BMI (body mass index) is a gross estimate of a person’s leanness or fatness based on their height and weight. It does have limitations, though, and should be used as only part of a patient's overall health assessment. In other words, having an elevated BMI does not necessarily mean that you are not healthy, and having a normal BMI doesn't necessarily mean that you are healthy (1).
In adults aged 20 and older, BMI is based on the standard weight status categories, which are the same for both men and women of all ages and body types.
The standard weight status and BMI ranges for adults according to the World Health Organization (WHO) are (2):
|18.5 – 24.9||Normal/Healthy weight|
|25 – 29.9||Overweight|
|30 – 39.9||Obesity|
In children and teens, BMI is based on age and sex, thus referred to as BMI for age. A high composition of body fat in children can lead to weight-related issues and other disorders. Being underweight is also bad as the body is devoid of nutrients, leading to health risks.
The BMI ranges for children and teens of ages two to 20, according to the US Centers for Disease Control and Prevention (CDC) are (3):
|BMI (percentile range)||Weight Status|
|Less than 5||Underweight|
|5 – 85||Normal/Healthy weight|
|85 – 95||Overweight|
|Equal to/greater than 95||Obesity|
BMI is calculated in the same way for both adults and children. Belgian mathematician Adolphe Quetelet devised the formula in 1830 (4). It is expressed in kg/m 2 universally. But, if imperial units (pounds and inches) are considered, then you require an additional conversion factor.
Let us understand both the calculation ways – Metric formula and the English Imperial formula (2).Calculating BMI in metric units:
Step 1 – Multiply your height (in meters) by itself. Make sure you convert any measurements in centimeters to meters by dividing it by 100.
Step 2 – Divide your weight in kilograms by the step one value.This example will help you understand better:
Consider your height as 180cm and weight as 75kg
Convert centimeters into meters - 180÷100 = 1.8m
BMI = 75 ÷ (1.8 × 1.8) = 75 × 3.24
Therefore, BMI = 23.15.
BMI = Weight (lb) ÷ height 2 (in 2 ) × 703
Step 1 – Multiply your height in inches by itself.
Step 2 – Divide your weight in pounds by the step one value.
Step 3 – Multiply the step two value by 703.This example will help you understand better:
Consider your height as 5 feet 11 inches and weight as 155 pounds.
Convert height into inches. One foot = 12 inches, therefore multiply 5 feet by 12 and add 11 inches to it. Therefore, your height in inches is ((5 x 12) + 11 =)) 71.
BMI = (155 ÷ (71 × 71)) × 703 = (155 ÷ 5041) × 703
BMI = 0.030747867 × 703
Therefore, BMI = 21.62.
BMI is not a highly accurate measure of health as it is an arbitrary calculation. Some research studies reveal that when BMI is compared with other measures of health, such as cholesterol, triglycerides, blood pressure, and insulin resistance, almost half the individuals considered overweight under BMI calculations were found to be healthy. Further studies are required to explore the BMI impact on a person’s health. But, doctors do believe that BMI is one of the many measurements to assess health (5).
BMI has significant limitations in evaluating an individual’s body fatness as it does not take into account the distribution of body fat. It does not distinguish between fat, muscle, or bone density (6).
The below variables influence the interpretation of BMI:
• Two individuals with the same BMI can have different patterns of fat distribution and have different risk-levels of developing type 2 diabetes and heart disease.
• Older adults have more body fat compared to younger adults with the same BMI.
• Women have higher amounts of body fat than men with the same BMI.
• Bodybuilders or highly trained athletes have a higher BMI because of their muscle weight.
BMI is interpreted differently for adults and kids. It is more difficult to calculate BMI in children as it needs to be sex and age-specific, as the body fat changes with age, and also varies between girls and boys. For adults, the BMI does not depend on sex and age (3).
A high BMI (over 25kg/m2) and less physical activity may increase the risk of developing cardiovascular diseases, gallbladder problems, hypertension, type 2 diabetes, osteoarthritis, cancers (colon and breast), and mental health disorders such as depression. Likewise, a low BMI (less than 18.5kg/m2) may indicate malnourishment, compromised immune function, and a higher risk of respiratory diseases, digestive diseases, cancers, and osteoporosis (7).
If you give the correct height and weight, BMI calculators can accurately calculate your BMI. However, it is not a highly accurate measure of body fat content or health in general.
Disclaimer:Remember, BMI is one of the many ways to assess weight but not the sole method. If you have any doubts or concerns regarding your child’s BMI, speak with your nutritionist or doctor before undergoing a new diet or fitness routine.
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