WHAT IS BMI?
Technically BMI is defined as weight in kilograms divided by height in meters squared or inches and feet for Americans. It's a fairly old measurement that has been used to indicate general health or »ideal weight«.
HIGH VS LOW BMI
A high BMI is considered a BMI greater than 25 kg/m2, which is defined as overweight, and + 30 kg/m2 is considered obese). Those tend to correlate with health risk.
On the other hand, very low values are claimed to be equally problematic. For example a value:
Below 18.5 kg/m2 is considered unhealthy or malnourished (possibly indicating an eating disorder or wasting disease).
Between 18.5 - 25 kg/m2 is considered optimal.
However, it's important to note that these are only averages. It has been established that individuals with a high BMI can also be healthy while those in the optimal range may be unhealthy.
IS BMI A DECENT TOOL FOR ASSESSING HOW "FAT" YOU ARE?
Two females who are 175 cm tall and who weigh 62 kg have the same BMI.
One is inactive with Body Fat (BF) at around let's say 30 %, and the other one is an athlete with 16 % BF.
Where's the catch?
Their body composition is different. Part of the reason for this is that BMI is not exactly speaking a measurement of BF % and it doesn't indicate body composition or how much fat or Lean Body Mass (LBM) someone is carrying. For example, bones can be highly variable even within the »general« population.
The same can be applied for active individuals, generally males.
Their BMI can be scored high (indicating they are overweight), although they are relatively lean.
Again, body composition. They simply carry more muscle mass.
"It works fine for general population."
That's the answer you will usually get by your doctor. Well, does it??
As you can see BMI is very simplified. Too simplified?
Let's look at the study from Mayo Clinic: (1)
They assessed the diagnostic performance of BMI using the World Health Organization (WHO) reference standard for obesity of BF % > 25 % in men and > 35 % in women. They tested the correlation between BMI and both BF% and lean mass by sex and age groups adjusted for race.
What they found out is that only 36 % of obese men actually had a BMI of over 30. The majority of people carrying around unhealthy amounts of fat actually had 'normal' or 'overweight' BMIs.
Moreover, there's another study with similar results.
They found out that people with a BMI of less than 30, actually correlates better to lean body mass than to body fat percentage.(2)
Well, is not that uncommon to find people with high BMI who also have a low BF % (for example athletes). Although people with a low BMI can often have a fairly high BF% (it's called skinny fat). It's also possible to have a high BMI and be metabolically healthy or a low BMI and be unhealthy.
So we can somehow conclude that BMI doesn't work so well for the »general« population either and that weight training is probably doing more for you than all that running is doing for a marathoner.
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1. Romero-Corral, A., Somers, V. K., Sierra-Johnson, J., Thomas, R. J., Collazo-Clavell, M. L., Korinek, J., . . . Lopez-Jimenez, F. (2008, June). Accuracy of body mass index in diagnosing obesity in the adult general population. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/18283284
2. Kennedy, A. P., Shea, J. L., & Sun, G. (2009, November). Comparison of the classification of obesity by BMI vs. dual-energy X-ray absorptiometry in the Newfoundland population. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/19360011