BMI (body mass index), which is based on the height and weight of a person, is an inaccurate measure of body fat content and does not take into account muscle mass, bone density, overall body composition, and racial and sex differences, say researchers from the Perelman School of Medicine, University of Pennsylvania.

Every few months the same comment is made by experts "BMI is flawed". The news hits the headlines, everybody agrees, and then all goes quiet for a while.

You are of normal weight if your BMI is between 18.5 and 25, overweight if it is between 25 and 30. Anybody with a BMI of 30 or more is obese.

Mitchell Lazar, MD, PhD, Professor of Medicine and Genetics and Director of the Institute of Diabetes, Obesity, and Metabolism, and Rexford Ahima, MD, PhD, Professor of Medicine and Director of the Obesity Unit in the Institute for Diabetes, Obesity and Metabolism, discuss the challenges health professionals face when studying the mortality risks and health of obese people in the journal Science1.

We all know that obesity increases the risk of developing heart diseases, type 2 diabetes, cancer, sleep apnea and other diseases and conditions.

However, according to recent studies, obesity may also protect against death from all causes, as well as death due to stroke, heart failure and diabetes.

In the Science article - "The Health Risk of Obesity - Better Metrics Imperative"2 - the authors refer to the pros-and-cons of obesity as the "obesity-mortality paradox". A paradox that generates a great deal of controversy.

Dr. Ahima said:

"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."

People with a BMI of 30 or more, i.e. obese individuals, have a significantly higher risk of eventually becoming diabetic, developing cancer, cardiovascular diseases, osteoarthritis, and liver and gallbladder diseases.

Being obese heightens the risk of premature death.

However, a number of studies have demonstrated that some obese individuals have lower cardiovascular risk and an improved metabolic profile, while a subset of "normal-BMI" people are metabolically unhealthy and have increased mortality risk.

A team of researchers at the University of Virginia, Charlottesville, found better post-surgical short-term survival rates among obese people than patients of normal weight3. Patients with a BMI of 23.1 or less were more than twice as likely to die within 30 days of surgery than those with a BMI of 35.3 or more.

Drs. Lazar and Ahima point out that the true impact of obesity may not be fully understood, because population studies focus on the link between BMI, health and mortality risks, without taking into account how unintentional/intentional weight loss/gain may affect these outcomes.

Dr. Lazar noted "Future research should be focused more on molecular pathways, especially how metabolic factors altered by obesity change the development of diabetes, heart diseases, cancer and other ailments, and influence the health status and mortality."

BMI exaggerates thinness in short people and fatness in tall people

Nick Trefethen, Professor of Numerical Analysis at Oxford University's Mathematical Institute, in a letter to The Economist explained that BMI leads to confusion and misinformation.

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BMI = weight in kilograms divided by height in meters squared.

Professor Trefethen believes that the BMI height2/weight term divides the weight by too much in short people and too little in tall individuals. This results in tall people believing they are fatter than they really are4, and short people thinking they are thinner.

BMI was devised in the 1830s by Lambert Adolphe Jacques Quetelet (1796-1874), a Belgian mathematician, sociologist, statistician and astronomer.

Trefethen explained that during Quetelet's time there were no calculators, computers or electronic devices - which is probably why he opted for a super-simple system. Trefethen wonders why institutions today on both sides of the Atlantic continue using the same flawed-BMI formula.

"Perhaps nobody wants to rock the boat", Trefethen added.

Trefethen believes a better calculation than the present weight/height2 for BMI would be weight/height2.5. "Certainly if you plot typical weights of people against their heights, the result comes out closer to height2.5 than height2."

Waist size linked to diabetes risk, regardless of BMI

Researchers from the Medical Research Council (MRC) Epidemiology Unit, UK, reported in PLoS Medicine that waist circumference is strongly and independently associated with type two diabetes risk5, even after accounting for BMI.

Study leader, Dr Claudia Langenberg and team suggested that waist circumference should be measured more widely for estimating type 2 diabetes risk.

They pointed out that a non-obese, overweight male with a waist circumference of at least 40.2 inches (102cm) has the same or higher risk of type 2 diabetes as an obese male. The same applies for females with a waist of 34.6 inches (88cm) or more.

A study published by the RAND Corporation showed that waist size explained the higher type 2 diabetes rate in the USA than UK, not BMI6. Co-author, James P. Smith said "Americans carry more fat around their middle sections than the English, and that was the single factor that explained most of the higher rate of diabetes seen in the United States, especially among American women. Waist size is the missing new risk factor we should be studying."

Waist-to-height ratio better than BMI

Dr Margaret Ashwell, an independent consultant and former science director of the British Nutrition Foundation, explained at the 19th Congress on Obesity in Lyon, France, May 2012, that waist-to-height ratio is a superior predictor than BMI7 of type 2 diabetes and cardiovascular diseases.

Dr. Ashwell said "Keeping your waist circumference to less than half your height can help increase life expectancy for every person in the world."

Thus a 6ft-tall man should have a waist circumference of 36 inches or less, while a 5ft 4in woman's waist should not exceed 32 inches.

The waist-to-height ratio should be considered as a screening tool, Ashwell added.

Ashwell explained that BMI does not take into account the distribution of fat around the body. Abdominal fat affects organs like the kidney, liver and heart more severely than fat around the bottom or hips. Waist circumference gives an indication of abdominal fat levels.

Dr. Ashwell and colleagues believe that the thought "keep your waist circumference to less half your height" is an easier one to hold on to that BMI.

An example of the biggest flaw in using BMI

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Body Mass Index' biggest flaw is that it does not take into account the person's body fat versus muscle (lean tissue) content.

Muscle weighs more than fat (it is denser, a cubic inch of muscle weighs more than a cubic inch of fat). Therefore, BMI will inevitably class muscly, athletic people as fatter than they really are.

A 6ft-tall Olympic 100 meter sprinter weighing 90kg (200lbs) may have the same BMI (26) as a couch potato of the same height and weight.

A BMI calculation would class both of them as overweight.

That calculation is probably right for the sedentary couch potato, but not for the athlete.

The athlete's waist circumference, at 34ins, is well within "healthy weight" - if his height is 72 inches, his waist is less than half his height.

However, the sedentary person's waist of 40 inches is more than half his height.

Authorities still promote BMI

The Centers for Disease Control and Prevention (CDC) says "BMI is a fairly reliable indicator of body fatness for most people."8

The National Institutes of Health (NIH) says "A good way to decide if your weight is healthy for your height is to figure out your body mass index."9

Perhaps they should consider revising their statements.