A new study has come out that hit headlines everywhere arguing that there's no safe level of caffeine intake during pregnancy I think it's worth a bit of a peer-review on twitter 1/npic.twitter.com/2PdMhgDz05
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12/n But, even if you look at just a few, some issues emerge. The single biggest study on caffeine and miscarriage, for example In this review, it's cited as evidence that maternal caffeine consumption increases risk of miscarriagepic.twitter.com/CB0p6w4oSo
13/n But actually, Gaskins et al found something a bit different. In their study, decaf coffee was associated with a higher risk and caffeinated tea was not Therefore, caffeine was NOT the likely culprit!pic.twitter.com/lHPzevG6MA
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
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
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
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
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
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
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
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
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
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
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
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
26/n This number - called the Population Attributable Fraction (PAF) - is actually calculable. The formula is pretty simple:pic.twitter.com/eoTJ5euQVv
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
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
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
30/n More broadly speaking, do we know that caffeine causes bad pregnancy outcomes? That is a VERY tricky question to answer
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
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
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
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 
35/n Anyway, regardless of what you think of the review, it's probably worth correcting the inaccurate table @BMJ_EBM
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
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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