They're back! Not
long after the furor caused by the Centers for Disease Control's
revised "figures" which showed that not 400,000 but about 26,000
Americans died annually from obesity and that being overweight might
well be protective against premature death, the fat police are at it
again with their scary headlines about the dangers of fat. This time
the story is perhaps even more dramatic since it is about how middle
age obesity "causes" late life dementia. So the supposedly bad news
isn't just that being fat will likely kill you before you reach old
age, it's also that if by some chance you do survive an early death
you can worry about spending your golden years
crazy.
That's the startling news from
gerontological epidemiologist Rachel Whitmer and colleagues at the
Kaiser Permanente Division of Research whose study, "Obesity in
middle age and future risk of dementia: a 27 year longitudinal
population based study" was published in the British Medical
Journal. Supported by funding from the US National Institutes of
Health, Whitmer followed 10,276 people from their mid-forties, in
1964, until 2003. From 1994 until 2003, 713 of these individuals
were diagnosed with dementia, including Alzheimer's disease. Of
these 713, the prevalence of dementia was "significantly higher for
those who were obese or overweight at mid-life." According to
Whitmer, obese people (BMI>30) were 74% more likely than normal
weight individuals to develop some from of dementia, while the
overweight (BMI 25-29.9) were 35% more likely to develop it. Hence
all those frightening headlines, earnest talk show musings, and very
happy proponents of massive government interventions against fat. As
Philip James, head of the International Obesity Task Force, told the
Associated Press "This adds another major reason for concern about
the obesity problem and it now unfolds yet another area where… we
have to say, 'for God's sake, we better get
cracking.'"
But before we get, as it were, cracking,
perhaps the Whitmer study deserves a quieter and more careful
reading in order to determine whether millions of middle-aged and
overweight Americans -- roughly half of the middle-aged -- really
are destined for an Alzheimer's cursed old age. To begin with, it is
important to understand exactly what the study is, namely a
small-scale, longitudinal epidemiological study that is being
interpreted as saying that middle age + obesity=old age dementia.
Such a study, however, is unable by its very nature to produce a
cause and effect conclusion in which middle aged obesity causes old
age dementia. That's because risk factor epidemiological studies --
which is what this is -- are statistical exercises, not laboratory
experiments. They are not carefully controlled clinical trials in
which two groups are identical except for one controlled variable.
Rather they are purely observational studies in which the researcher
does not control exposure to either the independent variable -- in
this case obesity -- nor to the possibly dozens of confounding
variables that might influence the conclusions which are drawn. As
Robert Heaney of Creighton University writes "… since
observational studies cannot adequately control for other factors --
known and unknown -- that may obscure or exaggerate the sought for
relationships, they can never establish causal relationships…."
These risk factor observational studies tell us nothing more than
how likely it is that an association between two things, like being
obese and having dementia, is real, as opposed to chance.
The world is full of associations, for
example, between the number of births in certain European countries
and the prevalence of storks, between the price of beer and the
salary of priests in Chicago, between the wearing of
shoes and the incidence of malaria. But that doesn't mean that these
associations represent causal connections in which storks cause
births or a lack of shoes causes malaria. As the authors themselves
tell us "Obesity and overweight in middle age as measured by body
mass index and skin-fold thickness were strongly associated with
risk of dementia in later life." Notice those words, "strongly
associated with risk." This isn't a study that presents any
scientific evidence that being overweight or obese CAUSES dementia
-- whatever the hysterical headlines might say. It is a study which
has simply found that there was an association between midlife
obesity and latter life dementia. Indeed, as in many instances of
association, there might well be another risk factor that produces
both midlife obesity and dementia.
Second, it is important to keep in mind
just how small this study really is. In all there were 713 persons
who were diagnosed with some form of dementia, 354 of whom were
either obese or overweight. But 359 of those suffering from dementia
were either normal weight or underweight, a fact that goes unnoted
by the authors. In fact, even with the women, where the authors tell
us there is a strong association between obesity and dementia, 237
subjects who had dementia were normal weight or underweight, while
just 159 were overweight or obese, a very curious finding for such
an allegedly strong connection between obesity and
dementia.
Third, there are enormous gaps in this
study. The only time the participants received anything like a
comprehensive medical examination was from 1964-1973 when they were
in their forties. From 1973 until 1994 we know nothing about these
participants in terms of health status in general or more
importantly their exposure to dementia risk factors. It is unclear
from the study whether, for instance, the original health histories
included family history for dementia, a factor which increases the
risk for some forms of dementia fourfold. More importantly, at the
time of their initial medical examination, none of the participants
was assessed for cognitive functioning to determine whether they
suffered from early onset dementia.
Fourth, risk-factor epidemiological
studies are judged on how reliable their associations are -- in this
case how strong is the linkage between obesity and latter dementia.
The authors tell us that "obesity and overweight in middle age…were
strongly associated with the risk of dementia", but this simply
isn't the case. The strongest association that they found, between
obese women and the risk of dementia (adjusted for mid-life and late
life co-morbidity) was 2.07, which counts as a weak association.
Most of their associations between obesity and risk of dementia were
between one and two, which are considered very weak associations. In
short, the statistical evidence that the connection between obesity
and dementia is a real one as opposed to a chance finding is
weak.
But that isn't the end of this study's
troubles. Not only are their findings weak, they only apply to one
sex -- women, though you have to read a considerable way into the
study to discover this. In fact, if you didn't understand the
language of epidemiology you would be unlikely to find this out at
all. As the authors note quickly in passing: "overweight men had a
non-significant 30% increase in risk" which means there was no
increased risk of dementia for men who were overweight or indeed
obese at middle age.
And this creates additional problems. One
of the things that helps strengthen the case for a genuine
association is what epidemiologists call biological plausibility. In
other words there has to be some biologically compelling reason an
alleged cause -- in this case mid-life obesity -- is associated with
dementia only in women and not in men. The fact that the authors
were unable to provide a biologically plausible reason obesity is
not associated with dementia in men further weakens their case.
Biological plausibility also undermines
the study's conclusions in another way. The authors controlled for
such risk factors as high cholesterol, diabetes, hypertension, heart
disease and stroke which are often linked with obesity and which
some have suggested might be associated with dementia. Yet even
after controlling for these conditions, there was still supposedly
an association between obesity and dementia according to the
authors. This means there is no recognized biologically plausible
explanation for the link between obesity and dementia. And given
that the link between obesity and heart disease, stroke and even
diabetes is tenuous, the claim that obesity now somehow causes
dementia is decidedly improbable.
Nor is the study's validity helped out by
the fact that it is contradicted by much of the established research
on dementia, particularly Alzheimer's disease. Take for example the
work on risk factors for Alzheimer's disease. These risk factors
include age, family history, gene mutations on chromosomes 21, 14,
and 1, which are associated with early onset of the disease, while
the gene ApoE4 has been identified with the more common late onset
form of the disease. Additional risk factors include Down syndrome
and head injury. Possible risk factors might include Type 2
diabetes, heart disease, high blood pressure and stroke. Notice that
obesity is not a risk factor. Again, there is no indication in the
study that the authors controlled for any of these established risk
factors except for age, a fact which further undermines their
alleged association. Some dementia research has suggested that
elevated homocysteine levels might be a risk factor and that
antioxidants might have a protective role. Both homocysteine and
antioxidants are connected with life-style factors such as diet and
physical activity yet the study had not information from
participants on these factors.
More importantly, the evidence about the
alleged connection between dementia and obesity and fat is extremely
limited and contradictory. For example, the Personnes Agees Quid
study in 2003 found no relationship between BMI and the incidence of
dementia and the Rotterdam study found no increased
risk of Alzheimer's with increased cholesterol levels. And the
previous study that looked at weight and Alzheimer's disease in
women that is cited by the authors found that there was no
significant difference in BMI between women who developed
Alzheimer's before age 79 and those who did not, nor any significant
difference in BMI between women who developed other forms of
dementia and those who did not. All of which further weakens the
author's claims about the link between midlife obesity and latter
dementia.
Finally, the lead author, Rachel Whitmer,
published another article on dementia and so-called midlife risk
factors in January of this year ("Midlife cardiovascular risk
factors and risk of dementia in late life" Neurology, 2005) that
uses the same population base of the Kaiser Permanente medical care
program of northern California. Only in that study, she and her
co-authors concluded that such things as hypertension, high
cholesterol, diabetes and smoking "substantially increases risk of
late-life dementia." That list is interesting in itself since it
mixes real diseases -- hypertension and diabetes -- with mere risk
factors like high cholesterol and smoking. It looks as if dementia
is becoming a bit like smoking in that everything is somehow
connected to it. Simply take your pick of the Whitmer studies as to
whether it is obesity, smoking, diabetes, hypertension or high
cholesterol that leads to late life dementia.
So just why did this study, which proves
nothing about the relationship between obesity and dementia, receive
so much fear-inducing media attention, both in the
US and internationally,
particularly right after the obesity numbers debacle? The answer is
to be found in a combination of the general scientific illiteracy of
the media and the determination by those who claim that obesity is
killing us by the millions to prove their case. Whitmer and her
colleagues make it easy for a deadline pressured reporter and her
headline writer to do their work with such claims as "Failure to
contain the present epidemic of obesity may accentuate the expected
age related increase in dementia." And what could be more headline
grabbing than linking America's favorite
obsession -- weight -- with one of its greatest worries -- the risk
of dementia, which is neatly summarized in the author's conclusion
that "obesity in middle age increases the risk of future dementia…."
Never mind the boring technical stuff of whether the study's data
actually supports its claims, it is the claims that are useful. The
result is another new and terribly useful "fact" not only to be
endlessly and mindlessly repeated but, at the right moment, to be
used to further the war on fat. And that's what makes all the
difference.
John Luik is a health policy analyst
currently writing a book about obesity in America.
