Does the data show ‘epidemic’ obesity risks?

It is sur­pris­ing­ly dif­fi­cult to find an answer to these ques­tions in the ABS data. The Bureau pub­lish­es only age-strat­i­fied preva­lence data from its sam­ples; not basic sta­tis­tics such as the mean and vari­ance of the adult pop­u­la­tion BMI. I can’t tell from their pub­li­ca­tion how the BMI isdis­trib­uted across the whole pop­u­la­tion or how the ‘cen­tral ten­den­cy’ of that dis­tri­b­u­tion changed between the mea­sured sam­pling in 1995 and 2008. This is cru­cial infor­ma­tion, how­ev­er, because it is like­ly to show what is actu­al­ly going on in the trend and whether the appar­ent increas­es in the pop­u­la­tion indices are due main­ly (as I sus­pect) to the greater vari­ance in the sam­ple or due to an alarm­ing ‘gross­ing-up’ of the aver­age Aus­tralian (as some inter­est­ed groups want us to believe).

Peo­ple are liv­ing longer and BMI tends to increase with age, as the ABS preva­lence data shows. So it’s quite pos­si­ble that a larg­er pro­por­tion of the total pop­u­la­tion has a high­er BMI just because there is a big­ger ‘spread’ (vari­ance) in the fre­quen­cy dis­tri­b­u­tion curve. If that were so, the ‘mean’ (cen­tral ten­den­cy) of the dis­tri­b­u­tion may not have changed to the extent that the increase in the pro­por­tion of ‘over­weight and obese’ in the sam­ple seems to indi­cate. In oth­er words, not so much ‘fat­ter on aver­age’ as ‘fat­ter at the extremes’.

So let’s accept, for the moment, the pre­miss that there has been an explo­sion of fat. What can we do to check whether we should be alarmed? The obvi­ous thing is to check whether the ‘risks’ that are said to be increas­ing are vis­i­ble in the chang­ing inci­dence of mor­tal­i­ty. That is, has there been an alarm­ing increase in actu­al deaths from the dis­eases that over­weight and obe­si­ty are said to cause?

It turns out that there’s no appar­ent cor­re­la­tion between the claimed trend in the obe­si­ty risk-fac­tor and actu­al deaths from obe­si­ty-asso­ci­at­ed dis­eases. Here’s the rel­e­vant infor­ma­tion on mor­tal­i­ty trends from the Aus­tralian Insti­tute of Health and Welfare’s Australia’s Health 2008 pub­li­ca­tion.

“Australia’s mor­tal­i­ty from cere­brovas­cu­lar dis­ease has been declin­ing in recent decades… since the mid-1970s, con­sis­tent declines have been not­ed for both males and females. Age-stan­dard­ised death rates for cere­brovas­cu­lar dis­ease fell by 37% (males) and 35% (females) over the peri­od 1996–2005.”
“There have not been major changes in the death rate from dia­betes (as an under­ly­ing cause) over the last 25 years… For males, the death rate rose by an aver­age of 0.7% per year. In con­trast, the rate for females fell by an aver­age of 0.5% per year.”
“From 1997 to 2005, the mor­tal­i­ty rate for CKD [chron­ic kid­ney dis­ease] as the under­ly­ing cause of death fell from 13 to 11 deaths per 100,000 pop­u­la­tion.”

This absence of cor­re­la­tion between the obe­si­ty-trend and the mor­tal­i­ty-trend in Aus­tralian now looks less mys­te­ri­ous, thanks to a unique and detailed Unit­ed States study pub­lished in Feb­ru­ary by the senior sta­tis­ti­cians of the Unit­ed States Cen­ters for Dis­ease Con­trol and Pre­ven­tion. Using a very large sam­ple of the U.S. pop­u­la­tion, and tak­ing BMI and death data since 1988 into account, Fle­gal and Graubard found BMI is not sig­nif­i­cant­ly cor­re­lat­ed with excess deaths from all-caus­es, from ‘obe­si­ty-relat­ed’ caus­es of death or from oth­er caus­es. Looks like the same might be true in Aus­tralia.

But what about the dis­ease bur­den? Excess mor­tal­i­ty is the only final arbiter of the degree of risk, but the bur­den of dis­ease on indi­vid­u­als and the com­mu­ni­ty (indi­vid­u­als, tax­pay­ers, car­ers) is also impor­tant. What’s going on there?

As far as I can tell from the semi-obscure way that ABS presents the data, the sto­ry for the dis­ease bur­den is con­sis­tent with the much-less-than-alarm­ing mor­tal­i­ty data. I went back to the ear­li­est report of the Nation­al Health Sur­vey data avail­able on the ABS site: as it hap­pens, the 1995 sum­ma­ry report that con­tained the last mea­sured BMI data. That report also con­tains some ‘com­par­i­son’ data on dis­ease preva­lence from 1989–90.

Here’s a sum­ma­ry of what I found:

Long-term dis­ease inci­dence (self-report­ed), per­cent of pop­u­la­tion200819951998
Dia­betes mel­li­tus (A-type and B-type)5.72.21.1
All cir­cu­la­to­ry dis­eases (car­diac dis­ease, hyper­ten­sion, cere­bro-vas­cu­lar e.g. stroke)16.420.013.1

The dia­betes inci­dence seems to have risen sharply—possibly because of more active diag­no­sis and because the deter­mi­na­tive blood-sug­ar thresh­olds have been low­ered in that time—but the inci­dence of cir­cu­la­to­ry dis­ease has appar­ent­ly fall­en (while every­one was get­ting fat).

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