Weight loss programs are gaining popularity because of the increasing awareness about health. However, weight alone is not an accurate measurement of one's health status, which is why health experts use body mass index (BMI) instead, as a useful predictor of one's health.

In one study presented at the annual meeting of the Endocrine Society in Boston, it was found that BMI is an effective tool to predict whether a child can develop obesity early in life.

What Is BMI?

BMI or Quetelet index is measured by obtaining the person's weight in kilograms divided by the square of height in meters. It attempts to quantify the mass of a person including muscle, fat, and bone. The resulting value groups individuals into underweight, normal weight, overweight, or obese.

The BMI was initially an economic tool but it later evolved as a health indicator. It is an effective health screening tool but it does not diagnose whether an individual is healthy or not, as it does not directly measure body fat.

A study published in the International Journal of Obesity, which correlated BMI values against health markers such as triglyceride, cholesterol, blood glucose and blood pressure levels, found that those who are tagged as obese by the BMI are not necessarily unhealthy. In fact, those found to be underweight are seen to be unhealthy.

Why Use BMI?

As an economic tool, BMI is an affordable and easy tool for doctors and other health experts to screen the general population in terms of body fatness alone.

There are other ways of measuring body fatness, such as underwater weighing, dual-energy x-ray absorptiometry (DXA), skinfold thickness using calipers, and isotope dilution, but they are either expensive or not easily available. Additionally, the above mentioned methods oftentimes require highly trained personnel and are quite difficult to standardize.

How Is BMI Calculated?

BMI calculation for adults and children uses the same formula. In kilograms and meters, the formula is weight (kg)/ [height (m)]2. For those using centimeters, value of height can be obtained by dividing the height (cm) by 100 to get the height in meters.

In pounds and inches, the BMI can be obtained by using the formula: weight (lb) / [height (in)]2 x 703.

How Is BMI Interpreted?

For adults 20 years old and older, BMI interpretation is based on weight categories, which is applicable for both men and women.

BMI values and its weight status are the following: less than 18.5 are underweight; 18.5 to 24.9 are normal or healthy weight; 25.0 to 29.9 are overweight; and 30.0 and above are obese.

Although BMI calculation for children uses the same formula as those for adults, BMI interpretation is specific for age and sex, as body fat changes are different between boys and girls. The Centers for Disease Control and Prevention (CDC) BMI-for-age growth charts take this into consideration and categorize children's BMI using percentile rankings.

For example, for a child or teen aged 2 to 19 years old to be classified as obese, BMI should be at or more than the 95th percentile of children of the same sex and age.

BMI And Body Fatness

The correlation between body fatness and BMI is significant but it does not mean that if two people have the same BMI, they would have the same body fatness.

Generally, females have more body fat than males even if they have the same BMI. The same is true for older people compared to younger adults. Athletes may have the same BMI with a non-athlete, but it does not mean that they have the same body fatness. In fact, athletes have lesser body fat than non-athletes, as their weight could be attributed to muscle mass rather than body fat.

BMI And Obesity

Measurement and identification of obese people is important in terms of monitoring health risks. Obese individuals are at a higher risk of developing diseases such as Type 2 diabetes, hypertension, coronary heart disease, and mental illnesses to name a few.

Photo: Dani Lurie | Flickr