3/n The study itself is here, in BMJ Evidence-Based Medicine:https://ebm.bmj.com/content/early/2020/07/28/bmjebm-2020-111432#DC1 …
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14/n This gets to the heart of the issue with a narrative review. It is, essentially, the author's opinion of each study and what they mean - no systematizing, no group of authors carefully reading through each point to be careful
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15/n One person's evidence of the risk from caffeine is another person's evidence of safety Case in point - this systematic reviewpic.twitter.com/GJDfYXSyoT
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16/n In the narrative review, it's cited as evidence that any amount of caffeine is dangerous, but the authors actually concluded that their results were consistent with the WHO recommendation to limit caffeine consumption to <300mg per day during pregnancypic.twitter.com/SdnVZErUva
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17/n Similarly, the Greenwood et al meta-analysis is cited as evidence that even small amounts of caffeine are probably harmful, but the authors don't really agree!pic.twitter.com/2tmUUjXVMh
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18/n All of this is fine, and just the limitations of the narrative review. The entire purpose of such a document is to provide a narrative decided on by the author, which is neither good nor bad - it's just how these things work But that's not all of the issuespic.twitter.com/gV8iSLplzc
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19/ If you get to the last part of the paper, you'll see this table It contains a fundamental error that makes the numbers in the last column incorrectpic.twitter.com/3cQNOmY64R
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20/n This table is calculating the population attributable fraction of various pregnancy outcomes. The last column is the context - for the ~1 million miscarriages in the US each year, 280,000 are caused by caffeine! Shocking. And untruepic.twitter.com/irjABFVFwb
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21/n This table has several errors. The first is in column (i) This is cited as the risk per 100mg. In fact, it is the RELATIVE RISK INCREASE per ADDITIONAL 100mg of coffee consumedpic.twitter.com/GSrBhX27sf
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22/n This is then used to calculated the risk AT 200mg of miscarriage - double the risk at 100mg Except, this is incorrectpic.twitter.com/9qoYmqckYW
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23/n The BASELINE risk of miscarriage, for a woman who drinks NO coffee, is around 16%. The RELATIVE increase is 14% per 100mg of coffee. So the ABSOLUTE risk for a 200mg per day drinker is: 0.16*1.28 = .205 = 20.5% Thus, all the numbers in column (ii) are wrongpic.twitter.com/eZaChnR0Ls
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24/n The last column calculates the number of caffeine-related events by multiplying the risk in column (ii) with the total number in column (iv) So, 28% risk and 1 mil miscarriages means 280k miscarriages related to caffeinepic.twitter.com/JEqsAM7Q3A
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25/n Think about this for a second. Even if that number was correct - i.e. 20.5% rather than the wrong 28% - it doesn't make any sense because it assumes that 100% of women who have had miscarriages drank 200mg of caffeine a day
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26/n This number - called the Population Attributable Fraction (PAF) - is actually calculable. The formula is pretty simple:pic.twitter.com/eoTJ5euQVv
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27/n In this case, with a prevalence of ~35% for caffeine intake of 200mg or higher, the number of miscarriages attributable to this is: 0.35*(1-(1/1.28)) = 0.763 = ~76,300 miscarriages So, about a quarter of what's in this study
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28/n This actually makes perfect sense when you think about it. Remember, the ABSOLUTE risk increase of miscarriage for women who drink caffeine is pretty small - 1-2% - so it doesn't really make sense to say that up to a third of miscarriages are caused by caffeine
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29/n And yet, these figures were picked up in the media and thrown around as if certainly true, which is very worrying!pic.twitter.com/5v3SQPeKtn
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30/n More broadly speaking, do we know that caffeine causes bad pregnancy outcomes? That is a VERY tricky question to answer
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31/n This narrative review argues yes, absolutely, and while most people would agree a LOT of caffeine is a problem, much previous research has said that small amounts are probably not so badpic.twitter.com/50siPcJ8HD
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32/n This narrative review argues that all such studies are the product of industry funding, which is not entirely untrue, but also doesn't really tell the whole story given that it disagrees with non-industry studies as wellpic.twitter.com/ynC0gKO1nj
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33/n Ultimately, reviews like this come down to opinions Do we think that Prof James is correct and caffeine is definitely bad? Well, maybe. It depends on your perspective I think
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34/n What we can't say is that this is new evidence, or even that we've proven anything here. It's one well-researched opinion that you may or may not agree with
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35/n Anyway, regardless of what you think of the review, it's probably worth correcting the inaccurate table
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36/n Also, for the record, I think the conclusions are pretty massively overstated, and also miss out a key part of recommendations which is the question of whether they'll be followed!pic.twitter.com/uPDQfv4aHI
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37/n I should also say that I thoroughly dislike opinion pieces and media attention like this The evidence hasn't changed, and quite frankly I think pregnant people have enough to worry about without the fear that a cup of coffee will cause a miscarriage
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