Call for blog post volunteer: Why BMI is unreliable

Question

I noticed a comment today by BackInShapeBuddy that body mass index (BMI) is not a reliable measurement of someone's physical fitness level. I've been seeing comments like that here and there that all the same thing. To me, this sounds exactly like one of the physical fitness topics that is perfect for the blog.

There is one question about conducting a BMI test and its answers do delve into bits about why BMI is unreliable. I believe this is something that we can further explain on the blog and spread better information about how to go about determining your level of physical fitness.

Answer 1

I don't have access to what Dave has already put together. I'll just add my 2 cents that I think a distinction should be made between:

• BMI as a screening tool for health risks associated with being overweight: diabetes, heart disease, cancer, metabolic syndrome, sleep apnea and arthritis; vs.

• BMI as an unreliable or insufficient assement of "physical fitness": strength, endurance, cardio-vascular endurance, power, agility, flexibility, body composition, bodyfat percentage etc.

  The screening tool for health risks is where its use is valuable, the physical fitness assessment is where it falls short imo.

Answer 2

As with all models, there are always limitations. Whether you can live with the limitations depend on your overall goals.

The advantage of BMI is its simplicity and it is probably relevant for a large part of any population to indicate over/under weight. The disadvantage is that it might give the wrong signal to someone that is very muscular and short (you will get a large nominator and a small denominator -> a high BMI for a person that probably isn't fat and in the danger of getting diabetes).

However, as an indicator for over/under weight it seems to be ok from a physicians point of view (based on best practice among Swedish general physicians). My father - a retired GP - used to say that almost all medications could be avoided if you first checked the BMI and asked the patient to lose enough weight to get below BMI 25.

Also, if the argument would be that it is unreliable then a substitute/complement (e.g. hip to waist, shoulders to waist measurement) should be included in the blog post. Only arguing that an model that was a simplification to start with is unreliable for all cases is a bit like kicking in wide open doors.

Answer 3

How do you define unreliable? BMI suffers from the same type I/type II errors as any other measure. For more information see this Examine article on BMI. BMI has a Low sensitivity and high specificity with regards to testing for body fat.

The short of it is if your BMI indicates you are obese, you are most likely obese, however an indication that you are in the "normal range" doesn't mean you aren't also obese. Obviously, there are outliers, such as very professional athletes, but if you know you are already quite fit and 'obese', then you aren't going to be testing your health or body fat using BMI, you've probably already done a few DEXA scans, numerous cardio stress tests and are in a camp somewhere.

For beginners, BMI is a fine metric (even at an individual level) and saying otherwise is a reckless misuse of statistics.

I'm just going to straight up quote the conclusion from the Examine article:

If you are normal weight or overweight according to BMI (18.5-29.9) there is still a chance you are actually obese, and thus is primarily due to low levels of lean mass (muscle, water, and glycogen).

If you are obese according to BMI, you are most likely obese according to body fat percentage as well. When sampling from the general population, over 95% of men and 99% of women identified as obese by BMI were obese via body fat levels.

Outliers to this dataset, those who have enough lean mass to be classified as obese by BMI but not by body fat percentage, are far and few in society. These persons would normally be highly active athletes or dedicated 'weekend warriors', and it is unlikely sedentary persons or those with infrequent exercise habits would be these outliers.