I hope everyone understands that there is NO reliable evidence that any dietary supplements, including vitamins, provide any health benefits whatsoever to healthy people.[1] Occasionally individual studies will find an association between a particular positive outcome and a particular supplement, say breast cancer survival and vitamin D.[2] But it is just as likely to find an association with a negative outcome, say higher breast cancer risk in regular vitamin takers.[3] However none of these results has ever been found to be reproducible AFAIK.
For example a 2009 thorough review of all research on vitamin D and calcium supplements by US DHHS concluded:
The majority of the findings concerning vitamin D, calcium, or a combination of both nutrients on the different health outcomes were inconsistent.[4]
Most if not all of the studies initially finding these spurious correlations are large studies looking at lots of factors and outcomes. Unfortunately due to the nature of statistics there is always a small chance of a false positive or negative relationship and if many possible relationships are examined, such as in these large exploratory studies, it is almost a certainty that some false correlations will be found. This is why almost all of these studies say that confirmation in additional studies is required but this warning does not always make it into press reports. Occasionally a new relationship will be found that is confirmed in follow up studies. Unfortunately this confirmation has yet to be found for any supplement for any outcome in healthy people AFAIK.
FYI - Peter Norvig has a nice writeup of what to look out for when considering the results of a study.[5]
[1] Some supplements can help people with specific health issues but ask an expert such as your doctor as there are many false claims about supplements helping with specific conditions.
"I hope everyone understands that there is NO reliable evidence that any dietary supplements, including vitamins, provide any health benefits whatsoever to healthy people."
I sort of agree with that, but I also know it is kind of a tautology. 'Healthy people' can't become 'healthier people', by definition. Yet, every healthy person can imagine himself, but slightly better: better vision, more willpower, stronger, faster runner, more intelligent, etc.
Because of that, there are no 'Healthiest people'. Even the hypothetical person who wins the decathlon at the Olympics at the age of 40 in the year they won their fifth Nobel will have something to desire (a bald spot? Feeling more tired after exercise than he used to be?). In that sense, nobody is truly healthy.
This is not a tautology. Every "healthy" person will someday die and will often develop cancer or heart disease or have a stroke or have one of thousands of rarer medical conditions. If any supplements actually worked they would prolong life or reduce the occurrence of any of these diseases. All of these out comes can be measured in a properly designed study. So far there is no reliable evidence that supplements help with any of these outcomes.
For example a 2009 thorough review of all research on vitamin D and calcium supplements by US DHHS concluded:
The majority of the findings concerning vitamin D, calcium, or a combination of both nutrients on the different health outcomes were inconsistent.[4]
Most if not all of the studies initially finding these spurious correlations are large studies looking at lots of factors and outcomes. Unfortunately due to the nature of statistics there is always a small chance of a false positive or negative relationship and if many possible relationships are examined, such as in these large exploratory studies, it is almost a certainty that some false correlations will be found. This is why almost all of these studies say that confirmation in additional studies is required but this warning does not always make it into press reports. Occasionally a new relationship will be found that is confirmed in follow up studies. Unfortunately this confirmation has yet to be found for any supplement for any outcome in healthy people AFAIK.
FYI - Peter Norvig has a nice writeup of what to look out for when considering the results of a study.[5]
[1] Some supplements can help people with specific health issues but ask an expert such as your doctor as there are many false claims about supplements helping with specific conditions.
[2] http://www.medpagetoday.com/HematologyOncology/BreastCancer/...
[3] http://www.medicalnewstoday.com/articles/183880.php
[4] http://www.ahrq.gov/research/findings/evidence-based-reports...
[5] http://norvig.com/experiment-design.html