What You Should Know About Your Body Mass Index (BMI)

The body mass index (BMI), or Quetelet index, is a value derived from the mass (weight) and height of an individual. The BMI is defined as the body mass divided by the square of the body height, and is universally expressed in units of kg/m2, resulting from weight in kilograms and height in metres. If pounds and inches are used, a conversion factor of 703 (kg/m2)/(lb/in2) must be applied. When the term BMI is used informally, the units are usually omitted.

The BMI may also be determined using a table or chart which displays BMI as a function of mass and height using contour lines or colors for different BMI categories, and may use two different units of measurement.

The BMI is an attempt to quantify the amount of tissue mass (muscle, fat, and bone) in an individual, and then categorize that person as underweight, normal weight, overweight, or obese based on that value. However, there is some debate about where on the BMI scale the dividing lines between categories should be placed.

Commonly accepted BMI ranges are: 

• underweight: under 18.5 
• normal weight: 18.5 to 25 
• overweight: 25 to 30 
• obese: over 30

There are criticisms of using the BMI to define obesity in individuals. One is that the BMI was designed for population studies, not individuals. Another is that body fat percentage (BFP) is a more reliable indicator of obesity than BMI: very muscular, lean (low body fat) individuals can be classified as obese using BMI, but are classified as having a normal weight using BFP. An even simpler alternative to the BMI is to define obese individuals as those whose waist circumference is greater than 50% of their height, indicating excess intra-abdomimal fat.

BMI CHART

The Measurement of Choice
BMI is the measurement of choice for most health professionals. ''I think BMI is a very good and easy screening tool,'' says obesity expert, Cathy Nonas, a spokeswoman for the American Dietetic Association. 

But while it is a simple, inexpensive method of screening for weight categories, it is not a diagnostic tool. Health professionals need to do further assessments to fully evaluate health risks. These assessments would include measurements of body fat percentage, diet history, exercise patterns, and family history.

Check adult BMI here.

Further, BMI does not take into account age, gender, or muscle mass. Nor does it distinguish between lean body mass and fat mass. As a result, some people, such as heavily muscled athletes, may have a high BMI even though they don't have a high percentage of body fat. In others, such as elderly people, BMI may appear normal even though muscle has been lost with aging. Take for example, basketball player Michael Jordan: ''When he was in his prime, his BMI was 27-29, classifying him as overweight, yet his waist size was less than 30,'' says Michael Roizen. That's one reason some experts think waist circumference can be a better overall health measurement than BMI.

Check children BMI here.

Another is that your health is not only affected by excess body fat, but also by where the fat is located. Some people gain weight in their abdominal regions (the so-called ''apple'' body shape.) Others are ''pear-shaped,'' with excess weight around the hips and buttocks. People with apple shapes are at higher risk for health problems associated with being overweight.

Fat around your waist is more biologically active and can do more damage to your body than weight around your hips. The data show that waist circumference is more reliable and more closely correlated with diseases associated with obesity. According to the National Institutes of Health, a bigger waist circumference (greater than 40 inches for men and 35 inches for women) is linked to a higher risk of type 2 diabetes, high blood pressure, abnormal cholesterol levels, and heart disease when BMI is 25 to 34.9.


To properly measure your waist, no math is needed. Just use a soft tape measure around your bare midsection at your belly button. Find your upper hip bone, and measure around the abdomen above the bone. The tape should be snug, but not dig into your skin.

Nonas argues that waist circumference is not a better tool than the BMI ''because we do not have good criteria or cut points for levels of overweight, obesity, age or height.'' She also thinks that properly measuring the waistline is a little more difficult than measuring height and weight. One thing that experts agree on is that weight is only one factor in our risk for disease. When it comes to evaluating weight and its impact on health, your percentage of body fat, waist circumference, BMI, and physical activity patterns are all important.

The National Heart, Lung, and Blood Institute recommends that health care providers assess BMI, waist circumference, and any other risk factors for obesity-related conditions. Combining all of the information provides the best assessment.

What Can You Do?
The first step toward shrinking your waistline and getting your BMI in line is to start eating a healthier diet and getting regular exercise. Preventing any further weight gain and slowly reducing weight into a healthier range is an excellent goal. And while you might want to lose more, dropping as little as 5%-10% of your body weight can bring dramatic improvements in blood pressure, blood cholesterol, and blood sugar.

Four steps to a healthy lifestyle

1. Be physically active.
2. Make healthy food choices.
3. Avoid overeating.
4. Schedule an annual physical examination.

These are the vital parts to maintaining a long and healthy life.

Culled and edited. Credits: Wikipedia, WebMD, CDC