A Word (Or More) About BMI
In future articles, we will discuss screening for under-nutrition. Some of the “tools” used to do this give prominence to measuring the BMI of individuals in care and in hospital. This is not always appropriate or helpful.
BMI stands for Body Mass Index. It provides a “ready reckoner” in the form of a ratio between height and weight to see if one is unhealthily underweight, unhealthily VERY overweight, or somewhere in between. And when I tell you how it’s measured, your eyes might glaze over. The values can go from around 11 to over 40 and are attained by dividing a person’s weight in kilograms by the square of that person’s height in metres. KG/M². I’m afraid this doesn’t work with “old money” – feet, inches and stones!
Most people are now familiar with the coloured bands on a BMI chart, starting with pale green for the underweight, going through to emerald green, yellow, orange, brick red and dark red for the morbidly obese. (just Google "Body Mass Index" to see the charts for yourself). However, these have their limitations: interpretations should be made with care and this is where my concern lies.
For example, they don’t take into account genetic tendencies or the constituents of the weight. For someone who is highly muscular and fit – and there are many sportsmen and women who fall into this bracket – the BMI might veer into the overweight band but they are NOT FAT. It is body FAT that is the health problem. Further, they are a poor indicator of health risks in adult individuals.
Children aged 0-18 years have their own set of growth and development charts. (Institute of Child Health)
Older people do not conform to these tables either. Body composition changes with age: we lose muscle and cartilage; bones get thin. Some medical conditions cause fluid retention. Such changes are reflected in body weight. Many dietitians who work in elderly care keep a check on residents’ weight changes but dismiss converting them to BMIs, as they are fraught with inaccuracies and mean precious little. Unfortunately, non-clinicians don’t always take these into account or appreciate that a care home resident, for example, could lose a significant amount of weight if she/he had been prescribed “water pills” (diuretics). Accurately measuring the height of someone who is very bent is virtually impossible. Various ways of overcoming this are proposed but methods depend on the ability to either stretch one arm out straight or sit upright, with the knees at a 90-degree angle. Many older and disabled people find these antics nigh impossible!
Anecdotally, enthusiastic doctors researching heights and weights of old people in care, have been tempted to go to dramatic lengths by suggesting that they lie out flat on their backs on their beds and thus be measured. It doesn’t take much imagination, even with a dash of dementia and a smidgen of confusion, to guess what these poor souls are thinking…
“We are being measured for our coffins!”