Calculating BMI

May, 2018

A resident’s nutritional status/diagnosis is important to assess within Aged Care. There is no one clinical tool or measure used but a number of measures are considered in conjunction, when making a judgement on nutritional status.

These include the nutritional screening tool, food diary and the monthly weighs used to assess weight loss and calculate BMI.

A nutritional screening tool is the initial tool, which identifies a resident’s risk of malnutrition so nutritional intervention and monitoring can be part of the dietary care plan for the resident.

There are a number of screening tools available. In the aged care setting the Mini Nutritional Assessment Tool (MNA) or the Malnutrition Screening Tool (MST) are commonly used.

Three Day Food Diary

A 3-Day Food Diary (3DFD) measures a resident’s food intake and highlights some factors influencing the food intake of the resident. Each meal and snack is recorded and a comment is written on how well this food was consumed.

This may include if food was refused or if the resident was asleep, or away on appointment e.g. dialysis, has poor dentition, on a special diet such as modified texture, independent or requires hand feeding. If possible including ongoing discussion with resident’s family is valuable.

The monthly weighs are the ongoing measure showing changes in weight over the time the resident has been living in the facility. It is used to assess unintended weight loss and calculate body mass index (BMI), both required to assess nutritional status.

Malnutrition is associated with unintended weight loss of at least 5% of usual body weight and/or 10% loss in usual body weight within 6 months even if BMI is in the healthy weight range.

Consider visual observations of residents’ weights informally as well, for instance, observation of loose or tight-fitting clothes, or loose-fitting dentures, which may not be so evident with the weight measure.

Measuring Health

Body mass index (BMI) is a measure of health risk. However, in the older person it is more a measure of life expectancy. BMI is just one measure of health status and should always be used in conjunction with other clinical measures when making a judgement on nutritional status. Height is required and if the resident does not know their height an estimate can be made using ulna length.

BMI = monthly weight (kg)
Height (m) 2

The position of the Australian and New Zealand Society for Geriatric Medicine (2011) is to use a BMI between 23-29.9 m2 as an appropriate healthy range for older Australians - see table 1 below.

In the Aged Care setting it is more important to focus on the residents with a BMI of less than 22.9m2 as their health and nutritional risk is so much greater than those residents who are overweight.

There is no significant mortality increase with any degree of overweight in people aged 70 years and older. However, there is an increase in death rate in younger residents who have a BMI >35 kg/m2 who are aged 60-69 years.

There are some diseases which benefit from being lighter such as Type 2 diabetes and insulin resistance, gall bladder disease, hypertension, dyslipidemia, heart disease, poor mobility, sleep apnoea and breathlessness and daytime sleepiness/fatigue.

BMI can be different for different ethnic groups such as Asians, Maori, Pacific Island and some European ethnic groups and clinicians will consider this when considering nutritional status.

Table 1: BMI and indicators of weight ranges.

BMI Range Indication
Below 22.9m2 BMI Underweight
Between 23 and 29.9m2 BMI Recommended weight range
Above 35m2 BMI Increased in mortality if 60-69years only

1: Winter JE1, MacInnis RJ, Wattanapenpaiboon N, Nowson CA. BMI and all-cause mortality in older adults: a meta-analysis. Am J Clin Nutr. 2014 Apr;99(4):875-90.
2: Flegal KM, Graubard BI, Williamson DF, Gail MH. Excess deaths associated with underweight, overweight, and obesity. Jama. 2005; 293: 1861-7.

Australian and New Zealand Society for Geriatric Medicine. (2011). Position Statement No 19, Obesity and the older person.

Therefore, it is important to consider a number of clinical measures when assessing the nutritional status and health risk of your resident.

Dietitians Australia

Don’t let your residents or budget experience the side effects of malnutrition or dysphagia.
Call us on 1300 850 246 or email and request a call back.

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