Is it really all that bad to be slighly underweight?

paperpudding wrote: »

Let's be honest here: you really don't have to study medicine to see that the younger Arnold Schwarzenegger was not obese. Anyone who insists he was, just of because of the BMI is an idiot. Also, how many of those athletes do we really have? I don't see very many of them when I go out.

In addition, why not also look at the other side: an obese person who had her/his legs amputated and who, as a consequence, is now in the normal range despite being obese?

Furthermore, on the level of an individual patient, BMI is needlessly complicated. Just look at a weight chart. Unless something really tragic happens to you, your height will stay the same for most of your adult life. Calculating the BMI is a waste of time and energy.

Seriously???

of course elite body builders and amputees are outliers - nobody here is disputing that at all.

and just looking at a weight chart without height is meaningless - obviously what is a healthy weight range for somebody 6 ft 10 in is not the same as somebody 4 ft 10 in even though both are within normal adult height range.

I think you either misunderstood what I wrote, or I haven't been clear about "an individual patient". Individual patients do have heights, so of course it is part of the total equation.

It is just that, barring extraordinary (and usually tragic) circumstances, your height will not change. So, once you have determined it, you no longer need to calculate your BMI, just look at the weight scale. There is no need to calculate the BMI. It is a needless complication that brings zero additional information to the equation.

This is not the same as, for example, your weight (obviously) or your blood pressure or your GFR or most any other medically important marker and make no mistake, even those have to be interpreted in very judicious ways and it is quite possible to be diagnosed with a certain disease despite the fact that the generally accepted markers for that disease are perfectly normal or even excellent.

Medicine is not something that can be learned from the back of a package of popcorn. There are reasons why medical studies are gruelling and take years and why even then mistakes and misinterpretations are common and there are reasons why the professional medical literature has numberless controversies about what the public considers completely uncontroversial and settled. Modern medicine, both the evidence-based and the science-based versions, is an astonishingly new field. Even the terms themselves did not exist yet when I was in med school.

As for nobody disputing the outliers, you forget that BMI is often attacked publicly and even in medical circles precisely for that reason. It is the standard criticism trotted out to disparage BMI. It is important to take that into account. And again, BMI is a population-level measurement. On the level of the population, averages and other techniques like BMI are historically important, because of the massive amounts of data that have to be taken into account. That is slowly changing, thanks to ever larger available space for data storage, but we haven't quite reached the stage yet where we could be looking at real-time massive data sets without giving it a second thought.

In short, BMI is still very useful for people in insurance, or public health and similar fields, but it is a needless complication on an individual level and while I haven't seen any explanation from MFP, I suspect that is at least part of the reason they decided to retire their BMI calculator. If am wrong about that, I am sure (or at least hope) someone from MFP will set the record straight.

paperpudding wrote: »

It is just that, barring extraordinary (and usually tragic) circumstances, your height will not change. So, once you have determined it, you no longer need to calculate your BMI, just look at the weight scale. There is no need to calculate the BMI. It is a needless complication that brings zero additional information to the equation.

snip

Medicine is not something that can be learned from the back of a package of popcorn. There are reasons why medical studies are gruelling and take years and why even then mistakes and misinterpretations are common and there are reasons why the professional medical literature has numberless controversies about what the public considers completely uncontroversial and settled. Modern medicine, both the evidence-based and the science-based versions, is an astonishingly new field. Even the terms themselves did not exist yet when I was in med school.

I disagree.

Nobody is claiming your height will change - but of course you still need your height to calculate BMI. You dont have to re measure it but you do need it in the equation.

and many basic things are basic and havent changed - including BMI range. One doesnt need a medical degree nor is the basic concept of healthy weight range an astonishingly new field.

MFP may have cut out its calculator, who knows why - but BMI calculations certainly havent been cut from websites such as Heart Foundation nor has it stopped being used in places like GP surgeries.

You can disagree. That is your right. The point is that those websites are working on a population level, not on the level of an individual patient. Even IF you use BMI for yourself, which any statistician will tell you is a questionable practice, it does not mean you have to recalculate it every time. Using a 1.71 m person as an example, the "healthy" BMI range is between 18.5 and 25. That is not equivalent, but identical to a weight between 54.1 kg and 73.1 kg as given by the MFP calculator (and any other).

These are the two procedures you can follow:
1. You stand on the scale, you notice you weigh 67.2 kg.
1a. You use any BMI calculator (MFP = 23.0), or you calculate it yourself: 67.2/(1.71^2) = 22.98
1b. You look it up and see that this is over 18.5 and under 25.0.
1c. You happily conclude that you have a healthy weight.
OR
2. You stand on the scale, you notice you weight 67.2 kg.
2a. You look it up and see that this is over 54.1 kg and under 73.1 kg.
2b. You happily conclude that you have a healthy weight.

Question: what is the difference?
Answer: the amount of work you have to do. There is NO difference between the two. Calculating your BMI is an extra step that you do not need.

On top of that, as said, any statistician will tell you that BMI is a tool for people who study populations. It is all but useless to individuals. It is, sadly, true that most people do not know this, for no other reason than that they don't learn it at school. It is why, and THIS IS AN OPINION: it is something that people should learn at school. There is a big difference between the value of markers at a population level and the value of those same markers at an individual level.

Medicine at a population level is essentially a branch of statistics because it is impossible for a researcher or a policy maker to look at every individual case, so they have to simplify. Medicine at an individual level is still very much of an art. It is why doctors do gruelling studies that take years, and then still manage to disagree.

That is why people are always being encouraged to talk to a doctor, a real one i.e. an MD, before making any health-related decisions. This is a complicated field, and people can get killed or (arguably even worse) harmed for a very long time when they make the wrong one. A doctor should always be your primary source of information.

AnnPT77 wrote: »

I understand from your posts here that you went through medical education a few decades back. I'm thinking you may not have IT experience?

That is a reasonable assumption, but it is wrong. I actually have over 40 years of IT experience, as a programmer, and if you look on the Internet, you will discover I have written a number of published books on the subject (that's a quite a while ago).

As for screening tools, they are ways to deal with populations, not individuals. It is the type of thing that is used, for example, to determine whether or not people potentially qualify for drug trials. If they don't pass the screening, they are not considered. If they do, they are interviewed and if they pass that stage, they are invited for a medical examination. Only after that, are they possibly included.

Screening is a way to make an expensive and/or difficult process simpler. Nothing more.

For the rest, unfortunately, it *is* necessary to repeat certain things ad nauseam. One of my favourite examples of that is homœopathy. Even though it was debunked almost as soon as it was created, in the late 1700s, We *still* have to instruct people on why this is nonsensical quackery and there are *still* people dying because they believe in it anyway.

Something similar is the case with BMI. It was created by a 19th century Belgian statistician. Yet, so many people, especially in the English-speaking world, are still applying it in the wrong way. That is what lack of science education does to people.