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Understanding BMI and its Limitations

There is a lot of gravity attached to the Body Mass Index (BMI) when applied to a person's weight and as a gauge as to whether the person is in the correct place, overweight or indeed underweight.

In most cases, it is a useful guide and starting place to assessing whether you need to lose some or not but it does have some limitations that many people may not be aware of.

In other words, the BMI is not the be all and end all of your physical health. You may take a BMI calculation and it may be telling you that you are overweight when in fact you are not, or it could be giving you a false reading if it is saying you are underweight or "normal" when you are carrying too much fat in your body.

So as a way of getting a better understanding of how your own personal BMI can help or hinder your weight loss efforts and indeed your state of health, here is an explanation of what it is and how it can sometimes be an inaccurate measure of your physical body mass.

What is the BMI?

Simply put, the BMI is a way of measuring body fat that is based on a person's height and weight and it applies to both adult men and women. The original formula was developed by Adolphe Quetelet (1796-1874), a Belgian statistician.

For that reason, it was initially named the Quetelet Index. Nowadays, it has been relabeled the BMI and is sometimes referred to as the Body Mass Indicator. BMI has now become an internationally applied measure of obesity.

How to Calculate BMI

In its original guise, the BMI was calculated using the metric system due to its creator being Belgian. The formula using metric measures is far simpler to apply and is this:

BMI = weight in kilograms divided by height in metres squared

or in shorthand:

BMI= W kg / H m²

To calculate your own BMI in imperial measurements, the formula is a little more complex but should be no problem to anyone with average arithmetic skills or who can use a calculator! It is this:

BMI = (weight in pounds times 703) divided by height in inches squared

or in shorthand:

BMI= (W lbs * 703) / H ins²

The result is a number that can then be used to assess if you fall into the bracket of underweight, normal, overweight or obese and is given as this:

It is also worth noting that different nations and certain organizations will use different ranges of BMI results to classify the status of a person's weight. The above readings and limits are in keeping with the US Dept of Health & Human Services published weight status categories.

Limitations of BMI

While the BMI is a good indicator of your state of body size by providing you with an instant visual indicator as to where you are in the spectrum of underweight to obese, it can only provide that information from the information that has been provided it. In some cases, there will be mitigating factors that will cause the BMI rating to be false. These are:

While some of these points can be obvious to anyone who thinks about them, such as those with a missing limb, others are not so obvious, as in the case of a person suffering with acute edema (water retention) as this can cause the weight to be much higher die to the excess fluid and not excess fat. It also does not discriminate between men and women and there is a physical difference in the way we store fat and what limits are deemed as normal as opposed to being potentially dangerous.

BMI Indicator of Health

The BMI is a reasonably good indicator of potential health problems that are associated with a person's size and can be the first step in diagnosing potential problems that may need further medical investigation. One good visual indicator of a potential health problem is the circumference of the waist.

Waist Circumference

Knowing the measurement of your waist circumference can help you screen for any potential health risks that are associated with being overweight and more so with being obese. If the majority of fat is stored around your waist instead of at your hips, you could have a higher risk of developing heart disease and also type 2 diabetes.

The level of risk increases if your waist size is more than 35 inches if you're a woman or 40 inches if you're a man.

If you can see that your waist is larger than you think it should be, you should measure it to be sure. To correctly do this, place a tape measure around your waist while standing up, placing it just above your hips. Take the measurement just after breathing out and write it down.

It is worth noting that a large waist measurement can be an indicator of potential weight related risks even in people who have a "normal" BMI. The risks for developing type 2 diabetes, hypertension and CVD (Cardiovascular Disease) increase with higher a BMI and are especially high risk for those who fall into the obese category.

Other Risk Factors

There are other risk factors that can be associated with having a high BMI in addition to those mentioned in the previous paragraph. These include high levels of LDL cholesterol ("bad" cholesterol) and conversely low levels of HDL cholesterol ("good" cholesterol), high triglyceride levels, high blood glucose (sugar) levels.

Further health risks can be brought into play for those with a high BMI if there is a family history of premature heart disease or hypertension, if you lead a sedentary lifestyle and get little or no exercise or physical activity and if you smoke.

Naturally, the best recommendation you can have if you feel you are at risk or are just concerned about having a high BMI (and you are sure it is not caused by water retention) and your physical health is not up to par is to lose some weight through a weight loss diet and exercise program. Even as little as a five to ten percent loss in body fat through a sensible slimming program can reduce the health risks enormously.

If your BMI puts you in the overweight bracket but you do not have a high waist measurement, it is still recommended that you make changes to your diet and lifestyle to prevent further weight gain.

A sensible, nutritionally balanced and calorie controlled diet combined with some form of daily exercise is the best way to avoid weight gain by an increased metabolism. It also helps you to maintain a healthy weight and keep you within the normal BMI range.

You should consult your doctor if you are concerned about your size and weight and the possibilities of associated health risks.

Source: BMI Limitations


Posted on Tue, 03 Jan 2012 in weight-loss | 4 Comments

4 thoughts on "Understanding BMI and its Limitations"

Tony Rovere says:

In my opinion, BMI is one of the WORST devices you can use to see if you are overweight or not. This is because when you weigh yourself... you are weighing the combination of muscle, bone, muscle and fat that you are composed of, as you correctly point out.

Instead, a much better measure is to compare your weight to your waistline.

Measure your waistline and then compare that to your height, both in inches or centimeters.

When you divide your waist by your height, it should be at a ratio of below 0.5. Anything over that indicates that you are carrying too much of a spare tire and need to start to lose fat around the waistline.

Janice says:


I am extremely overweight and need to lose fast because my wedding is just 3 months away! How do I lose weight so fast, except without starving??

Thanks for your help and advice!

Terry says:

Janice, all the advice you need is right here in the articles on this website. You can lose a lot of weight in 3 months, but you have to work at it. There's no magic bullet, just determination, a positive attitude and a combo of eating a healthy, low calorie diet and plenty of daily exercise while drinking lots of plain water. It sounds basic, but that's pretty much it aside from all the extra tips and stuff that you'll find here.

Michael says:

I have found that healthy eating begins with the right types of foods. There are certain foods out there that can help or hinder weight gain. This winter I was pushing 240 and since I educated my self about what I should eat and how I should exercise I have lost pounds quickly and healthily.