The measurement mess in Britain is in itself reason enough for the discontinued use of stones and pounds for personal body mass (commonly weight), but is there a case for using kilograms that goes beyond this?
This article proposes that there are possible implications for those trying to lose or maintain weight from a poor choice of measurement units.
To make the case we look at the assessment and subsequent planning and monitoring of changes.
Working out if we need to lose weight
(Please note: For those who don’t like maths or have never understood algebra please don’t be put off by the use of letters instead of numbers. This is kept to a minimum and alternative explanations in ordinary language are included).
Current main stream health advice tells us that we should have a body mass index (BMI) of between 20 and 25. This is in fact measured in kg/m2 and calculated as follows:
BMI = m/h2
where m is body mass in kilograms and h is height in metres. In other words divide the weight in kilograms by the square of height in metres. For example suppose someone 1.7 m tall is weighing in at 113 kg. First we need the square of height (h2):
1.7 × 1.7 = 2.89 m2
Then divide that into the figure for mass:
113 ÷ 2.89 = 39.10
A BMI of 30 or more is considered obese so this person needs to lose a significant amount of weight. But what weight should this person be?
We look at this question shortly but first it must be acknowledged that it is quite possible to measure BMI in units other than metres and kilograms; the concept itself does not depend on the actual units used. The resulting numbers though would be different and the scale for a healthy range something other than 20 – 25.
But it must also be understood that the use of kg/m2 for BMI is universal even in countries that are not fully metric like the US. It would therefore be irresponsible for any health authority or advisory body to adopt a scheme based on other units.
What weight should we be?
If we re-arrange the formula for BMI we can see how to determine our ideal weight from our height and a chosen value of BMI as follows:
m = BMI × h2
where BMI is a value between 20 and 25. In other words multiply the required BMI by the square of height in metres.
Taking the previous example of someone 1.7 m tall and a desired BMI of 25; we know the square of height (2.89 m2) so just need to multiply this by 25:
25 × 2.89 = 72.25 kg
Typical bathroom scales are not accurate to anything better than about 0.5% so let’s call this 72 kg
Planning a weight loss programme
There is good medical evidence that deliberately losing weight through dieting should not be any more rapid than 1 kg a week on average. Crash dieting can cause problems such as anaemia, irregular heart rhythms and muscle loss. A safe and satisfactory regime is between 500 and 1000 g per week so if we assume say 700 g that works out at 100 g/day. That means 3 kg over 30 days i.e. 3 kg/month.
In our previous example there are 113 – 72 = 41 kg to shed. That will take about 14 months. It is quite achievable though with the right approach.
It is quite extraordinary that respected organizations in the UK that offer people help with this do nothing to encourage the use of the kilogram. Instead they perpetuate stones and pounds leaving people forever burdened with that awkward ratio of 14 lb to a stone, making these kind of calculations unnecessarily difficult.
Shorter term goals
In the previous example the ultimate aim of getting personal weight down to a safe limit requires quite a long term commitment and a good healthy regime of eating and, advisedly, exercise. It would naturally seem quite daunting in that case.
Recent UK public health advice is telling us that for people who are seriously overweight, losing 10% can have significant benefits. This emphasis on percentages is worthy of note. It makes it all the more important that we have a handy way of reckoning it.
It is not difficult to see that 10% of 113 kg is 11.3 kg.
It is also a straightforward operation to calculate the total percentage loss represented by the ultimate goal as:
41 ÷ 113 × 100 = 36.3%
If the weight were in stones and pounds then any percentage would be an awkward calculation because there are two numbers requiring a conversion to one before the ratio can be calculated. This cannot be done with a single operation even with a calculator. Worse still, many people wouldn’t know how to do it anyway.
Needless to say, it is dead easy to calculate weight change in kilograms. In imperial units, small changes are mostly straight-forward unless the larger unit, the stone, also changes.
The general advice on personal weighing is to do it no more frequently than once a week, on the same scales and at the same time of day. Body mass varies naturally for obvious reasons, such as gaining from food and fluid intake and losing from perspiring and going to the loo etc. It is helpful therefore to be able to relate the typical mass values of these elements to that of the whole body in order to predict the effect it may have on scale readings.
Strangely, people in the UK are not shy of measuring food portions in grams and fluid volume in millilitres. Why then do they not simply extend this to whole body mass?
Perhaps people are not generally aware that a litre of water and most of the fluids we drink weigh a kilogram (within the limits of accuracy of everyday weighing equipment), or the amount in millilitre is numerically the same as its weight in grams. What better way then to asses the immediate effect on body mass after eating or drinking?
For example if we have drunk the recommended 1.5 litres of fluid throughout the day it will, potentially, add 1.5 kg to our body mass. If we go to the loo and pass say 400 ml of water it reduces by 400 g, and so on.
The foregoing has, hopefully, demonstrated that the use of the kilogram offers greater simplicity to the process of assessing and monitoring changes to personal body weight.
The human body is an animal organism like any other and doesn’t warrant the use of obscure units like stone and pounds especially in circumstances where it is important to easily recognize what constitutes a healthy body weight and how it can vary with easily measured components like food and drink.
This article has not touched upon the much more complex topic of how we actually go about losing and maintaining weight because the issues there have more to do with energy balance involving other units. This author is planning another article at a future date that will consider that very important topic and the implications for how we measure it.