Why BMI?

Why do doctors so religiously depend on BMI formulas and calculations? Why is it so important?

Why is it "bad" to have a bmi of less than 18 or slightly over 25? I understand there are "health risks" involved, but arent these applicable to the greater extremes of underweight such as a BMI <17 and overweight >27?

Can they let a person who looks fine at a slightly underweight/overweight mass remain the same?

Just something ive been wondering lately.

I'm not really too sure, but I think it has something to do with that even if a person looks fine, there could be things going wrong inside of their body.

It's measurement of risk....  There is a direct correlation between where someone appears on the BMI scale and their risk of developing certain medical conditions in the long-term.  A normal, healthy weight-range is BMI 20 - 25 which is about a 28lb spread, typically.   It doesn't mean that if you're 19.5 or 26 you instantly flip from being well into being unwell.... that would be silly.   However, if your BMI is continously over 30 (obese) you are significantly more at risk of cancer, for example.  If your BMI drops below 19 you are significantly more at risk of amenorrhea... that's a statistical fact.  Body-weight reflects general health....  The chronically overweight person is also likely to be sedentary (unhealthy) and probably eats too many fatty or sugary foods.  The chronically underweight person is likely to have malnutrition (unhealthy) from their poor/inadequate diet.  Both situations will harm the body over time.

Any responsible doctor has to recommend to a patient that they try to get their weight in the normal, healthy range as a routine preventative medical practice.  Same as they recommend someone should stop smoking or drinking heavily.  It won't kill the patient tomorrow or next week.... but it will harm their health long-term.

It seems like the medical community sees things as black or white.  That has been my experience time and time again.  Rather than listening and understanding a patient as a whole, it's easier to just shortcut and go by the numbers.  It's a shame, but I guess that is the way it works when $$ is the bottom line

Original Post by huggitbear:

 It's a shame, but I guess that is the way it works when $$ is the bottom line

 In the UK, where healthcare is universally free at the point of need, doctors also work with the BMI scale and will also advise patients outside the parameters to change their lifestyles accordingly.   It isn't affecting insurance premiums because no-one pays them.   Treatment isn't denied if someone is outside the normal parameters although the obese woman seeking fertility treatment, for example, will be asked to make an effort to get to a healthy weight first.... because it increases the chances of success.  However, patients don't have to listen to this advice, of course, and many don't.   

The BMI is a good tool, but it's not the tool to end them all. You can use it, and doctors do use it to evaluate quickly weither you're inside an healthy weight range, but used alone it's not actually enough to evaluate someone's health relative to their weight.

Fat percentage is also very important, because muscles weight more than fat and for two people that have the same BMI, you can  have one that has a much more substantial muscle mass and therefore, a fat percentage that is much lower than the average people for that BMI.  (that's my case - I was a competitive figure skater and I still have a very important muscle mass (plenty of fat around it now...), so I have a different body shape and fat percentage than the majority of people who share my BMI.

Blood tests can also evaluate health (ex: most people my BMI have very high cholesterol, I don't have high cholesterol at all, mine is perfectly average).

And then you have cardio vascular tests - I can go easily half an hour to an hour on a cross trainer/elliptical machine without being out of breath, despite the fact that my BMI puts me in the obese category.

Doctors use all of those to evaluate health - if your doctor only uses the BMI to give you advise about your weight, I really advise you to get a second opinion if you think there's nothing wrong with your weight/health.