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Calculating BMI - from Chris Forbes-Ewan

The standard screening test throughout the world for weight category ('underweight', 'normal weight', 'overweight' or 'obesity') is based on body mass index (BMI). BMI is calculated as weight (kg) divided by height-squared, where height is in m.

Underweight is defined as BMI <18.5
Normal is 18.5-24.9
Overweight is 25.0-29.9
Obese is 30.0 and above.

From: Zero Sum
Sent: Sunday, August 11, 2002 5:22 PM
Subject: B.M.I

> ... Please, Forbzy, understand
> that I am *very* upset and angry over this as based on a BMI
> measurement I have been told to "get more exercise".
>
<SNIP>
>
>  ... But I have no faith in BMI...
>
<SNIP>
>
> BMI is just plain silliness.  It may be a standard - but so is
> MS-Windows.
 

BMI is good for measuring what is happening to populations over time (e.g. it demonstrates that there has been doubling in obesity in Australia over the past two decades), but on its own it doesn't give much useful information about an individual.

Combining BMI with waist circumference is a lot more useful. If your BMI is over 25 and your waist circumference exceeds ~100 cm (for a male) or 90 cm (female), then there isn't much doubt that the 'overweight' is due to excessive body fat.

For more information on obesity, I recommend you read the text of my Ockham's Razor program on this. The program went to air on 12 Aug 2001. The complete text is below - 

TEXT OF OCKHAM'S RAZOR PROGRAM ON OBESITY

 The western world is experiencing an epidemic. This epidemic is directly affecting a greater proportion of the population than did either the Black Death of the 14th Century or the influenza epidemic that occurred during and after the First World War.

Many people are unaware of this epidemic...but they should be...because, for a majority of Australian adults, the evidence is right under their very noses!

I'm talking about the 'obesity' epidemic that is sweeping the developed world. For example, in Australia in 1983, about 40% of adults were above the normal weight range. By the year 2000, this number had increased to 60%. That is, in the space of just 17 years, Australia had gone from having a minority to a substantial majority of the adult population being overweight.

In this talk I'll address the following aspects of the obesity epidemic:

What is causing it? What is its significance to health? How can you tell if you are 'underweight', in the 'normal weight range', 'overweight' or 'obese'. And what can you as individual do if you want to safely lose weight and keep it off?

First then, the probable causes of the obesity epidemic:

A decade or two ago it was common to blame the individual for being overweight. 'Gluttony' and 'sloth'--two of the seven deadly sins--were often invoked as the causes. Today, a more enlightened approach is taken to the problems that people have with weight control. It's now believed that there are very strong and complex, but often quite subtle factors involved in the tendency to increase body fat.

For example, over the past decade, the average weight of adult Australians has increased by about 2-3 kilograms. This corresponds to a daily increase of only about one gram in body fat-a minuscule amount. This means that adult Australians are balancing food intake with energy output to better than 99.5%. But that tiny imbalance--less than half of one percent--has led, in the last fifteen years, to a doubling in the proportion of Australian adults who are obese.

So what is the cause (or what are the causes) of these subtle but, in the long term, very significant effects?

If your food intake is greater than your energy output, then your body is 'stuck' with that extra energy... it cannot make it miraculously disappear. Rather, the extra energy is converted into body fat.

Until a few thousand years ago, this was of great value to people. When the inevitable famine arrived, those who had built up a large reserve of energy--in the form of body fat--when food was plentiful, were better able to cope with the famine, and so were more likely to survive. This meant that they would pass their genes onto the next generation, while many of their naturally slim relatives and other members of their group would not survive the famine and so could not reproduce.

It is believed that this strong evolutionary pressure has led to a substantial proportion of the present population having a tendency to put on weight when conditions are favourable (that is, during periods when plenty of food is available).

But in the developed, Western world, the feasting periods are no longer interspersed with famines. We live in an era when food is available all year round. And the food that is available isn't wild berries, yams and low-fat game meat any more--it is food that has been made to appeal to taste buds as strongly as food manufacturers can make it, often by having added sugar and/or fat.

Compounding this is the reduction in the need to do physical work in both our jobs and in everyday life. Modern technology means that many people now drive or catch a train to work, take an elevator to the floor where their air-conditioned office is located, and spend the rest of the day seated in front of a computer.

This combination of a genetic predisposition to put on weight, ready availability of very tempting food, and diminishing physical work seems to be the major factor in the dramatic increase in overweight and obesity that is occurring in Australia and in other western nations.

The second aspect I said I would address is the health significance of the obesity epidemic.

Although being slightly above normal weight poses only minimal health risks, carrying a lot of extra body fat significantly increases the risk of several common diseases--especially heart disease and diabetes. It is almost certainly NOT a coincidence that the rise in incidence of diabetes in Australia (and other similar countries) parallels that of obesity.

Being obese also vastly increases the risk of conditions such as sleep apnoea--where people literally stop breathing for short periods during sleep. As a result, they have only broken sleep of poor quality, usually accompanied by episodes of loud snoring, and often feel exhausted during the day.

Obesity also exacerbates some existing conditions such as osteo-arthritis, because the joints now have to carry a much greater load than usual.

Obesity can also lead to psychological problems through social ostracism--particularly for children--and can adversely affect self-esteem throughout life.

So how do you know which category you are in: 'underweight', 'normal weight', 'overweight' or 'obese'?

Well, there is a simple test: your 'body mass index', or BMI, is a reasonable indication of which weight category you fit into. BMI is calculated as your weight (in kilograms) divided by the square of your height, where your height is measured in metres. A BMI of less than 18.5 is defined as 'underweight'; from 20 to just below 25 is 'normal'; from 25 to just below 30 is 'overweight'; and 30 or above is defined as 'obese'.

To take a simple example, one that is simple enough for me to do by mental arithmetic: if a man--perhaps a rugby forward or Australian Rules ruckman--is two metres tall and weighs 100 kg, his BMI is his weight (100 kilograms) divided by the square of his height (where his height is two metres).

So his BMI is [100 divided by two-squared], which equals 25. That is, our hypothetical footballer has a BMI at the low end of the 'overweight' category.

Although BMI is useful, it does have limitations, because it doesn't take into account body build. In fact, our hypothetical footballer--who may be very muscular--illustrates this problem. He is categorised as 'overweight' without actually being overfat.

Another problem with BMI is that it doesn't determine where the excess fat lies on the body. Having a lot of fat in the abdominal area (that is, the 'beer gut' that many Australian men so fondly cultivate), is a much better predictor of diabetes and heart disease risk than is high fat in the area of the hips and thighs, for example.

The circumference of your body at the level of your waist is a reasonably good predictor of abdominal fat, and it will also indicate if your BMI is high because of excessive body fat rather than unusually high muscularity. To measure your waist circumference, pass a tape measure horizontally around your body at the level of the navel. Breathe out and then measure the circumference before you breathe in again. The maximum recommended waist circumference for a man is ~100 cm, for a woman it is ~90 cm.

So if you have a BMI above 25 AND a waist circumference of greater than about 100 cm (for a man) or 90 cm (for a woman) you are almost certainly carrying more body fat than is good for you. What can you do about this?

The orthodox view is that to lose weight--and to keep it off--you need to reduce your total food intake slightly (a reduction of about 5-10% should be enough), eat fewer fatty foods, and increase consumption of fruits, vegetables and cereal foods such as bread, pasta and rice. This is not a recipe for a short-term weight loss diet--to be fully effective, these changes must be maintained for life.

But, of equal importance to dietary change is the need to increase energy expenditure. This does NOT mean taking up marathon running, or anything like it. You can substantially increase your energy output--tipping the balance in favour of reducing body weight--by simply doing about 30 minutes of moderately-vigorous activity each day. This activity should be in addition to any incidental activity that you do, such as walking to the train station rather than driving, taking the stairs rather than the elevator or escalator, and so on.

The 30 minutes of extra activity can be virtually any form of moderately-vigorous exercise. Walking is excellent (provided that it is brisk walking). So too are swimming, riding a bike, light weight training, playing tennis, or any other activity that uses up energy at a moderate (or greater) rate.

It's important not to try to lose weight too quickly. A rate of weight loss greater than about one kilogram per month carries with it the risk of losing muscle as well as fat, and also means that you are probably not eating enough food to obtain all essential nutrients.

So that's the orthodox position; but what about all the diets available in the popular press and on the Internet? How reliable are these?

Well, unfortunately, most of these diets seem to be aimed squarely at making money for their authors, rather than providing sound, scientifically-based advice to people who need to lose some excess body fat. Instead of going laboriously through each diet, indicating its pros and cons, I thought I would give you some general guidance on how to judge whether or not a popular diet is likely to be helpful:

Perhaps the first question to ask is 'does this diet promise very rapid weight loss?' If the promise is for any more than about one kg per month, then the diet is inappropriate, for reasons already given.

Second, does the author make extravagant promises about other benefits, such as a 'wonderful inner glow', 'vastly more energy', 'better liver function', and so on? If the answer is YES, then the diet may be claiming to do things that a simple change in eating patterns should not be expected to achieve.

Third, is there emphasis on the value of physical activity? If not, the diet is covering only one of the necessary modifications in behaviour: food intake. It is neglecting the equally important role that physical activity plays in weight control.

The fourth and final question I suggest you ask is: does the author use a lot of jargon in an attempt to 'blind the reader with science'? Orthodox, reputable authors of books aimed at the general public avoid using too many technical terms, and make their writing comprehensible to anyone with a high school education.

Before you embark on any weight loss program, one important question to ask yourself is: 'are the likely benefits of losing weight greater than the potential risks?'  If you have previously made many unsuccessful attempts to lose weight, you may be better off accepting that you are a 'large person' by design, throwing away the bathroom scales, modifying your diet along the lines suggested earlier in this presentation, and increasing your level of moderately-vigorous physical activity. Even if the kilograms do not melt away as quickly as you would like, adding activity to your lifestyle will almost certainly improve your health, and make you feel as though you have more energy. Someone once said that even if exercise doesn't add years to your life, it will at least add life to your years.

Finally--if you have access to the Internet--for orthodox, scientifically-based information on nutrition, I recommend the Nutrition Australia website: www.nutritionaustralia.org