5 questions about BMI

It’s pretty easy to step on a scale and find out how much you weigh. But that scale won’t answer a crucial question: Is your weight healthy?

You might rely on your clothing size to tell you that. Or look for signs of expansion like tight-fitting jeans. Or compare yourself to impossible standards presented on TV or in magazines.

There is a way to make sense of the numbers and find out if your weight is right for you. It is by using the body mass index (BMI). We talked to Lindsey Wohlford, a wellness dietitian at MD Anderson, and got her answers to five questions about BMI.

What is Body Mass Index?

Body mass index, or BMI, is a way to measure your size. It allows you to see if you are a healthy weight for your height. For example, take someone who weighs 165 pounds: That could sound like a healthy weight. But what if the person was 5 feet tall?

In reality, just about any weight can fall into a category of overweight, underweight or healthy, depending on your height. BMI is a measure that accounts for this difference.

How and when did BMI get started?

Believe it or not, at one time, it was considered good to be overweight. High body-fat was seen as a sign of good health and a robust diet. That changed in the mid-20^th century when insurance companies noticed that their overweight customers actually had more health problems.

The search for a way to measure healthy weight began.

The body mass index was first used under that name in 1972. It is based on the work of Belgian math expert Adolphe Quetelet, who died in 1874. Quetelet figured out that your weight should be in proportion with your height and created the formula used for BMI.

Should I ignore my weight and concentrate on my BMI?

For most people, BMI is a good tool because it’s easy to understand, and it can shed light on a problem that you are not aware of. Maybe you’re not sure where your weight should be.  Or maybe your current weight is unhealthy, but it has come to seem normal to you over time.

But a true assessment of what’s right for you should include talking with your doctor. He or she may consider other measures, like waist circumference.

Is BMI a useful measure of health?

Studies do show people with a high BMI have a higher risk of disease. And people who improve their BMI by losing weight also improve their health.

Think of it is as a starting point to help you figure out what range you fall into. It should not be used by itself to judge how healthy you are.

For example, BMI does not factor in blood pressure, cholesterol, blood sugar or lifestyle factors like smoking or your family medical history. All of these are very important clues to how healthy you are.

BMI also does not tell you if you are eating enough fresh, plant-based foods, or if you are doing enough exercise.  It is possible to have a BMI in the healthy range but still be physically unfit and short of the essential nutrients provided by fruits and vegetables.

Is the BMI scale affected by race, age or other factors?

There are limitations to the body mass index.

When the BMI tables were originally created by life insurance companies, there were charts for small, medium and large body frames. This meant someone who had a large frame could be heavier and still be rated as healthy because of their frame.

The current BMI scale does not take body frame into account. It also does not account for gender, race, ethnicity or age differences, which also could influence your BMI.

If your BMI is high, it can be a sign that something is wrong. Whatever your race, ethnicity or gender, talk to your doctor. He or she can combine your BMI with other health measures and let you know what action to take.

Remember, BMI can signal that you are overweight or obese, but it's just a starting point.

Does the BMI scale work for people who are very athletic?

Not always.

Body builders and some athletes or fitness enthusiasts build up a high level of muscle mass.

This shows up as extra weight and gives some athletes a BMI in the unhealthy range. But an increased risk of disease only comes with extra body fat, not with muscle, so the result is wrong.

By the same token, there are people who have excess body fat and very low muscle mass. Their BMI could suggest they are a healthy weight when they are not. It may give them a false sense of reassurance.

What numbers should I know about my health?

Your doctor will look at many different things to assess how healthy you are. Keep track of the big items on the list like BMI, weight, blood pressure, waist circumference and minutes of exercise per week.