A Case Against BMI

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The world’s rising obesity epidemic is hardly news. According to the World Health Organization (WHO), 200 million adults worldwide were classified as obese in 1995. A figure that ballooned to more than 300 million adults in a short five years – a sobering statistic indeed.

And while the seriousness and scale of this epidemic is undeniable, some questions regarding the method used to classify the body fat of individuals has come to light in recent years. Do all persons classified as being “obese” in terms of BMI really carry around too much body fat? And are all of these individuals really at a higher risk of developing potentially deadly chronic diseases? Let’s find out.

The Ins and Outs of BMI

What is BMI?

BMI, or body mass index, is basically an indication of relative size or body fat based on an individual’s height and mass. And while it’s not a direct measure of body fat, BMI is currently used as a screening tool to determine whether a person is at a healthy weight, underweight, overweight or obese.

How to Calculate Your BMI

You can calculate your BMI by dividing your weight, in kilograms, by your height in meters squared, i.e. weight (kg)/height (m)2. There is, however, a variety of easy-to-use online BMI calculators available that will do the calculation for you once you’ve entered your weight and height. Alternatively, you can also look up your BMI on a BMI chart, the latter which can be easily located through a simple online search.

What is a Healthy BMI?

According to the WHO, adults aged 20 and up can be classified into the following principal groups based on BMI scores:

  • A BMI score of <18.5: Underweight
  • A body mass index score of 18.5 to 24.99: Normal Weight
  • A BMI score of 25 or more: Overweight
  • A body mass index score of 30 or more: Obese

For children aged 2 to 20, a BMI percentile is currently believed to be a good indication of body fat. This method takes into account the child’s age and sex.

What are the implications of a less than ideal BMI score?

So what are the implications of having a less than ideal BMI score? In short, a BMI score that falls outside of the healthy or “normal weight” range is believed to be associated with a significant increase in health risks. Individuals with an “overweight” or “obese” BMI classification, for example, are believed to be at an increased risk of a number of diet-related chronic diseases, including the following:

  • Stroke
  • Dislipidemia
  • Hypertension
  • Diabetes mellitus
  • Certain cancers (including endometrial, breast and colon cancer)
  • Coronary heart disease
  • Gallbladder disease
  • Sleep apnea
  • Respiratory problems
  • Osteoarthritis

Many of these illnesses may even increase the risk of premature death.

Keep in mind, though, that since BMI is not a direct measure of body fat, it isn’t a diagnostic tool. If therefore, after initial screening, your BMI score is too high, a health care provider will normally conduct a number of further assessments, including one or more of the following:

  • Skin fold measurements
  • A diet evaluation
  • An evaluation of physical activity
  • A family history screening
  • Any other health screenings deemed appropriate
 Some additional notes on BMI

Note that BMI classifications are both age-independent (for adults aged 20 and over) and identical for men and women. At this stage, BMI classifications also make no differentiation between different population groups, irrespective of different body proportions.

Criticism against using BMI as an indication of body fat content

And while BMI is a quick, simple, inexpensive and non-invasive way to give an indication of an individual’s body fat content and overall health, its use hasn’t been without criticism. Several experts have expressed concern about relying on BMI too heavily, mainly based on the following perceived shortcomings:

  • BMI fails to take gender into account. A woman generally tends to have more body fat than a man with a similar BMI. This is a naturally occurring difference between the two sexes and does not necessarily point to health-related issues.
  • BMI fails to take the age of adults into account. Older people naturally tend to have more body fat than younger persons with the same BMI.
  • BMI gives no indication of the distribution of fat in the body. Waist circumference, in particular, is independently and strongly correlated with type II diabetes risk. BMI does not account for this.
  • Individuals who are very muscular may receive inaccurate BMI scores. Since muscle weighs more than fat, a muscular person may get a high BMI score, but still be perfectly healthy. It’s worth noting, though, that cases like this are few and far between and normally only limited to highly active athletes. Likewise, an inactive person with little muscle mass may receive a BMI score that falls within the normal range, all while carrying around too much body fat.

  • BMI does not make provision for lactating or pregnant women.
  • Body mass index does not account for the natural differences in height, build and weight ratios between different races.
  • BMI does not take bone density into account.
  • Body mass index may exaggerate fatness in tall people and thinness in short people. Professor Nick Trefethen from the Mathematical Institute of Oxford University reckons that the formula used to calculate BMI may be flawed. The formula was originally devised in the 1930s by Lambert Quetelet, a time during which there obviously weren’t any fancy calculators or electronic devices around. As such, it is believed that Quetelet wanted to keep the formula as simple as possible. Professor Trefethen feels that a slightly more complex formula, i.e. weight/height 2.5, would be more accurate than the original version. He also feels that the improved formula will eliminate the exaggeration of fatness in tall people and thinness in short people.

So where does that leave us?

So where does this leave us? Is BMI a good indicator of being under- or overweight? And does it give a good indication of the potential presence of increased weight-related health risks? According to Rexford Ahima, a physician from the University of Pennsylvania School of Medicine, “BMI as a number alone may not be sufficient to predict health and risk of death.” Ahima adds that BMI should always be viewed within context.

So while it is vital that you are aware of your BMI, as well as the potential implications of a too high or too low score, keep in mind that a single number isn’t necessarily a death sentence. If your BMI is too high or too low and you feel that this isn’t a true representation of your personal situation, discuss this concern with your physician. He or she may, in turn, recommend some additional tests and screenings.

Sources

  1. Christian Nordqvist, Why BMI is inaccurate and misleading, Online publication, Aug 25, 2013
  2. Tanya Lewis, BMI Not a Good Measure of Healthy Body Weight, Researchers Argue, Online publication, Aug 22, 2013
  3. BEC Staff, The Healthiest Weight Might Actually Be 'Overweight', Massive Study Finds, Online publication, May 16, 2016
  4. Medical News Today Editorial Team, BMI (Body Mass Index), Online publication, Jan 05, 2016
  5. WHO Staff, BMI Classification, Online publication,
  6. WHO Staff, Controlling the global obesity epidemic, Online publication,
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