WHAT IS BODY MASS INDEX (BMI)
Body Mass Index (BMI) is an index or number which is calculated from ones
weight and height. Knowing your BMI gives one a fair idea about his/her body
fatness. Research indicates that BMI correlates with direct measures of body
fat, such as underwater weighing and dual energy x-ray absorptiometry (DXA).
BMI can be considered as an alternative for direct measures of body fat.
BMI is an inexpensive and easy-to-perform method of screening for weight
categories that may lead to health problems like Hypertension, Dyslipidemia
(high Low Density Lipoprotein (LDL) cholesterol, low High Density Lipoprotein
(HDL) cholesterol, or high levels of triglycerides), Type 2 diabetes, Coronary
heart disease, Stroke, Gallbladder disease, Osteoarthritis, Sleep apnea and respiratory
problems and some cancers (endometrial, breast, and colon).
One of the best methods for population assessment of overweight and obesity is
the calculation of BMI. Its calculation requires only height and weight; it is
also inexpensive and easy to use for clinicians and for the general public. The
use of BMI allows people to compare their own weight status to that of the
general population.
Other methods of measuring body fatness include skinfold thickness measurements
(with calipers), underwater weighing, bioelectrical impedance, dual-energy
x-ray absorptiometry (DXA), and isotope dilution but these methods are
expensive, not always readily available, need highly trained personnel and can
be difficult to standardize across observers or machines, complicating
comparisons across studies and time periods.
HOW IS BMI CALCULATED AND INTERPRETED
Measurement Units | Formula and Calculation |
Kilograms and meters (or centimeters) |
Formula: weight (kg) / [height (m)]2 With the metric system, the formula for BMI is weight in kilograms divided by height in meters squared. Since height is commonly measured in centimeters, divide height in centimeters by 100 to obtain height in meters. Example: Weight = 68 kg, Height = 165 cm (1.65 m) Calculation: 68 ÷ (1.65)2 = 24.98 |
Pounds and inches |
Formula: weight (lb) / [height
(in)]2 x 703 Calculate BMI by dividing weight in pounds (lbs) by height in inches (in) squared and multiplying by a conversion factor of 703. Example: Weight = 150 lbs, Height = 5’5″ (65″) Calculation: [150 ÷ (65)2] x 703 = 24.96 |
Interpretation of BMI for adults
For adults 20 years and older, BMI is interpreted using standard weight status
categories that are the same for all ages and for both men and women. For
children and teens, on the other hand, the interpretation of BMI is both age-
and sex-specific using growth charts.
BMI | Weight Status |
Below 18.5 | Underweight |
18.5 – 24.9 | Normal |
25.0 – 29.9 | Overweight |
30.0 – 34.9 | Obese (Class I) |
35.0 – 39.9 | Obese (Class II) |
40 and Above | Morbid Obese (Class III) |
HOW IS BMI FOR AN ATHELETE OR A MASCULAR PERSON INTERPRETED
The BMI weight status categorizes individuals with BMI over 25 and 30 as
overweight and obese respectively. It is however important to remember that BMI
is not a direct measure of body fatness. An individual’s weight includes both
muscle and fat. In this regard, some individuals may have a high BMI but not
have a high percentage of body fat. For instance, highly trained athletes may
have a high BMI because of increased muscularity rather than increased body
fatness. Although some people with a BMI in the overweight range (from 25.0 to
29.9) may not have excess body fatness, most people with a BMI in the obese
range (equal to or greater than 30) will have increased levels of body
fatness.Weight is only one of the factors related to risk for disease. If you
have questions or concerns about the appropriateness of your weight, you should
discuss them with your Nutritionist.
How reliable is BMI as an indicator of body fatness?
The relationship between BMI and body fatness is fairly strong but varies by
sex, race, and age. These variations include the following examples:
• At the same BMI, women tend to have more body fat than men.
• At the same BMI, older people, on average, tend to have more body fat than
younger adults.
• Highly trained athletes may have a high BMI because of increased muscularity
rather than increased body fatness.
One should bear in mind that BMI is only one of many factors related to risk
for disease. For assessing someone’s likelihood of developing overweight- or
obesity-related diseases, the National Heart, Lung, and Blood Institute
guidelines recommend looking at two other predictors:
• The individual’s waist circumference (because abdominal fat is a predictor of
risk for obesity-related diseases).
• Other risk factors the individual has for diseases and conditions associated
with obesity (for example, high blood pressure or physical inactivity).