Body mass index (BMI) is a measure of body fat based on your weight in relation to your height

Adults – applies to most adult men and women aged 20 and over.

Children – aged 2 and over, BMI percentile is the best assessment of body fat. [Read: Childhood Obesity]

BMI does not measure body fat directly.

Why Rely On BMI At All?

A. BMI is an inexpensive and easy-to-perform method of screening for weight category.

  1. underweight,
  2. overweight,
  3. obese
  4. Healthy weight for their height.

B. It Acts As A guideline to indicate whether bariatric surgery is indicated or not

Calculating Body mass index

BMI is calculated by dividing a person’s weight (in kilograms) by his or her height (in meters, squared).

BMI can also be calculated by multiplying weight (in pounds) by 703, then dividing by height (in inches, squared). People with BMIs of 25 and above are considered to be overweight

In 1997, a WHO expert committee classified a

A. Overweight – BMI of 0 – 29.0kg/m2

B. Obesity – BMI 0f 30 – 34.9 kg/m2

C. Morbid obesity – BMI of ≥ 35 kg/m2

Table 1: The International Classification of adult underweight, overweight and obesity according to BMI

Ethnic specific BMI

Cut off values, especially for Asian populations, have been proposed to

a. Address the higher prevalence of diabetes and cardiovascular diseases and
b. The differing associations of BMI with body fat in different populations.

The revised guidelines categorize

  • Overweight as a BMI of 0 – 24
  • Obesity as a BMI ≥ 25

using values lower than the ethnic specific BMI previously advocated for Asian Indians.

How is BMI used?

A high BMI can be an indicator of high body fatness.

BMI can be used as a screening tool but is not diagnostic of the body fatness or health of an individual.

To see if a high BMI is a health risk, need to perform further assessments.

These assessments might include

  • skin fold thickness measurements,
  • evaluations of diet,
  • physical activity,
  • family history,
  • appropriate health screening.

 

How good is BMI as an indicator of body fatness?

The correlation between the BMI and body fatness is fairly strong

But even if 2 people have the same BMI, their level of body fatness may differ.

In general,

At the same BMI, women tend to have more body fat than men.

At the same BMI, athletes have less body fat than do non-athletes.

The accuracy of BMI as an indicator of body fatness appears to be higher in persons with higher levels of BMI and body fatness16.

 

Metric BMI Formula

Weight (kg) / Height (m)2

Imperial BMI Formula

(Weight (lbs.) * 703) / Height (inches)2

 

BMI limitations

  1. It is not an accurate measure of body fat or health.
  2. BMI fails to take age and sex into account.
  3. No way of measuring where body fat is located in the body.

Belly fat – is more dangerous than peripheral fat beneath the skin in other body areas.

If you are obese according to BMI, you are most likely obese according to body fat percentage also.

When sampling from the general population, over 95% of men and 99% of women identified as obese by BMI were also obese via body fat levels.10

Health consequences of overweight and obese adults

The BMI ranges are based on the relationship between body weight and disease and death.

Overweight and obese individuals are at increased risk for many diseases and health conditions, including the following

  • Hypertension
  • Type 2 diabetes
  • Stroke
  • Gallbladder disease [Read: Gallstones]
  • Sleep apnea and respiratory problems
  • Dyslipidemia
  • Coronary heart disease
  • Osteoarthritis
  • Cancers (endometrial, breast and colon) [Read: Breast Cancer]

Related links

WHO

WHO Department of Nutrition
WHO Global Database on Child Growth and Malnutrition
Publications on obesity and nutrition transition
Chronic diseases and health promotion. STEPwise approach to surveillance (STEPS)

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