Let's break it down
Published in JAMA and titled “Multivitamin Use and Mortality Risk in 3 Prospective US Cohorts”, this new study attempted to more rigorously assess the link between multivitamin use and mortality by addressing some of the biases that have been present in previous observational studies, namely, the healthy user bias and the sick user effect.
To control for the sick user effect (the possibility that people who get sick will then start using a multivitamin, making them less healthy than nonusers), any participant who had cardiovascular disease, cancer, diabetes, or kidney disease at baseline was excluded.
In an attempt to control for the healthy user bias (the possibility that people who take multivitamins may have better lifestyle habits than nonusers), the statistical analyses were adjusted for confounding factors like age, sex, education, alcohol and tobacco use, BMI, physical activity, diet quality, family history of cancer, and the use of other supplements.
A total of 390,124 participants were pooled from 3 large prospective cohort studies. The participants were asked at least once whether or not they used a multivitamin and were then separated into one of three groups based on their answer:
- Daily multivitamin users (n=163,307)
- Nondaily multivitamin users (n=47,151)
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Nonusers (n=179,666)
The participants were followed for a median of 23 years, during which deaths from all-causes, heart disease, cancer, and cerebrovascular disease were recorded at two time points: the first 12 years (follow-up number one) and 15 years and beyond (follow-up number two).
Is daily supplementation risky?
During follow-up period number one—people who used multivitamins daily had a 4% greater risk for all-cause mortality than people who never used multivitamins, while people who used multivitamins less than daily had a 9% increase in all-cause mortality. No differences in all-cause mortality were observed during the second follow-up period—which was 15 years or more after the baseline assessment.
Other than heart disease mortality—the rate of which was 6% higher in daily users compared to nonusers during the first 12 years of follow up—there were no significant differences between multivitamin users and nonusers in the risk of dying from heart disease, cancer, or cerebrovascular disease. The same was true for nondaily users vs. nonusers.
In other words, taking a multivitamin wasn’t associated with protection against dying from these diseases, but it wasn’t associated with any harm either.
The researchers also conducted an analysis that included a second assessment in which participants indicated their multivitamin use (on average, 6 years after their baseline assessment). This is important, because it’s a better indicator of multivitamin use over time than a single assessment at baseline—it helps to control for multivitamin use habits that may have changed during the study.
The result? A similar finding to the first analysis: a 4% greater all-cause mortality risk in daily multivitamin users in the first 12 years of follow up and no difference in all-cause or cause-specific mortality at the 15-year and beyond follow up.
