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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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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End of conversation
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