The FastDay Forum

Not strictly scientific, I suppose, but I've been musing about the rate of decrease of my waist measurement! Now it's been a long time since I've done any geometry, so please feel free to correct me, but I have been thinking that those of you who have lower BMIs and are actually both shorter & smaller than me, seem to be able to decrease your waist measurements relatively quickly, whereas mine (after I was brave enough to measure it nearly 2 months after starting 5/2), seems to decrease only very slowly. So I'm pondering the maths of this - if I assume for simplicity that I'm somewhat cylinder-shaped, and that weight (fat, hopefully) loss will be directly proportional to volume loss, then what I'm actually measuring is only circumference (2 x pi x r) (stupid iPhone has no pi symbol - how rude!), whereas what I'm losing is volume of the cylinder, pi x r squared x length). So assuming that I'm losing uniformly, then kg for kg, my waist will be decreasing much more slowly than someone who has a smaller waist to start with, but also, as I lose more weight, my waist will decrease faster per kg lost than it does at the moment.

Can anyone see any holes in my argument, or provide more maths or more detail? ****(sorry Moogie, can I say that?) I don't know whether this is of any interest to anyone except me, or whether I'll ever get to the magical "80 cm waist for women" promoted in the Australian recommendations. Oh well!

I haven't a clue jools, but I just wanted to let you know your post made me smile

Looks correct to me, you could try some numbers in a spreadsheet to convince yourself.
I guess BMI works because the height squared component balances the radius squared effect of increasing mass...

And now I'm wishing I'd thought to call this thread "contemplating my navel"

Oh thanks, @CreakyPete, I had never thought of that link before! I thought it was just a formula someone pulled out of somewhere ( sorry, trying to be politer this time).

Just looked at your tracker and it is a slippery slope shape to me so well done you, wouldn't have a clue on what you are talking about but you are on the right trajectory IMHO @jools7

I think I can help a bit @jools7!
About 10 days I said to my doctor something like 'when we look at the diabetes pamphlet it says I have to be under 80 cms to be healthy but on this forum carorees from the latest scientific data recommends half of your height. So for me at 170 cms my ideal waist should be 85cms or under. My wonderful doctor said white Australian women should aim for 88_cm. The 80 cm is for native Australian people who were mostly quite slim before white people turned up. So that makes sense for me as how can they have one measurement for people of all heights. 50% of each height makes much more sense to me. So I am aiming for 85_cm. 7 cms to go!!!
Xxx julianna

Am I just having a wank?

...............................

Em, you have either had a visit from the Predictive Text Fairy or it means something altogether different in antipodean speak..........mmmmm

Ballerina x

[Debs]

No ballerina, means exactly the same in Aussie lingo!!
And I had no idea what you're on about, but very admiring of your attempts to find out!! And I used and at the start of the sentence again
I had a giggle too!

Oh, apologies@Ballerina for causing cross-cultural offence! I'd like to blame a fairy, but in truth, I was just speaking plainly, blame my Aussie bluntness. I suspect it means the same in British and Australian culture, but just more acceptable in Australia, perhaps? Sorry!

Oh dear, goodness knows what will happen when the Americans turn up - time for an edit, methinks!

[Debs]

Mass confusion will occur!!!! But be very funny in the process!

Hmmm, just thinking again about the relationship between height & volume, wouldn't it be better to be something cubed, not squared then, CreakyPete? As in radius squared times length? Isn't it a bit bizarre that we look on these numbers as gospel, when their origin sometimes seems somewhat arbitrary? I did hear somewhere recently, I don't remember which way around it was, that BMI isn't terribly good at predicting risk for those of us who are at either end of the height nomogram. I should look it up I guess, but now that Federer has beaten Andy Murray (sorry, CreakyPete), I'm off to bed! Goodnight!

No ballerina, means exactly the same in Aussie lingo!!
And I had no idea what you're on about, but very admiring of your attempts to find out!! And I used and at the start of the sentence again
I had a giggle too!

@Debs

And you forgot to say 'And' I had a giggle too!

Ballerina x

[carorees]

Hmmm, just thinking again about the relationship between height & volume, wouldn't it be better to be something cubed, not squared then, @CreakyPete? As in radius squared times length? Isn't it a bit bizarre that we look on these numbers as gospel, when their origin sometimes seems somewhat arbitrary? I did hear somewhere recently, I don't remember which way around it was, that BMI isn't terribly good at predicting risk for those of us who are at either end of the height nomogram.

Hi @jools7
I did find a paper where they looked at using height cubed:waist cubed to predict obesity but they found it was only a bit better than BMI and standard anthropometric measures, and then only in women. http://www.ncbi.nlm.nih.gov/pubmed/16768846

Also, there's this paper: http://www.plosone.org/article/info%3Ad ... ne.0039504 which attempted to integrate height, weight and waist circumference into a 'body shape index' (here's a summary and comment: http://blogs.plos.org/obesitypanacea/20 ... ape-index/) which may give a better indication of mortality risk. The authors conclude:

Body shape, as measured by ABSI, appears to be a substantial risk factor for premature mortality in the general population derivable from basic clinical measurements. ABSI expresses the excess risk from high WC in a convenient form that is complementary to BMI and to other known risk factors

Here's a calculator for your body shape index if you want to try it: http://www.absi-calculator.com/