What is Body Mass Index (BMI)?

The body mass index (BMI) is an important index for relating weight to height. It is a measurement that is a weight-to-height ratio. It is a better way to see whether our weight is proportionate to our height. BMI, formerly known as the Quetelet index, is a metric used to assess an individual's nutritional status. A high BMI can indicate a high level of body fat. Indeed, knowing our BMI can help us determine any health risks we may face if it is above the healthy range. Adults with a BMI between 25 and 29.9 are classified as overweight; an adult with a BMI of 30 or above is classified as obese; a BMI less than 18.5 is considered underweight, and a BMI between 18.5 and 24.9 is considered healthy.

The significance of the Body Mass Index

BMI is a simple screening method for weight categories such as underweight, normal weight, overweight, and obesity. BMI is the most practical method of classifying body fat mass and obesity. Because calculation requires only height and weight, BMI is an inexpensive and simple tool.

BMI is used as a screening tool, but this does not diagnose an individual's body fatness or health. A health professional will conduct additional testing to determine whether BMI is a risk factor for health. Detailed dietary and physical activity evaluations, skinfold thickness measurements, and family history are examples of such evaluations. People with a higher body mass index are at risk of Type 2 diabetes, heart disease, high blood pressure, snoring and sleep apnea, joint inflammation and osteoarthritis, high blood cholesterol, cancers, and premature death. Obesity may lead to fatty material accumulating in the arteries (the blood vessels that carry blood to the organs). A heart attack can occur if the arteries that deliver blood to the heart become damaged and congested.

However, BMI is one of the factors used to assess health risk. Other factors considered when measuring health risk include blood pressure, cholesterol level, blood sugar level, family history of heart disease, age, gender, waist circumference, amount of physical activity, menopause status, smoking status, and so on.

Calculation of Body Mass Index

A mathematical method is used to calculate an individual's BMI. It can also be determined using tables in which the height in inches is compared to the weight in pounds to calculate BMI. There are other simple calculators available on the internet that assist in calculating BMI. BMI is calculated by dividing a person's weight in kilograms by their height in meters squared.

The formula for calculating BMI is as follows:

$BMI=\frac{Weight (in kgs)}{Height (in m^2)}$

A normal BMI score is between 18.5 and 24.9. This means that a person's weight is within the usual range for their height. A BMI chart is being used to determine if a person is underweight, normal weight, overweight, or obese.

How is BMI calculated for adults and children?

For adults aged 20 and above, BMI is calculated using standard weight status classification. For children and teenagers, the BMI is measured using the same formula as adult BMI, but BMI is interpreted differently than adults. The BMI of children and teenagers must be age and sex-specific because the amount of body fat varies with age and varies between girls and boys. For the calculation of BMI in adults, gender and age are not taken into account. 

The table shows the weight status based on BMI:

Adult BMI calculator

The adult body mass index (BMI) is calculated by using our height and weight. We will get a score when we enter our height and weight into a BMI calculator. That number can help to determine whether we are at a healthy or unhealthy weight. We employ the BMI chart to decide whether we need to gain weight, retain our weight, or reduce the weight when we have our BMI score. The adult BMI calculator is used for people aged 20 or older. This calculator measures body mass index (BMI) and the related BMI weight status group.

BMI calculator for children and teenagers

The BMI calculator for children and adolescents differs from the one for adults. It takes into account not only height and weight but also age and gender. This is because male and female height and weight vary during growth and development. BMI in children is reported as a percentile in comparison to other children of the same age and gender.

Body mass index (BMI) and the associated BMI-for-age percentile are calculated for children and teenagers aged 2 to 19 years using CDC (clinical) growth charts. The results of this calculator may vary slightly from BMI-for-age percentiles estimated by other applications due to possible rounding errors in age, weight, and height.

Clinical growth charts

Pediatricians and other health care practitioners use growth charts to track a child's development over time. Growth charts were created by monitoring the growth of a large number of healthy children throughout time. The clinical growth charts are formatted differently than the individual charts, with two separate charts on a single page and data entry tables included. The grids on the clinical charts are scaled to metric units (kg, cm), with English units (lb, in) serving as an alternative scale. There are clinical charts for both boys and girls.

The following are included in clinical charts:

• Two to 20-year-old children and adolescents: Weight-for-age, stature-for-age, and BMI-for-age.
• Preschoolers (ages two to five): Weight-for-stature.
• Infants aged zero to 36 months: Weight-for-age, length-for-age, weight-for-length and head circumference-for-age.

BMI for various categories

BMI is a good indicator of obesity for the majority of people. However, BMI does not provide accurate information on muscle mass and bones. BMI is a more accurate estimate of body fat in certain people than in others. Exceptionally muscular persons, for example, maybe classified as overweight despite being healthy and athletic. These people with extremely low body fat percentages may have the same BMI as someone who is overweight. Similarly, an aged or feeble person may be considered normal weight if they have minimal muscular mass and a high amount of body fat.

Context and Applications

This topic is significant in the exams at school, graduate, and post-graduate levels, especially for

• Bachelors in Biology/Physiology
• Masters in Biology/Physiology

Practice Problems

Question 1: What is the formula for BMI calculation?

1. Height/weight
2. Weight/height
3. Height/muscle mass
4. None of the above

Answer: Option 2 is correct.

Explanation: A mathematical method is used to calculate an individual's BMI. BMI is calculated by dividing a person's weight in kilograms by their height in meters squared.

The formula for calculating BMI is as follows:

$BMI=\frac{Weight (in kgs)}{Height (in m^2)}$

Question 2: Obesity increases the risk of developing which of the following diseases?

1. Heart disease
2. Type 2 diabetes
3. Cancer
4. All of the above

Answer: Option 4 is correct.

Explanation: People with a  higher body mass index or obesity,  are at risk of Type 2 diabetes, heart disease, high blood pressure, snoring and sleep apnea, Joint inflammation and osteoarthritis, high blood cholesterol, cancers, and premature death.

Question 3: BMI is used for ______.

1. Indicator of high body fat
2. Screening tool for overweight and obese people
3. To determine obesity-related health risks
4. All of the above

Answer: Option 4 is correct.

Explanation: BMI is used as a screening tool, but this does not diagnose an individual's body fatness or health. A health professional will conduct additional testing to determine whether BMI is a risk factor for health.

Question 4: What are the factors used to calculate adult BMI scores?

1. Height and weight
2. Age and gender
3. Both 1 and 2
4. None of the above

Answer: Option 1 is correct.

Explanation: The adult body mass index (BMI) is calculated by using our height and weight. The BMI calculator for children and adolescents differs from the one for adults. It takes into account not only height and weight but also age and gender.

Question 5: A BMI score of 32 would be classified as ______.

1. Obese
2. Underweight
3. Overweight
4. Normal weight

Answer: Option 1 is correct.

Explanation:

BMI weight status:

A normal BMI score is between 18.5 and 24.9. This means that a person's weight is within the usual range for their height. A BMI chart is being used to determine if a person is underweight, normal weight, overweight, or obese. So the BMI score of 32 would classify as obesity.