How Useful is the Body Mass Index as a Health Indicator?

Are there other more accurate measures?

By Dr Reginald Liew

Body Mass Index

There has been a lot of stir in the media and medical communities lately regarding the Body Mass Index Scale (BMI). Some professionals and public members hold it up as a gold standard measurement of health, while others shout about its uselessness. So which is it? Is the BMI scale a good measure of overall health, or should it be replaced?

What is the Body Mass Index?

BMI is a measurement of body fat based on your weight and height. It tells you if you are underweight, at an average weight, overweight, or obese, based on a calculation. The calculation takes your weight in kilograms divided by your height in centimeters squared. If you are more comfortable with pounds and inches, the formula is your weight in pounds divided by your height in inches squared, multiplied by 703. For example, if you are six feet (182.88 cm or 72 in) tall and weigh 200 pounds (90.72 kgs), your BMI would be 27.1. If you’d like an easy calculator to do the math, click here for the NIH BMI calculator.

What do the numbers mean?

Once you have calculated your BMI, you can then compare it to the BMI chart to find where your number weighs in. The ranges are as follows:

<18.5 = Underweight (high risk)
18.5-24.9 = Average (low risk)
25-30 = Overweight (low to moderate risk)
>30 = Obese (high risk)

If your BMI is over 40, you are considered at extreme risk.

Why is BMI bad?

The BMI itself is not necessarily bad. It does, however, have some issues. Looking at only your height and weight leaves out so many factors that determine health. The BMI does not consider a person’s age, race, gender, or muscle mass, all of which can contribute to differences in what is regarded as a ‘healthy’ weight. Additionally, some reports show that where you carry weight has more impact on overall health than how much body mass you have total.

In many cases, people who are considered overweight on the BMI scale have very healthy cholesterol levels, their blood pressure is normal, and there are no other warning signs of illness or disease. On the other hand, many times, a person considered to be at a normal BMI will have numerous health problems. Therefore, the BMI by itself is not an adequate measure to assess overall health.

Then there is the problem of bias. Healthcare providers are just like all other people - some of them are good; some are … less good. It’s these less good types that tend to have issues of bias in healthcare. These types are more likely to look at numbers on a chart and make those the sole motivator behind their care. This bias leads many overweight people to be leary of seeking medical care.

Why is BMI good?

Despite the many problems presented by using BMI as an overall indicator of health, it still has some beneficial uses. While it shouldn’t be the only thing to assess individual health, it can give advanced warning signs. For example, populations below 18.5 or above 30.0 on the BMI scale generally have higher rates of diseases and premature death. Additionally, if a person falls in the obese or overweight ranges, a loss in weight corresponds to fewer cases of serious illnesses. Again, while the BMI is useful as a single tool to help indicate risk levels, it should not be the only tool used.

What are alternatives to BMI?

Several alternatives, used in conjunction with other tests and the BMI, can give a more comprehensive view of your overall health. These alternatives include:

  • Waist-to-hip ratio - measure the circumference of the smallest part of your waist and divide it by the circumference of the largest part of your hip/buttock region. For women, a result of .80 or lower equals low health risks, .81-..85 is a moderate risk, and .86+ is high risk. For men, the risk numbers are: up to .95 = low, .96-1.0 = moderate, and 1.1+ = high.
  • Waist circumference - All you have to do is measure your waist at its smallest point. A measurement of over 35 inches for women and 40 inches for men equals a higher risk value.
  • Lab testing - Having a doctor perform blood tests and measure your vital signs are the most comprehensive way to check your health. However, it is costly and time-consuming to perform complete diagnostic tests.
  • Measuring body fat percentages - This test determines what ratio of your overall mass is body fat. Accurate measuring systems are rarely available, and less high tech alternatives are often faulty.

While the BMI is not a good indicator of overall health, it is useful when used as one of many tests. It is a quick and easy way to determine if you are overweight and possibly at risk for disease or illness. Just make sure the BMI is not the only factor you and your doctor look at when determining overall health.

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