Helpful or Stigmatizing? 5 Things Everyone Should Know About BMI

By Madeleine Ortiz

BMI, or body mass index was invented nearly 200 years ago, and although it can still be a valuable tool when it comes to an initial diagnosis of obesity, it’s not without flaws. Here are some things to consider when it comes to BMI and your health:

1. BMI is not a measure of body fat.

BMI can only give you a ratio of weight to height – but it can’t tell you how much of that ratio is composed of body fat. And when it comes to health risks, percentage of body fat tends to be far more telling than a number on the scale. Some better tests for body fat would be like hydrostatic weighing or skinfold calipers. If your doctor classifies you as overweight or obese based on your BMI, getting further information about body fat via one of these tests may be more helpful in tracking health improvements long term. 

2. It doesn’t predict cardiovascular risk.

According to the Canadian Obesity Guidelines, multiple reports have documented a subgroup of “metabolically healthy” patients with obesity, characterized by the absence of any objective evidence of increased cardiometabolic risk despite having an elevated BMI. In simpler terms, just because your BMI places you in an overweight or obese category does not mean you are automatically at risk for cardiovascular disease or heart attack. If you fall into an overweight or obese category but eat well, exercise and feel healthy – ask your doctor for additional screeners (such as blood pressure measurements and blood tests for cholesterol / triglycerides) which may be better indicators of risks and overall health.

3. It doesn’t take fat distribution into consideration.

Excess adipose tissue, especially around the abdominal region, is a key indicator for diagnosing obesity, but BMI does not take fat distribution into account. In other words, though it might give a general idea of a “healthy” weight to height ratio – it can’t indicate whether or not the weight on a person’s body is dangerously concentrated around the waist or more evenly distributed, which usually indicates it’s more metabolically healthy. This is why it’s important for healthcare providers to use BMI in combination with body measurements which can give patients and providers a better idea of fat distribution and associated risks.

4. It can’t take muscle mass into account.

The benefits of muscle can feel virtually endless. Increased muscle mass can help increase metabolism, improve bone density and even help maintain weight loss, among other benefits. Muscle tissue is also more dense than fat tissue – meaning it takes up less space per kg. People with large amounts of muscle mass often weigh more than those with average or less than average amounts of muscle mass. This extra muscle can sometimes skew BMI and make it higher than what might be considered “healthy.” If your doctor wants to diagnose you as having obesity or excess weight and you feel like an above average muscle mass might be to blame, ask for a test that can specifically measure your body composition. This can help alleviate any confusion.

5. It doesn’t distinguish among men, women, and ethnicity.

No doctor would ever deny the biological differences between men and women, but neither the BMI calculations nor the categories that classify someone as “overweight” or “having obesity,” distinguish between genders, ethnicities or special populations. This sometimes leads to women being diagnosed with obesity more often than men or specific populations of people being under-diagnosed. And if you are someone who falls into one of Obesity Canada’s “special populations” categories- elderly, very muscular, extremely tall or short stature – you’ll want to be especially cautious of taking BMI at face value. Be sure to ask your doctor for the additional screenings and tests mentioned above.

The Canadian Obesity Guidelines make it clear that despite the drawbacks of BMI, its simplicity, objectivity, and reproducibility make it an efficient and effective screening measure which can help identify individuals who may benefit from obesity management. It should never be the only way a diagnosis is established, but instead used as a piece of the larger picture. And for those already diagnosed with overweight or obesity, lowering BMI should not be the focus of your weight journey. Instead, you and your healthcare provider’s goals should be finding the root cause of the weight gain, identifying barriers to success and establishing healthy lifestyle changes and an individualized treatment plan.