and easy to measure and calculate, and is generally acceptable to patients. It may not be as quick or easy with older patients compared to younger people, but there are ways to solve these problems.

Interpretation is reasonably straightforward in younger people, but more complex for older people. The classification from the WHO is widely used, but whilst underweight values are similarly linked with increased mortality, the detrimental effects of higher values seen in younger people may not apply for older people.

Table 1. Classification of BMI values by the WHO

BMI Nutritional Status
Below 18.5 Underweight
18.5-24.9 Normal
25.0-29.9 Pre-obese
30.0-39.9 Obese
Above 40 Very obese 

Author

Lin Perry, PhD, MSc, RGN, RNT, is senior research fellow, Research Centre for Gender Health and Ageing, University of Newcastle, New South Wales

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