Introduction to BMI and Its Importance
Body Mass Index (BMI) is mentioned frequently in the context of healthcare and disease prevention. It is a way of classifying the body weight of adults and assessing any associated health risks.
Since 1995, BMI has been the World Health Organization’s official criterion for weight classification. However, it has its limitations, and it’s important to know what they are in order to optimise its use. In this article, we’ll take a look at those limitations, as well as explaining how BMI is calculated and how it can be used as a starting point for setting health goals.
Using the BMI Calculator
BMI is calculated by dividing a person’s weight (in kilograms) by their height (in metres squared). So, to work out your BMI score, you need to know:
- your height in metres
- your weight in kilograms
You can find an example of a body mass index calculation here. There are easy-to-use online BMI calculators such as this one on the Better Health Channel.
Understanding BMI Categories
The result of a person’s BMI calculation puts them in one of several categories.
These range from ‘underweight’ to ‘obese’.
- BMI less than 18.5 – Underweight: being underweight can be associated with malnutrition, vitamin deficiencies or an underlying health condition. It may lead to weakened immune system function.
- BMI 18.5 to 24.9 – Healthy weight: this category is associated with the lowest health risk.
- BMI 25 to 29.9 – Overweight: being overweight can start to increase the risk of chronic health conditions such as hypertension (high blood pressure), type 2 diabetes and cardiovascular disease.
- BMI 30 to 34.9 – Obese Class I: this is associated with being at a higher risk of serious health conditions, including type 2 diabetes, cardiovascular disease and some types of cancer.
- BMI 35 to 39.9 – Obese Class II: this category signals a higher risk of the health conditions that relate to Class I obesity.
- BMI 40 or more – Obese Class III: being in this category brings the highest risk for health complications. It can lead to a significantly reduced life expectancy.
Interpreting BMI Results
In order to interpret a BMI result correctly, other factors such as a person’s muscle mass, age and gender should be considered.
As muscle is denser than fat, people with a high percentage of muscle mass may have a higher BMI that places them in the ‘overweight’ or ‘obese’ categories despite being healthy. Another factor is age. As people grow older, their muscle and bone density tend to decrease, which can lead to an underestimation of body fat when using BMI. Gender also needs to be considered. Women tend to have more body fat than men with the same BMI. Also, men and postmenopausal women are more likely to develop excess fat in the waist area, which carries a higher risk of chronic conditions.
Factors Influencing BMI
As we’ve seen, weight is a key part of the BMI calculation, and factors that influence a person’s weight will also affect their BMI. These include diet, exercise, genetics and lifestyle choices.
Eating a balanced diet that is rich in vegetables, fruits, whole grains, lean proteins and healthy fats can help maintain a healthy weight. High-calorie diets that contain too much sugar, saturated fats and processed foods can lead to an increase in BMI.
Regular exercise helps to burn calories and boost the metabolism, both of which contribute to a healthy BMI. The Australian Government recommends that adults engage in at least 150 minutes of moderate-intensity physical activity or 75 to 150 minutes of vigorous-intensity physical activity each week.
Some people may be predisposed to a higher or lower BMI by their genetic makeup. While a person’s genetic predisposition cannot be changed, understanding it can help in creating diet and exercise plans that will be effective in managing their BMI.
Things like sleep patterns, stress levels and alcohol consumption can also impact BMI. Getting enough high-quality sleep and practising stress management techniques are vital for maintaining hormonal balance, which affects appetite and weight. Limiting alcohol is also beneficial as alcoholic drinks can be high in calories and excessive consumption can disrupt sleep and affect a person’s judgment, which could lead to consumption of unhealthy foods.
Setting Realistic Health Goals
The Australian Government’s National Obesity Strategy 2022–2032 outlines a comprehensive approach to managing weight and health. A key part of this is setting achievable and personalised goals. Knowing your BMI can be a useful starting point for establishing these goals and then as a way of monitoring your progress towards them.
BMI does have its limitations, the main one being that it does not account for fat distribution. Carrying fat around your middle is associated with higher rates of type 2 diabetes and cardiovascular disease. Therefore, waist measurement is also recommended as it can help to predict the risk of chronic conditions like type 2 diabetes, heart disease and certain cancers. Regular blood tests are also beneficial in terms of predicting risk.
BMI should be used alongside other measures and within the context of a person’s overall health. Diet, exercise, mental health and lifestyle factors should also be considered.
You can read about setting health goals here.
Seeking Professional Guidance
When it comes to weight management, it is helpful to speak with healthcare professionals. They can provide tailored advice that considers a person’s medical history, dietary needs, physical capabilities and personal goals. This personalised approach ensures that weight change is effective and sustainable in the long term.
It is beneficial to seek professional help in these situations:
- If your BMI puts you in the ‘underweight’, ‘overweight’ or ‘obese’ categories, a healthcare professional can help you understand the associated health risks and develop a plan to reduce them.
- Before starting any weight management program (especially if it involves significant dietary changes or increased physical activity) seek professional advice to ensure the program is safe and suitable for you.
- If you have health complications or chronic conditions, professional guidance will help you to manage your weight and your existing health conditions.
- If you have been trying to lose weight without success, a healthcare professional can help identify any underlying issues and adjust your plan accordingly.
The Better Health Channel provides useful information on weight management services and providers.
BMI and Its Limitations
As we’ve seen, BMI does not take into account how much of a person’s weight is fat and how much is muscle.
This means that a standard BMI calculation is not accurate for:
- people under 18 years
- pregnant women
- older people
- people living with eating disorders
- athletes and other people who have a lot of muscle
- certain ethnic groups, including Aboriginal and/or Torres Strait Islander peoples and people of South Asian and Asian descent
This is why the Life! program also considers a person’s waist circumference when looking at BMI scores. Measuring waist circumference gives a good estimate of body fat, especially visceral fat (fat around your kidneys, liver, digestive organs and pancreas) and therefore your likelihood of developing weight-related diseases.
How the Life! program can help you
Life! is a free healthy lifestyle program that helps you improve your eating habits, increase your physical activity and manage stress. The program is for people who are at risk of developing type 2 diabetes, heart disease and stroke.
Life! is run by experienced health professionals, including dietitians and exercise physiologists, who guide and support you to make healthy lifestyle changes. The program includes 7 sessions delivered over a 12-month period and you can choose from a group course or our telephone health coaching service.
Check your eligibility for the Life! program by taking a quick online test here.
