During the course of the study, 15,994 of the women
died. Those who died from injury, accident, or suicide were not included
in the study results, since it's unlikely that supplement use could
have played a part in those deaths.
The researchers analyzed the data for any association between
supplement use and mortality, using validated statistical methods and
adjusting the results for other possible factors known to affect
mortality including age, lifestyle, smoking, alcohol, and certain medical
factors. And different statistical treatments gave different results.
Different Statistical Methods, Different Meanings
The statistical model used by the researchers adjusted the results for
participants' age and energy intake, nutritional factors (alcohol use;
fruit and vegetable consumption; saturated fatty acid consumption; and
consumption of fruits, vegetables, and whole grain products) and
non-nutritional factors (educational level, place of residence,
diabetes, high blood pressure, body mass index, waist to hip ratio,
hormone replacement therapy, physical activity, and smoking status). But
the different adjustments gave different results:
- When the results were only adjusted for age and energy intake,
taking vitamin B complex, vitamins C, D, E, and calcium all gave a
significantly lower risk of mortality, while only copper gave
significantly higher mortality.
- Adding in adjustments for the non-nutritional factors listed above,
only calcium gave a significantly lower risk of death. Copper,
multivitamins, vitamin B6, and iron gave a significantly higher risk.
- Adding adjustments for both nutritional and non-nutritional factors
gave the results the researchers' conclusions were drawn from, with six
supplements raising the death risk and calcium lowering it.
These different results are why some people have drawn different conclusions from the study than the authors did.
In general, when an effect is present in some statistical models and
absent in others, it means that if the effect actually exists, it's a
The results suggest the possibility that vitamin
and mineral supplements may be doing people who eat a standard diet more
harm than good, an idea that simply doesn't enter into the minds of
most people. They also suggest that taking additional calcium is
beneficial, though calcium's effect on the heart is controversial. It will take studies that are more controlled to confirm or disprove both of these ideas.
The researchers are particularly concerned about the effect of iron
because they observed a dose-response relationship between the amount
taken and the death rate: the higher the dose taken, the greater the
risk of mortality.
The results should not be interpreted to mean that vitamin and
mineral supplements can cause early death. One possible scenario is that
women were taking supplements in response to illness, which may have
been the reason for their earlier deaths. But since most people can get
all the nutrients they require from eating a balanced diet, the study
does raise the question of whether routinely taking a vitamin or mineral
supplement is a good idea.