3.4 Body Mass Index (BMI)
Body Mass Index (BMI) is an essential health parameter used to determine whether an individual has a healthy body mass relative to their height. Calculating BMI helps identify whether a person is underweight, at a desirable weight, overweight, or obese.
Learning Objectives
- Define Body Mass Index (BMI) and state its calculation formula with correct units.
- Calculate BMI and classify the result into categories: underweight, desirable weight, overweight, or obese based on standard reference ranges.
- State the importance of knowing one's BMI.
- Identify the health risks associated with being overweight or obese, as well as being underweight.
- Describe practical ways to increase body mass for underweight individuals and decrease body mass for overweight individuals based on nutritional and physical guidelines.
Body Mass Index (BMI) Reference Scale and Health Guidelines
Map your calculated BMI score to the correct weight category and view the corresponding recommended dietary and lifestyle actions.
Short Explanation
Definition
Body Mass Index (BMI) is the measurement of body mass against height.
BMI is a method used to determine whether a person has a body mass problem or not. It relates a person's mass in kilograms directly to the square of their height in metres.
BMI Formula
\[ \text{BMI} = \dfrac{\text{Body mass (kg)}}{(\text{Height})^2\text{ (m}^2\text{)}} \]
The unit of BMI is \(\text{kg m}^{-2}\) (or \(\text{kg/m}^2\)).
Why is BMI important?
An individual whose BMI falls between \(18.5\) and \(24.9\text{ kg m}^{-2}\) enjoys a desirable weight, which correlates with greater longevity. Falling outside this ideal range poses specific health hazards.
BMI Classification Chart (Table 3.2)
| Category | BMI Range |
|---|---|
| Underweight | \(< 18.5\text{ kg m}^{-2}\) |
| Desirable weight | \(18.5 - 24.9\text{ kg m}^{-2}\) |
| Overweight | \(25.0 - 29.9\text{ kg m}^{-2}\) |
| Obese | \(30.0\text{ or more kg m}^{-2}\) |
Studies show that the lifespan of a person with an ideal BMI of between \(18.5 - 24.9\text{ kg m}^{-2}\) is longer than that of a person with a lower or higher BMI.
Health Risks Associated with BMI Status
Risks of Being Overweight / Obese
Heart attack, high blood pressure, stroke, diabetes mellitus, and complications of joints and bones.
Risks of Being Underweight
Heart diseases, decrease in body's defence against diseases, fatigue, anaemia, and depression.
Ways to Manage Body Mass
Ways to Increase Body Mass
Practise a healthy and balanced diet based on the Malaysian food pyramid and the Malaysian Healthy Plate. Eat at fixed times. Consume snacks that are packed with nutrients to increase calories. Eat more nutritious food.
Ways to Decrease Body Mass
Set a realistic goal such as reducing half a kilogram of mass weekly. Monitor and record body mass once a week. Eat healthy foods in moderation based on the Malaysian food pyramid and the Malaysian Healthy Plate. Make changes to eating habits, such as changing food preparation methods. Reduce or avoid food with high sugar or fat content. Do moderate exercises such as brisk walking, cycling, and swimming for at least 30 minutes, three times a week. Gain support from those who are close.
The Malaysian Healthy Plate
The Malaysian Healthy Plate consists of key food groups:
1. Fish, poultry, meat, and legumes
2. Rice, noodle, bread, cereals, cereal products, and tubers
3. Fruits
4. Vegetables
Try to Answer First
Answer in your mind, then press “Check Answer”.
What is the category of an individual with a BMI of \(30.0\text{ kg m}^{-2}\)?
State the formula used to calculate Body Mass Index.
List two health risks associated with being underweight.
Common Mistakes
- Using height in centimetres (\(\text{cm}\)) instead of converting it to metres (\(\text{m}\)) before squaring it in the formula.
- Forgetting to square the height: dividing body mass directly by height instead of \((\text{height})^2\).
- Misclassifying boundary values: classifying a BMI of \(25.0\text{ kg m}^{-2}\) as desirable weight, when \(25.0\text{ kg m}^{-2}\) marks the beginning of the Overweight category (\(25.0 - 29.9\)).
- Assuming underweight means completely free from illnesses: believing only overweight individuals face health risks, ignoring problems like reduced immunity, fatigue, and anaemia seen in underweight individuals.
Concept Misunderstandings
Being underweight is always healthier and safer than having any other BMI value.
Being underweight also carries significant health risks, including heart diseases, weakened immune defence, fatigue, anaemia, and depression.
Rapid, drastic weight loss of several kilograms per week is the recommended approach to reduce body mass.
The textbook recommends setting a realistic, gradual goal, such as reducing half a kilogram (\(0.5\text{ kg}\)) of mass weekly.
Summary
- BMI measures body mass against the square of height (\(\text{kg m}^{-2}\)) to evaluate whether an individual has a healthy weight.
- The categories are Underweight (\(< 18.5\)), Desirable weight (\(18.5 - 24.9\)), Overweight (\(25.0 - 29.9\)), and Obese (\(30.0\text{ or more}\)).
- Maintaining a desirable BMI helps prevent conditions such as stroke, diabetes, high blood pressure, joint complications, anaemia, and compromised immunity.
- Management involves balanced nutrition (based on the Malaysian food pyramid and Malaysian Healthy Plate) alongside regular physical exercise.
Short Activity
A. Objective Quiz
1 What is the BMI range classified as “Desirable weight”?
2 Which of the following is a recommended method to decrease body mass in overweight individuals?
B. Fill in the Blanks
3 In the formula for calculating Body Mass Index, body mass in kilograms is divided by height in units of .
4 An individual with a BMI of 30.0 or higher is classified into the category of .
C. Drag and Drop: Match Each Health Issue or Management Method to Its Correct BMI Condition
Drag each description to the correct category or action. On a phone, tap an answer first, then tap a matching box. Tap the Answer Choices area to return a selected item.
C. Match Each Health Issue or Management Method to Its Correct BMI Condition
Write the correct description next to each category or action.
| Category / Action | Description / Condition |
|---|---|
| 1. Health risk of being overweight or obese | |
| 2. Health risk of being underweight | |
| 3. Way to increase body mass | |
| 4. Way to decrease body mass |
Answer choices: Complications of joints and bones • Anaemia and decreased body defence • Consume nutrient-packed snacks to increase calories • Reduce or avoid food with high sugar or fat content
