There has
been some buzz lately about a new study into endometriosis - it’s been in news
reports on a number of blogs and websites (such as this
one and this
one), you may have already heard about it. From the headlines in the news
article you’ve probably gathered this new study is about the relationship
between body size and endometriosis, specifically leaner women were found to be
more at risk.
So, this
study has found that thinner women are more likely to have endo; does this mean,
in order to decrease your endo risk, you should all start reaching for the cakes
and pies, forgo the use of even the most rudimental cutlery and just shovel
food in your mouths with your hands like a mad combine harvester ploughing
through a buffet?
Well no,
definitely don’t do that. Most of the news outlets have done a pretty decent
job of explaining the findings, but this month I’ll be looking over the
original study and finding out exactly what was done, what was discovered and what we can learn from it.
Before we
start though I know this study is controversial to some, so I will say I'm not
going to comment on the worthiness of this study. I'm only going to give you an
explanation of the findings and my interpretation thereof, then let you make up
your own mind.
To start
with it’s always a good idea to see if any other research like this has been
done in the past and it turns out it has, and I’ve written
about it before, way back in the dark murky past of 2010. So the finding
that women with a low BMI are more likely to have endo isn’t a particularly new
discovery.
What’s
different about this latest study? For a start it’s bigger, much bigger. The
same US research group conducted a similar study in 2010 with 1,817 women with
endo, this latest study includes 5,504 women with endo and the more people you
have in your study, the more accurate your results. So where did they get all
these women from? Well in the US since 1989, 116,430 female nurses from 14
different states have been filling in questionnaires about their health every 2
years. All this information together forms the Nurses’ Health Study II – a vast
mine of information available to researchers looking into relationships between
lifestyle and environment factors and the risk of all manner of illnesses.
This current
study looked at all the questionnaires from 1989 to 2011, picked out the
information from women who were diagnosed with endo and compared it to women
who weren’t. The major factor being examined here was BMI, which is a ratio of
height to weight. My BMI is around 20, which is classified as normal, but I’m a
gangly leviathan, so someone who was a foot shorter than me but the same weight,
would have a BMI of 28 and be classified as overweight (in my last post on this
subject I air my grievances about BMI as a measure of body size).
BMI on its
own isn’t a very reliable measure of body size, basically because a body
builder and a fat guy could have the same BMI as it doesn’t take into account
body shape. Fortunately BMI wasn’t the only factor these researchers were
looking at; in order to get a better picture of body size they also took
measurements of waist/hip ratio and waist circumference.
Anyway,
enough of the methods, onto the results. The central finding of this study was
that women with a higher BMI were less likely to be diagnosed with
endometriosis – in fact morbidly obese women (i.e. with a BMI over 40) were 39%
less likely to have a diagnosis of endometriosis than women with a BMI between
18.5 and 22.4.
What the
researchers found was that there was an inverse
correlation between endometriosis and BMI. What this means is that as BMI
went up, endometriosis risk went down. Below is a graph from the paper which
shows that relationship (the solid black line is the important one, don’t worry
about the dashed lines).
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| Source:
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Interestingly
this study found that this association was stronger in women who were also
infertile, but there is an important point to take into consideration first. The
average age of women at the time this information was collected was around the
mid-thirties, so it is very likely they’ve been living with endo for a long
time. BMI tends to increase with age and, more importantly, many women who have
suffered endo long term take medications that cause weight gain. Therefore,
wouldn’t it be better to look at the BMI of these women when they were younger?
Yes, and that is exactly what the researchers did - they looked at the BMI of
these women aged 18. Below is the graph of the result.
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| Source: http://humrep.oxfordjournals.org/content/early/2013/05/09/humrep.det120/F2.large.jpg (click to enlarge) |
As you can
see it looks pretty much the same and, despite the fact that the result was
slightly attenuated, morbidly obese women were still 25% less likely to be
diagnosed with endo.
Now, one
thing we must be careful of here is not to assume this means women with a higher
BMI can’t get endometriosis, it just suggests they are less likely to have it when they are younger. For example, if we
look at the results of this study, women with a BMI of around 21 had the
‘average’ rate of endo, which is 1 in 10. However women with a BMI of over 40
had a 39% reduction in their endo rate, which is equivalent to about 0.6 in 10.
So, in women with a normal BMI, around 10 in 100 would be expected to have
endo, however in women who are morbidly obese, 6 in 100 of them would be
expected to have endo.
So what of
the other body size factors being investigated? The results of this study
showed that a larger waist circumference was also associated with a lower risk
of endo, but only for infertile women and there was no association with height
and endo. Waist-to-hip ratio was a harder result to call as a lot of women
didn’t have that information recorded. Although the researchers found that
women with the smallest waist-to-hip ratio had a 3 fold increased risk of endo,
this was only based on data from eight women, so it’s probably best not to pay
much attention to that result until it can be verified.
Now we know
the results, the next big question is, why might this be the case? At first
appearance it doesn’t make sense, fat cells can produce estrogen, so the more
fat cells you have the higher levels of estrogen you have. Being as endometriosis
is an estrogen dependant disorder, shouldn’t women with more fat (and therefore
a higher BMI) be more susceptible to the disease? Similarly, being underweight
as a child is associated with later menarche and less menses, yet women with
endo are known to have, on average, and earlier menarche so that
doesn’t seem to add up either.
There are
several ideas as to why we are seeing these results. It is known that obese
women are more likely to have Polycystic Ovarian Syndrome (PCOS) and some
researchers think that PCOS may be ‘protective’ against endo due to a reduced
number of menstrual cycles and higher levels of androgen hormones which could suppress endo lesions. Although
without knowing which women had PCOS in this study we can’t tell if this is the
case. The androgen link may be important though. Studies have suggested that
androgen hormones are elevated in obese young girls and women regardless of them
having PCOS. Therefore, a higher level of androgens in obese girls and women
may suppress endo and explain why we see a lower risk of endo in the high BMI group.
The authors
of this study also suggest there may be a diagnostic bias for obese and
morbidly obese women. Surgeons may be less likely to want to perform a laparoscopy
on these women as it is just more difficult, therefore they would be less
likely to be confirmed with endo. However, the authors point out that if this
were the case there wouldn’t be such a strong association between endo and BMI
in infertile women compared to women with pain symptoms alone.
There are several
other factors that can modify BMI, such as medications, lifestyle factors and
co-morbid conditions and these all need to be considered when examining a
relationship between endo and BMI, which as far as I can see, this present
study did not take into account. Another consideration about this study is the
population of women from which it was drawn. All the women from whom the
information was taken were nurses employed in that profession for at most 20
years. As endometriosis presenting with severe symptoms would hinder a woman’s
ability to maintain long-term employment (especially a physically demanded job,
like nursing), then it is likely that women with severe endo symptoms were not
proportionally represented in this study, which may skew the results to favour women
with less severe forms of the disease, who in turn would be more physically
active and hence, have a lower BMI.
Of course it
could be that whatever makes a woman susceptible to endo also makes her more
likely to have a lower BMI at a younger age. A growing body of evidence
suggests that endometriosis is condition you are born with and is hereditary -
so it may, or may not, be that whatever genetic/epigenetic factors result in a
woman developing endo, also affect her metabolism or fat deposition resulting
in her having a lower BMI before she started taking hormonal medication. Unfortunately
there are still a lot of questions that need to be answered before we can make
any definite conclusions here, but when we eventually answer those questions,
we will have a better understanding of this paradoxical disease.
This is my
interpretation of the current study, as ever though I encourage everyone to do
their own research and draw their own conclusions, so below is a link to the study
I have been talking about this month, which you can read and download, in full,
for free.
