Evidence reviews

One question per page, read from the trials themselves, with the search protocol shown.

• Do vitamin D supplements prevent fractures? What the large trials found
  Four large randomised trials and two meta-analyses covering more than 90,000 people, read against each other: where vitamin D supplements prevent fractures, where they do nothing, and where higher doses made bone worse.
• Iron in pregnancy: what 57 randomised trials in 48,971 women actually found
  A Cochrane review of 57 trials, a meta-analysis in women who were not anaemic, and four recent trials of intravenous iron, read against each other: what iron in pregnancy reliably changes, what it barely moves, and where daily dosing is the wrong question.
• Do calcium supplements harm your heart? Two meta-analyses that disagree
  A 22-year follow-up of 36,282 women, two meta-analyses of the same trials that reach opposite conclusions, and 35 cohorts on calcium from food: what is actually known about calcium supplements and cardiovascular risk, and what the disagreement is about.
• Does magnesium help you sleep? Three trials in 151 people is the whole evidence base
  The most-sold sleep supplement in the world rests on three small randomised trials in 151 older adults, rated low to very low quality. What they actually found, what magnesium does do, and how to tell the two apart.
• Do zinc or vitamin C shorten a cold? What the Cochrane reviews found
  Two Cochrane reviews covering 8,526 and 11,306 people, read side by side: zinc shortens a cold that has already started, vitamin C does not once symptoms begin, and neither of them stops you catching one.
• Does folic acid before pregnancy prevent birth defects? The strongest result in nutrition
  Ten systematic reviews covering 296,816 women, trials and observational studies agreeing on the same number: what folic acid before conception prevents, when it has to be taken, and what it does not do.