Key Takeaways
- BMI is a quick health check, not a full diagnosis
- The World Health Organization sets standard BMI categories
- BMI doesn’t work well for everyone - those with excess muscle for example
- Alternative tools can provide a clearer picture than BMI if you want to delve farther
- BMI helps track health trends in populations
Body mass index, also known as BMI, is a ratio of a person's body mass to their height. According to the World Health Organization, it is one of the main resources for assessing nutritional status. However, it is not the most accurate way to determine whether a person is healthy or not. Let's understand how BMI works. Want to learn more about how to compute body mass index? Our health fitness professionals guide explains everything about fitness and nutrition.
How to Figure Out Body Mass Index
Your BMI is an indicator to determine whether your weight is "suited" to your height. It is calculated by dividing a person's weight in kilograms by the square of their height in meters. However, research has shown that BMI does not accurately reflect a person's overall health. Indeed, it is unreliable because it provides very little information about a person's nutritional status. The results are based only on total body weight, without taking into account the percentages of muscle, fat & body water.
Therefore, a person with a high body fat rate but short and thin could still fall within the normal weight range and have a "normal" BMI. However, that percentage of fat could be higher than recommended. Therefore, a healthy/normal BMI does not necessarily indicate good health.
🏥 Why Use the BMI Calculation
🧮 The BMI Formula
Here is the formula: weight (kg) / height (m)²
You should get a two-digit number, possibly with a decimal, if done correctly. Depending on the result, a person may be:
- Underweight: <18.5
- Normal weight: 18.5–24.9
- Overweight: 25–29.9
- Obese: >30
Take these results with a pinch of salt! It's an important statistic, but a generalized one too!
Understanding BMI Categories: What Do the Numbers Actually Mean?
🍃 Underweight: <18.5
You are sliding into the dangerous field of underweight, which could indicate a serious health condition or malnutrition.
✅ Normal weight: Between 18.5 - 25
Your body has the optimum weight for healthy living. You can sustain this weight by maintaining your current calorie intake.
⚠️ Overweight: Between 25 - 29.9
You would be considered overweight and have a possible risk of weight-related diseases.
🚨 Obese: >30
You would be considered obese and with a high risk of weight-related diseases. Before you engage in any fitness program, you should undergo a health assessment.
Find a good online personal trainer today!
BMI for Adults vs Children: Key Differences
BMI is a calculated measure of weight in relation to height. When BMI is used to assess a child's physical growth, because children and adolescents continue to grow, BMI values must be expressed in relation to other children of the same sex and age, which is called age and sex percentiles.
| Adults | Children and adolescents | |
| Calculation 📊 | Weight/height² | Weight/height² |
| Interpretation 📏 | Based on fixed ranges (underweight, normal, overweight, obese) | Based on age and sex percentiles |
| Factors considered ✅ | Does not consider age or sex | Considers age and sex because growth varies |
| Use 📈 | Rapid assessment of health risks in adults | Growth monitoring and early detection of weight problems |
| Limitations 🚫 | Does not distinguish between muscle and fat | Affected by rapid growth phases |
Limitations of BMI: What It Can't Tell You About Your Health?
Although BMI is a practical tool adopted by the WHO, using the body mass index calculation may not be appropriate for everyone.
Asian, African & Hispanic people may have a higher percentage of body fat or different patterns of fat distribution, which can result in lower BMI values compared to Europeans. Thus, it is not advisable to use BMI to assess the nutritional status in this population, as they may be at a greater risk of metabolic disorders, such as cardiovascular disease and diabetes, even if they have a low or normal BMI.
Another great example would be athletes. Athletes have much more muscle mass and less fat than the general population and could have a high BMI despite being high-performance athletes. Because muscle weighs more than fat, these individuals could have a very high BMI, which does not mean that they are obese or have a high percentage of body fat.
Another disadvantage of BMI is its inability to take sex & age into account. For example, a higher BMI may underestimate muscle mass in a younger athlete and overestimate it in an older person who may generally have less muscle and a higher percentage of fat.
Let's discover some limitations and why BMI may not be suitable for everyone:
- Large bone structure: Some people have a large bone structure, while others have a smaller or medium bone structure, which BMI cannot determine.
- High muscle mass: Not all people have the same muscle mass. A fit athlete will probably have a higher BMI than someone of the same height who is sedentary or does not exercise regularly.
- Fat percentage: Some short people may have a high body fat percentage and still fall within the normal or healthy BMI range.
- Individual's age: Although this measure is used for people of all ages, it could be argued that the average young adult has greater bone density and more muscle mass than older adults, even if they are the same weight and height. Age is also important to consider when assessing nutritional status.
- Fluids retention: Pregnant women and new mothers are likely to have an inexact BMI caused by fluid retention.
Alternative Measurements to Assess Your Health
For all these reasons, BMI is a parameter that may not be reliable in certain population groups. This is why many specialists today prefer to use the percentage of accumulated body fat to assess health status, which does increase the risk of metabolic diseases.
For example, visceral fat, which is stored around organs such as the liver, pancreas, and intestines, is more closely linked to cardiovascular disease than fat distributed in the lower extremities of the body.
It is also advisable to use other measures to assess health status, such as waist circumference to determine abdominal fat. Registered dietitians use specific instruments to measure skin folds (called skinfold measurements) at different points on the body, which allows them to assess the amount of subcutaneous fat.
Specialists also use other parameters such as cholesterol, triglyceride, blood glucose and blood pressure.
Nowadays, there are also body composition scales that work using electrical bioimpedance. These scales send a weak electrical current through the entire body. As it passes through different tissues, such as fat, muscle, and water, since electricity is conducted differently, this allows these scales to measure resistance and determine body composition. This is accurate for obtaining the % of body fat, muscle mass, water, visceral fat, and bone mass.
Other tools for assessing body weight and overall health include:
- Waist circumference
- Body fat percentage/Visceral fat
- Blood chemistry: Glucose levels, cholesterol, and triglyceride measurements
- Hematology panel
- Blood pressure
People will not change their eating habits overnight. Learn more about healthier food choices to stay healthy.
How Do You Figure Out Body Mass Index
You can determine your BMI by using the formula provided. However, due to all these factors, it is important to bear in mind that body mass index is a complementary indicator of health, but not a reliable parameter. That is why it is always advisable to contact specialists in the field if you are interested in assessing your nutritional status or undergoing a diet or exercise regimen, in order to:
1. Undergo a complete physical evaluation 🩺
This should include checking your weight, height, blood pressure, and heart rate, a general physical exam, and an assessment of your body composition. For better control, you should also have some blood tests done, such as a blood chemistry or hematology panel.
2. Find a certified personal trainer 🏋️♂️
A certified trainer will help you determine which types of training are most beneficial for you, based on your level of fitness, fitness goals, and availability. If you don't practice exercise regularly, start with light activities, such as stretching and aerobic exercises. Nothing that could cause musculoskeletal damage or overwork your heart and lungs. Once you have achieved a good level of physical fitness, the design of your program can go in several different directions:
- Endurance training: Using weights, bands, and even gravity to build muscle endurance.
- Strength training: To gain muscle mass.
3. Improve your eating habits with a professional 🍎
Nutrition professionals such as registered dietitians in Canada are the people to turn to for a proper eating plan. These experts will teach you which foods are recommended and in what quantities, based on your health and metabolism rate, to achieve your nutritional goals without putting your health at risk. For example, if you are looking to lose body fat, a registered dietitian may suggest that you reduce your calorie intake and choose healthy foods such as vegetables & fruits.
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