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Understanding BMI

Jun 3
2 min read

BMI, also known as Body Mass Index, is one of the most commonly used health measurements, but also quite misunderstood. Here is what you need to know about it.


What is BMI?


BMI is a simple calculation using your height and weight.


BMI = weight (kg) ÷ height (m)2 | Or in US units: BMI = (weight in lbs * 703) ÷ (height in inches)2


As you can see, when just your weight increases, your BMI increases, but when just your height increases, your BMI decreases.


The result of this places you in one of these 4 categories:


  • A BMI under 18.5 is underweight

  • A BMI of 18.5-24.9 is normal weight

  • A BMI of 25-29.9 is overweight

  • A BMI of 30+ is obese


Why do doctors use BMI?


BMI is popular because it’s fast, free, and non-invasive. It gives a rough estimate of whether someone’s weight may be affecting their health. At a population level, higher BMIs are statistically linked to higher rates than chronic disease.


What BMI Does NOT Tell You


BMI is a screening tool, not a diagnosis. Here are some limitations it has:

  • It does not distinguish muscle from fat. A muscular athlete may have a high BMI while having very low body fat.

  • It does not show where fat is stored. Abdominal fat, also called visceral fat, is more dangerous than fat stored elsewhere, and BMI cannot detect this.

  • It does not account for age. Older adults naturally lose muscle, so the same BMI may mean different things at different ages.

  • It does not account for sex. Women naturally carry more body fat than men at the same BMI.

  • It may be less accurate for certain ethnic groups. Some groups face higher health risks at lower BMI thresholds.


Alternative Measures of Health


Alongside BMI, healthcare providers may use:


  • Waist circumference: Abdominal fat is a stronger predictor of heart disease and diabetes risk

  • Waist to hip ratio: Another measure of fat distribution

  • Body  fat percentage: This is more accurate, but requires special equipment

  • Blood tests: Cholesterol, blood sugar, and inflammatory markers are more informative of one’s health than solely BMI


BMI can be a useful starting point for learning about your health, but it shouldn’t be the only factor you consider. It is important to focus on healthy habits, including good nutrition, regular movement, and quality sleep rather than just fixating on a number.

 
 
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