How is your BMI?

It surprises me how the BMI is still misinterpreted, possibly leading to suboptimal interventions.

The big issue is that the BMI creates lots of false-negatives. Many obese people are misclassified.

For example, a 2008 study published on the International Journal of Obesity (PMID: 18283284), assessed the SENSIBILITY and SPECIFICITY of the BMI in 13K people, after comparing with body fat percentage values obtained through Bioelectrical Impedance Analysis-BIA (a more precise form of assessing body fat).

Sensitivity is the capacity of a test to avoid false-negatives. In this case, to correctly identify obesity.

Specificity is the capacity of the test to avoid false-positives, or to correctly classify individuals who do not have the disease.

The results were shocking! When the BMI was utilized, 21% of men and 31% of women were classified as obese. But when the researchers assessed people’s body fat percentages through bioelectrical impedance (a more accurate method), the results showed that in fact 50% of men and 62% of women were obese.

So the sensibility of the BMI is very low to prevent false-negatives, equivocally misclassifying more than half of those people. The poor performance of the BMI happened mainly with the individuals on the 25-30 BMI range.

This happens due to the inability of the BMI to differentiate lean mass (everything that's not fat: bones, muscles, blood, skin, etc) and fat mass.

So you may think you have a healthy body composition but in fact have an elevated body fat percentage, which obviously is bad, since it can lead to many cardiovascular and metabolic diseases. This is exactly what the term skinny-fat refers to.

Nevertheless, the BMI is still a great tool to identify true-positives for people with values above 30. If your BMI is higher than 30, you definitely need to lose weight. Plain and simple. Unless you have a ton of muscles, like a bodybuilder, in this case you’re an exception to the rule.

So think of the BMI as a starting point. If higher than 30, you need to lose weight. If lower than that, it doesn't mean that you don’t. A more precise assessment is recommended, like skinfolds, BIA , DEXA, hydrostatic weighing, etc.

With that, you'll have more detailed information to determine the next steps of your training and nutrition program, getting more ownership over your health.

Willian Alba