Sunday 25 August 2019

BMI... just how useful is it to measure weight?

Resident dietitian Orla Walsh has all the answers to your questions

BMI - people love to hate it
BMI - people love to hate it
Leinster's Cian Healy. Photo: Stephen McCarthy/Sportsfile
Dublin's Stephen Cluxton. Photo: Piaras Ó Mídheach/Sportsfile

People love to hate the Body Mass Index (BMI). As a dietitian, when you mention BMI, you often hear passionate responses from those that feel like it's a terrible measure of a healthy weight - so what are the arguments against measuring with BMI?

* Argument 1 - What if I'm a muscular build?

One of the more common responses is that BMI doesn't account for muscle. In fact, people often refer to the Irish rugby team as a perfect example of why the BMI measurement is so inaccurate. The IRFU have information on the rugby team on their website, and sure enough their BMI scores put them in the overweight or obese range.

The begrudges of the BMI do have a point, as these are fit lads with body fat levels most males would aspire for. However, rugby is somewhat unique. The lads are on tailored programmes to make them stronger and more powerful, as this will enhance their performance. However, in other sports, players are often lighter and would not have the same level of muscle mass as rugby players. Take football for example. A profile was done on the Dublin GAA football team and their weights were published. As you can see from the table below, their BMIs are within or very close to the healthy range.

BMI is usually not taken into consideration in sport, but that doesn't mean that in most sports most people have a BMI that would fit into the healthy range. The truth of the matter is, unlike rugby players and some devout gym bunnies, the majority of people who have raised BMI scores weigh more than they should. This is due to excess fat on, and in their body and not because of layers of extra muscle on their skeleton.

* Argument 2 - My BMI is raised but I'm healthy

There is a lot of scientific evidence published that indicates that BMI is a good risk predictor of disease. It's not the only measure - but it's a good measure. A report from the World Health Organisation shows there is a lot of evidence supporting BMI as a measure of disease risk.

* Argument 3 - is waist circumference more important than BMI?

Although BMI is often used by practitioners, it would be better practice to consider BMI alongside another measure such as waist circumference. Even within a narrow range of total body fat and BMI, the amount of abdominal fat can vary greatly among individuals. If your waist circumference is raised due to excess fat around your middle, you are at greater risk of disease. Therefore waist circumference measurement complements the BMI measurement, and helps to correctly identify individuals at increased risk of obesity-related health issues due to the accumulation of abdominal fat. In other words, BMI is good, but BMI with waist circumference added in is an even better marker of health.

For further thoroughness it is advisable to compare waist to height as tall people will have wider waists that shorter people, naturally. As a general rule, your waist circumference should be less than half your height.Additionally it would thorough practice to compare waist circumference to hip circumference. This measure aims to get an idea of your shape, with a smaller waist compared to hip girth being preferable. A healthy woman's waist will usually be a lot smaller than her hips - with the difference between the two less dramatic in males. To assess this divide your waist by your height to obtain a ratio. The score you're looking for is less than 0.9 if male and less than 0.85 if female.

In a study called the INTERHEART - a case control study of heart attacks considering individuals from 52 different countries BMI, waist circumference and waist-hip ratio were all strongly associated with risk of having a heart attack. Interestingly the assessment of risk was more accurate when BMI was considered alongside waist-hip ratio.

* Argument 4 - The health of your body should be measured by callipers and not BMI

Callipers are used to measure body fat in individuals. It's a method where you pinch very specific sites in the body, and using an instrument that looks like a giant tweezers, you assess how much fat is underneath the skin. It takes about 30 minutes to measure seven sites. Therefore it's not practical from a time-sensitive point of you and you can't measure yourself.

For this method to be accurate the person conducting it has to be trained and experienced in measuring. Often people are not adequately trained.

The other big issue with callipers is that the more fat the person has on their body, the more inaccurate it is. Although I use callipers to measure fat mass within my clinic, it's not appropriate for everyone. The person being measured needs to be comfortable with being prodded, poked and pinched.

BMI is quick and involves less intrusion. BMI will check to see what weight someone should be for their height. There is a range given as it aims to account for different skeletal frames. It's not perfect, but it's fairly good.

Player  Height  Weight  BMI Range

* Cian Healy 1.85m (6ft 1) 112 (17 st 8 lb) 32.7 Obese 1 = BMI 30-35

* Jamie Heaslip 1.93m (6 ft 4) 110 (17 st 4 lb) 29.5 Overweight = BMI 25-30

* Rob Kearney 1.85m (6 ft 1) 95kg (14 st 13 lb) 27.8 Overweight = BMI 25-30

* Conor Murray 1.88m (6 ft 2) 94kg (14 st 11lb) 26.6 Overweight = BMI 25-30

* Johnny Sexton 1.88m (6ft 2) 92kg (14 st 6lb) 26 Overweight = BMI 25-30


* Stephen Cluxton 1.8m (5 ft 11) 82.6kg (13 st) 25.5 Overweight = BMI 25-30

* Philly McMahon 1.83 (6 ft) 82.6kg (13 st) 24.7 Healthy weight = BMI 20-25

* Rory O'Carrol 1.85 (6 ft 1) 82.6kg (13 st) 24.1 Healthy weight = BMI 20-25

* Johnny Cooper 1.83 (6 ft) 73kg (11st 11lb) 21.8 Healthy weight = BMI 20-25

* James McCarthy 1.85 (6ft 1) 88kg (13st 12lb) 25.7 Overweight = BMI 25-30

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