Next time you see the Irish rugby team run out onto the hallowed turf of the Aviva Stadium, bear in mind you have just witnessed 15 highly-paid overweight and obese men emerge from the tunnel. Because that's what their body mass index (BMI) will tell you and it is the metric upon which Healthy Ireland and the World Health Organisation relies in determining obesity statistics. (Healthy Ireland is a Government-led initiative that aims to create an Irish society where "everyone can enjoy physical and mental health".)

Take those lean speedsters in the backline to begin with: they are all overweight, apparently. And as for the forwards, some are technically obese, even though they are fit, fast, toned, muscular athletes with enviable percentages of body fat. So if an agile, uber-conditioned professional such as Irish international prop Cian Healy is officially obese, how many thousands of others out there are similarly miscategorised?

Although the origins of BMI can be dated back to 19th century Belgian mathematician, statistician and sociologist Adolphe Quetelet, its modern manifestation can be traced to the 1970s. In a paper published in the July 1972 edition of the Journal of Chronic Diseases, US physiologist Ancel Keys coined the term, "body mass index" and the name stayed.

But how does it work? BMI is the value derived from the mass (ie combined weight of muscle, fat and bone) and height of an individual, arrived at by dividing the former by the square of the latter. Aside from a GP visit, health-focused websites such as bmicalculator.ie can determine your BMI if you simply enter your height and weight. Depending on the value, you will be deemed underweight, normal weight, overweight or obese.

But body mass, just like individuals can’t be so clumsily pigeonholed. Muscle is heavier than fat and, even before recognising that fat, muscle and bone serve very different roles, some counter that even the BMI formula has been miscalculated for the last 45 years.

In 2013, noted mathematician and researcher at Oxford University Nick Trefethen observed that BMI divides the weight by too large a number for short people and too small a number for tall people.

Trefethan and his colleagues concluded that using the BMI index adds 10 per cent to a tall person and takes 10 per cent from a short person’s result.

So what might this suggest about the 25 per cent of children and 60 per cent of adults in Ireland who are categorised as overweight or obese?

Worldwide obesity levels have more than doubled since 1980, while official levels of overweight and obese people in Ireland have doubled in the past two decades. The data suggests we are on our way to being Europe’s most obese nation by 2030, by which time its cost to our economy could rocket from €1.3 billion to €5.4 billion.

As the Irish Heart Foundation puts it, "by targeting a 5 per cent reduction in the population's body mass index . . . the annual cost of obesity could actually be reduced by as much as €394 million by 2020." But why BMI?

Lean, fit and strong

"It's important to make a distinction as to where the fat is in your body. The crucial fat that does the damage is the abdominal fat sitting around your organs," says Dr Liam Glynn, a GP in Ballyvaughan in Co Clare and a senior lecturer in general practice at the College of Medicine in NUI Galway. "The pancreas is the crucial one, as it's hugely linked to diabetes, but also the liver."

Glynn says that there is no doubt in his mind we are in the midst of an obesity epidemic.

“BMI is a very crude measurement but, at the same time, when I sit in front of people as a GP, I need to be able to have a conversation with them where you need to have some guidelines or pointers. But there is some evidence to suggest that abdominal circumference is a much better measure, as it’s more about approximating the risk. So I use both in my practice,” says Glynn.

“I still think that BMI is useful as it’s something most people understand, and a way into the obesity conversation. And let’s not forget that having these conversations in the first place is not easy.”

Checking the scales

“BMI is archaic . . . because the industry I’m in is about building and strengthening joints and muscles, and their surrounding ligaments and tendons,” says Ronan Murphy, a strength and conditioning expert who owns TonedFit gym in Howth. “When you build those, they become denser and therefore naturally heavier. But that should never be confused with fat.

“People fixate on BMI, and so it starts to inform their training, or even their nutrition. And it tends to particularly affect women as, in my experience, they tend to spend more time than men checking the scales.

“When I start training women, first off I let them know there is going to be resistance training, including weight lifting. Then, by explaining the science, I debunk the myth that women must therefore get larger or bulkier. Lastly, I inform them that the scales will get heavier as they become more fit and lean, but their measurements will decrease.”

Murphy offers himself as one example of the BMI paradox. In spite of training intensively every day, a GP informed him he was obese, although he has just 6 per cent body fat.

Ainle Ó Cairealláin is founder and managing director of Aclaí personal training facility in Cork, and has worked with the senior Cork footballers as head strength and conditioning coach, as well as the Adelaide Crows Football Club in Australia.

“We want people to put on muscle mass, which is so important for their long-term health. The older you get the more important your muscle mass becomes,” says Ó Cairealláin, who adds that many clients of his produce great results and look drastically different from before they began their training programs, even though there is little difference on the scales.

Ó Cairealláin says in his opinion the Dexa scan is ideal for measuring body composition, but it requires a costly hospital appointment using very expensive, lengthy scans akin to an MRI, which is simply not feasible or affordable for the general public.

Measuring fatness

But how can we successfully tackle the obesity challenge when reliable measures of body composition are ignored by those collating the statistics? Measuring Obesity in the Absence of a Gold Standard, a 2014 study led by Donal O'Neill, professor of economics at NUI Maynooth, raised this anomaly. It states that "the lack of an acceptable gold standard for measuring fatness has made it difficult to evaluate alternative measures of obesity".

Using data on US adults the reports says it shows that measures based on body mass index and bioelectrical impedance analysis misclassify large numbers of individuals. “For example, 45 per cent of obese white women are misclassified as non-obese using body mass index. . .”

O’Neill says, “When competing measures of obesity give conflicting results, it is challenging to know how to reconcile these differences.

“However, to date the lack of an acceptable gold-standard has limited the assessment of the validity of field methods used to measure obesity . . . Arguing that BMI, although imperfect, is often the only measure available to researchers, and therefore is the best that can be done, is not a satisfactory response.”

As BMI does not take into account muscle mass, bone density, overall body composition, or racial and sex differences, researchers from the Perelman School of Medicine at the University of Pennsylvania came to similar conclusions in a 2013 article in the journal Science. As co-author Rexford Ahima, professor of medicine and director of the Obesity Unit at Perelman, put it, "There is an urgent need for accurate, practical and affordable tools to measure fat and skeletal muscle, and biomarkers that can better predict the risks of diseases and mortality.

“Advances to improve the measurement of obesity and related factors will help determine the optimal weight for an individual, taking into account factors such as age, sex, genetics, fitness, pre-existing diseases, as well novel blood markers and metabolic parameters altered by obesity,” said Ahima in a news release at the time.

Body composition

Ironically, it could be the factor that so skews BMI that offers the most hope. Muscle Mass Index as a Predictor of Longevity in Older Adults, a 2014 study by Dr Preethi Srikanthan, an endocrinologist and health sciences associate clinical professor of medicine at UCLA in California, is a good place to start.

“Compared to low muscle mass, higher levels of muscle decrease risk of mortality, whereas changes in total body mass measured as BMI do not predict risk of mortality,” Dr Srikanthan tells the Irish Times. “Changes in total body mass are not related to changes in mortality, thus rather than trying to achieve changes in body mass or weight, we should try to improve our body composition by increasing muscle mass.”

One alternative, more accurate metric to measure the components of body composition, especially body fat, is bioelectrical impedance, which was used to assess subjects in the study. “It runs an electrical current through the body. Muscle allows the current to pass more easily than fat does, due to muscle’s water content. This is how researchers could determine a muscle mass index – the amount of muscle relative to height – similar to body mass,” says Dr Srikanthan. “Bioelectrical impedance is easily measured in a physician’s office, using a device the size of a weighing scale with two grip handles and it costs a few hundred dollars.”

Bioelectric impedance has its detractors too. For example, the study in NUI Maynooth discovered that more than 50 per cent of non-obese white women are misclassified as being obese, using bioelectric impedance.

A quicker, simpler alternative to both BMI and bioelectrical impedance could be the old-fashioned measuring tape.

"Keeping your waist circumference to less than half your height can help increase life expectancy for every person in the world," Dr Margaret Ashwell told an obesity congress in France in 2012.

According to Ashwell, an independent consultant and former science director of the British Nutrition Foundation, after having analysed the health of 300,000 people, it was concluded that waist-to-height ratio was better equipped than BMI to predict high blood pressure, diabetes, heart attacks and strokes.

Waist-to-height ratio would mean someone who is 6ft (183cm), should have a waistline of less than 36in (91cm), while another who is 5ft 4in, or 163cm, should keep his or her waist measurement less 32in (81cm). The World Health Organisation recommends that to measure the waist circumference accurately, it should taken midway between the lowest rib and the top of the pelvic bone at the hip. Perhaps by those measurements, our understanding of the scale and nature of our obesity epidemic might be very different indeed.