But is BMI really an effective measure of obesity or just a convenient method of classification that doesn’t really reflect your health? We find out.
What is the BMI Calculation?
The principles of BMI are easy to understand. Essentially the calculation measures your body weight in relation to your height and sets a figure based on this height to weight ratio. Many people find they are too short for their weight!
It is easy enough to get your BMI by visiting any health provider’s website such as the NHS and typing in your height and weight into the appropriate boxes.
If you do it yourself, there is a formula to measure either in the metric system or in pounds.
The Formula for Imperial measurement (pounds and inches):
- Formula: weight (lb) / [height (in)]2 x 703
- Divide your weight, expressed as pounds (lbs) by your height in inches squared and then multiply by a conversion factor of 703.
- So for example: Weight 150 lbs., Height 5ft 5 ins expressed as 65 inches.
- Calculation: [150 ÷ (65)2] x 703 = 24.96.
The Formula for metric measurement:
- Formula : weight (kg) / [height (m)]2
- Divide your weight expressed as kilograms by height in metres squared. As height is usually measured in centimetres, divide by 100 to obtain the measurement in metres.
- So for example Weight 68 kg, Height 165 cm (1.65 m)
- Calculation: 68 ÷ (1.65)2 = 24.98
What are the BMI Guidelines for Obesity?
- You are classed as underweight if you have a BMI of under 18.5
- Seriously underweight if your BMI is less than 16
- Normal healthy weight is anything within the parameters of 18.5 to 25
- Overweight between 25 to 30
- Moderately obese between 30 and 35 (obese class 1)
- Severely obese between 35 and 40 (obese class 2)
- Very severely obese over 40 (obese class 3)
History of BMI
This formula was invented at some time between 1830 and 1850 by a Belgian mathematician and sociologist Adolphe Quetelet as an accurate way to measure body mass ratio. It was invented to work with the metric system so is easier to calculate in this way because you don’t have to multiply by 703. Quetelet was not interested in health or obesity – he just wanted to define the average man.
Quetelet’s formula was taken up in the 1970s an American scientist Ancel Keyes who was interested in the effects of the diet upon health and he renamed it the Body Mass Index.
He published several papers in which he proved that the Body Mass Index was an accurate reflection of the state of a person’s health and more relevant than his or her height or their weight alone.
BMI a Tool for Assessing Life Insurance
Although BMI became well known in the 1970s and 1980s as a guide to health, the interpretation of the figures is based on business reasons rather than health and relates to life insurance.
The figures and the interpretation of the BMI were implemented back in the 1940s by Louis Dublin a statistician and vice president of Metropolitan Life Insurance Company (Met Life).
He noticed a correlation between height to weight ratio and longevity based on statistics of around 4.9 million policies issued by Met life and other insurance providers and he interpreted the figures based on these statistics.
The BMI seemed to prove that there was a standard body size that tended to live longer and of course, it meant that people who did not fall within the ideal standard could be charged extra for their insurance premiums.
The Met Life tables were approved by the Royal College of Physicians in a report on obesity in 1973 and over the next couple of decades, other health authorities followed.
This led to the concept of a standardised perfect body shape and size deemed healthy, which discounted any individual cases who did not fall into the BMI criteria as unhealthy, overweight or underweight.
Are 1949 Mortality Figures Relevant Today?
The problem with the original statistics provided by Louis Dublin is that they are not that accurate and do not reflect true health and mortality.
These statistics were only based on people affluent enough to buy insurance, the weight of the individuals was provided by the insurance customers – many who may have weighed themselves inaccurately or while wearing clothes and shoes – adding vital pounds onto the statistics.
Some people often had more than one life insurance policy so that skewed the figures and health concerns such as smoking and heart disease were not entered into the equation. The figures were based on a mainly Caucasian group and did not take the differences in build of other races into consideration such as Afro-Americans and Asian people.
Back in 1949 and the 50s when the statistics were complied, life style, diet and health concerns were very different from today. For example, you only have to see an old film to see how widespread the smoking habit was and smoking had a huge impact on mortality.
Today life insurance still takes the BMI into consideration and you will pay more for your cover if you fall outside the normal 18.5 – 25 BMI readings.
When Ancel Keyes began his research in the 1970s, he based his findings on BMI figures and Dublin’s statistics in order to look at the general health of different groups and populations with relation to diet and lifestyle. The BMI was not intended to be used to assess an individual person’s absolute health. It was a tool to look at a population as a whole.
The fact that the insurance industry has used it to evaluate risk and therefore drive up the cost of health insurance is based more upon finance than it is upon health.
BMI, Easy to Use
Despite the shortcoming of the BMI, it does provide a cheap, safe and non-invasive way of estimating the body fat percentage and assessing a person’s health and nutritional status. However it is important to remember that your BMI does not necessarily tell the whole story about your health and weight.
The BMI is a measure of weight, not fat (adiposity) so although the BMI is used for convenience, as everyone can provide his or her own height and weight data, measuring the true percentage of body fat is not as easy. The BMI only provides an estimate of this.
There are other factors that affect BMI but are not taken into consideration such as frame size and muscle density. In addition, it cannot calculate the levels of extra fat around internal organs, which might not show up on the scales or even make you look particularly overweight.
Super fit athletes with their dense muscle mass often show up as obese and many rugby players and football stars would be classified as obese due to their height weight ratio.
The BMI does not provide any calculation for your age either and how many of us will stay at the same weight aged 50 as we were at 25.
However, it can provide a basic and simplistic framework for assessing health, fitness and fatness and because it is so easy to calculate, it has been widely adopted.
The BMI began to be used by the World Health Organisation in the early 1980s to assess the level of obesity, optimal weight and nutritional health.
Scientists JD Webster and JS Garrow proposed the title of the categories –now classifying people as overweight or obese depending on their scores, following research into weight gain.
The Classification of Overweight
The boundaries of the BMI are a source of discussion and the most widely disputed area is the overweight classification of a BMI between 25 and 30. Many scientists believe that the overweight category should be redefined as normal and that it is not unhealthy to fall within this category.
A recent study carried out in the USA involving three million people seems to suggest that the overweight category have a slightly lower risk of premature death than those in the normal category and that a BMI between 25 and 30 is healthy. Prior to 1998, this reading was classified as normal by health experts. Having a BMI in excess of 25 but less than 30 has only seemed to be a matter for concern in recent years.
Changing the BMI Settings Made People Overweight
In 1998, the National Institute of Health lowered the cut off point between normal and overweight. They reduced the original reading of 28 for men and 27 for women to 25 for both sexes.
This bought the USA more in line with the rest of the world and made millions of American people technically overweight overnight. The BMI 25 cut-off has now become generally accepted worldwide. However, the cut off for obesity has never changed and remains at 30.
The problem with setting a one stop shop figure for all the world is that we are not all the same and we do not all have the same diet life style or physical characteristics. The BMI is based on averages and these figures will vary depending on where you live and your genetic make up.
In India, for example the cut off for obesity has been reduced to just 23 because the government are so worried about the rise of diabetes.
According to an Indian Government Paper published in 2013;
Researches over the last several years have shown that Indian bodies and genetics are different from their western counterparts. Indians suffer from abdominal obesity compared to people in the west whose bodies are uniformly obese.
The Indian body composition puts them in high risk for diabetes and hypertension
The BMI Is Here To Stay?
The BMI classifications appear to be fixed for now so if you have a BMI of over 25 you may be technically classified as overweight. However this is not such bad news because, current thinking is reassessing the true health impact of this score.
The BMI is only a measure of your weight relative to your height. It does not measure fatness, health or any other issues.
Dr Katherine Flegal from the US National Center for Health Statistics and the author of the American study looking into the health and mortality of 3million people within the 25 – 30 BMI range said in an interview:
“I think the moral of this study is that these BMI categories are weight categories and they’re not exact health categories.
For the health professional the advantages of using the BMI outweighs any of the system’s shortcomings. It is an easy to use system that is available to all and as yet there is not a viable alternative. Although it might be upsetting to fall within the overweight category, the rise of obesity is a global problem that is reaching epidemic proportions so you are not alone.
You don’t have to define obesity as a measurement with a score because a quick look down any British or American street will show you that we are bigger than ever before and redefining the boundaries will not really change this fact.
The good news is that if you fall within the overweight category, your few extra pounds may not make much difference to your long-term health. The important factor for health and longevity is fitness so if you manage to keep fit and eat healthily, being officially overweight may not be as bad as it appears.