Vitamin D has had big raps lately. We know that low levels of it correlate with higher levels of some diseases, but does taking a supplement help? An article in the Herald this morning by Martin Johnson nicely outlines a study being undertaken by Professor Robert Scragg of the University of Auckland. His is the quote in the title.
Why is there need for an expensive trial when lots of observation studies show low levels of Vit D mean you are more likely to get Cardiovascular (and other) diseases, high levels mean you are less likely? Isn’t it obvious that by taking supplements that health outcomes will improve? Sadly, no it isn’t. Correlation does not mean causation (or “Post hoc ergo propter hoc” for you latinistas out there – I learnt this from a re-run of West Wing this week). What this means is that there is more than one reason for the correlation ie:
- Illnesses are because Vit D is an essential component in the biochemical pathway’s that provide a defense against these illnesses (causation), or
- Low Vit D is a consequence of something else that has gone wrong that also causes the diseases (ie Vit D is a “flag” or “marker” for something else).
If 1 is true, then raising Vit D levels may help. If 2 is true, then raising levels probably won’t help. For the moment assume 1 is true, then the next question is “does supplementation help?” Again, most would think “Of course.” However, it is possible that by bypassing the mechanism by which the body makes its own Vit D (ie beginning with exposure to the sun) the body’s response to the increased Vit D is different. These, and others, are reasons why a Randomised Controled Trial (RCT) in which some participants get Vit D and some get Placebo (in this case sunflower lecithin) is conducted. There is some information about the trial in the Herald article, more can be found on the Aust NZ Clinical Trials Registry here. Briefly, participants (50 to 84 years of age) will receive 1 capsule a month for 4 years. The incidence rate of fatal and non-fatal cardiovascular disease is the primary outcome. Secondary outcomes include the incidence of respiratory disease and fractures. They need to recruit 5100 people (so get involved!).
Why so many people? This is because they want to avoid making two mistakes. They want to know with high certainty that if they see a difference in the rates of cardiovascular disease between the Vit D and Placebo group, the that it is not a difference that occurred randomly (ie seeing a difference when there really is no difference). It is most common to accept a 5% chance of seeing a difference by chance (tossing 4 heads in a row is about a 6% chance). The second mistake is if the trial were to show no difference between the groups, but for this to be a false conclusion (ie not seeing a difference when there really is a difference). It is common to accept about a 10% chance of this happening. Notice, I have talked about “difference” not Vit D being “better” than placebo. This is very important, because it is possible that Vit D is worse and scientists must take into account that possibility. That is why scientists also start with what we call the “null hypothesis” – the presumption, in this case, that there is “no difference” in the rates of cardiovascular disease between those taking Vit D and those taking placebo.
I liked the quote of Prof Scragg in the Herald:
“GPs are very supportive of it and I know they are prescribing it extensively to patients. Hospital specialists are sceptical. Me, I’m in the middle. My heart says I want it to work. My head says I have to keep an open mind.”
I too often find myself in the “middle” – hoping with my heart that something works for the good of all, but working with my head so that we don’t end up peddling false hope or worse.