Are BMI calculations any good?

**Stuart** · 31-01-2010, 17:55

This article set me thinking. We often hear from "health experts" that our BMI should be such and such level, but is the BMI level an accurate guide to obesity?

If it's calculated as in the about article (height / weight to the power of 2), then it actually discriminates against muscley people (muscle being denser and therefore heavier than fat). As the article says, a 6 ft pro baseball player who weighs 16 stone is certainly not unfit, where as someone who is 6 ft, weighs 16 stone and does no exercise would have the same BMI (both would be classed as obese) and would certainly not be fit.

I remember a few years ago, I consulted my GP about my weight. He told me that the charts and BMI calculations say I should be around 12-14 stone. He said that due to my large build, I would almost certainly be ill if I went down to 12 stone. He estimated I should be 15 stone based on my build.

I'll admit, I *am* overweight (currently 16.5st), but I do wonder at the sense of obsessing about BMI, and I wonder how our obsessing with it will affect future generations. One way is women who lose too much weight tend to become infertile.

Taf · 31-01-2010, 18:10

BMI seems to be ignored by our GP... he's just hot on the "37 inch or more waist and you're going to die" thing.

6ft3 and 95kg... I look like a pregnant goalpost... but only 36 inch waist so I'm "healthy".

Ignitionnet · 31-01-2010, 18:29

BMI is pointless as it takes no account of things like build, lean muscle mass, etc.

Body composition, fat, lean muscle, bone is a far better guide.

I'm not fat, but I'm 5'9" and 14st. My BMI is considerably over the norm, though I'd like to think my regular gym visits account for the extra tonnage, but according to BMI I'm obese.

martyh · 31-01-2010, 18:31

i don't worry about it, personaly i have much more important things to worry about ,haven't been to my doctor in about 7-8 years i eat what i want ,my job keeps me active and fairly fit so i have no interest in health fads

Smilie · 31-01-2010, 18:34

I was just reading a article about BMI aswell (3 year old Focus magazine article)
the writer pass every other examinations (blood pressure, body fat, etc) but his BMI was alittle high and he was saying theres reports saying life expectancy was highest if your BMI was around 26 (cut off is 25)

altis · 31-01-2010, 20:03

Quote:

Originally Posted by Stuart C

...If it's calculated as in the about article (height / weight to the power of 2)...

Actually, it's

mass / height ^ 2

http://en.wikipedia.org/wiki/Body_mass_index

I don't think the value is given a great deal of 'weight' but it does give a rough and ready estimate for body shape. Percentage body fat is a better measure but requires more instrumentation.

http://en.wikipedia.org/wiki/Body_fat_percentage

LondonRoad · 31-01-2010, 20:05

I've always been a sceptic where BMI is concerned. It doesn't take into account other, probably more important, factors such as natural frame/build and the percentage of body fat.

A mate of mine is the same height as me but for him to be within acceptable BMI levels, you'd have to sook all the fluid out of him

. He's at least 2 stone heavier than me but carries only 15% body fat.

At my annual medical in November my body fat was 22% but I was still OK on my BMI.

Gary L · 31-01-2010, 23:52

Quote:

Originally Posted by Stuart C

then it actually discriminates against muscley people (muscle being denser and therefore heavier than fat). As the article says, a 6 ft pro baseball player who weighs 16 stone is certainly not unfit, where as someone who is 6 ft, weighs 16 stone and does no exercise would have the same BMI (both would be classed as obese) and would certainly not be fit.

I know someone who is 17 stone but it's all muscle. he had a letter from his GP saying about doing weight watchers or Rosemary whoever. he was quite annoyed at the time.

they just go by your age/height/weight and assume you're overweight, obese.

rogerdraig · 01-02-2010, 00:53

its a tool to use not to be taken as a absolute measurement of perfection

bmi
weight
hight
lifestyle

all need to be taken into account

beeman · 01-02-2010, 12:06

Theres 2 methods of calculating BMI, The basic one is simply an equasion baised on weight and hight this method uses the "average" body shape to get the calculation, for the majority of people it would bee resonably accurate, but people with particlarlly high (or low) muscle mass will get an incorrect result.

The second method uses wight and hight but ALSO intruduces a small electric currect into your right hand (witch exits the left hand) which can determin mussle and bone mass to add into the equation.

The first method is simply a method that you can do in your own home to come up with a rough idea But doctors/hospitals/gyms etc should use the electrode method to give a true(er) result.

altis · 01-02-2010, 12:15

Quote:

Originally Posted by beeman

Theres 2 methods of calculating BMI,...

Erm, no there aren't!

BMI is mass / height ^ 2

The other thing you are on about is "percentage body fat". See my post above.

beeman · 01-02-2010, 12:43

Quote:

Originally Posted by altis

The other thing you are on about is "percentage body fat". See my post above.

Yes i knew it had a different name i just coulnt remember what that was, but it is still essentally the same thing (the the level of fat in your body compared to the level of non-fat mass)

**Stuart** · 01-02-2010, 13:39

Quote:

Originally Posted by beeman

Yes i knew it had a different name i just coulnt remember what that was, but it is still essentally the same thing (the the level of fat in your body compared to the level of non-fat mass)

The thing is (and actually my point) that BMI does not necessarily represent the percentage of fat in your body. It uses your body's mass and height, but ignores what that mass is made up of. If your body mass is primarily muscle, you are likely to be fit (although excessive muscle building can bring it's own problems), if your body mas is primarily fat, you are likely to be unfit.