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home:pathogenesis:vitamind:observational_bias [04.24.2009] – neldawhite | home:pathogenesis:vitamind:observational_bias [01.03.2012] – external edit 127.0.0.1 | ||
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+ | ====== Bias in observational epidemiological studies ====== | ||
+ | It is arguably impossible to sufficiently control for the socioeconomic factors, which drive a person to participate in a therapy or take a supplement. The case of hormone replacement therapy (HRT) is instructive. For decades, researchers thought that HRT prevented disease, but it was ultimately shown that it caused it. | ||
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+ | Studies of vitamin D's efficacy are especially fraught with challenges. For one, the secosteroid is palliative and the negative side effects can only be seen after decades of use. Also, people who take vitamin D are demonstrably different than those who don't. They almost always have a higher socioeconomic status. | ||
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+ | Not all studies on vitamin D's efficacy are observational, | ||
+ | ===== Hormone replacement therapy: a cautionary example ===== | ||
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+ | In a widely-studied example, numerous epidemiological studies as recently as 15 years ago showed that women who were taking combined hormone replacement therapy (HRT) also had a lower-than-average incidence of coronary heart disease (CHD), leading doctors to propose that HRT was protective against CHD. One researcher' | ||
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+ | There must have been some mistake, because this statement has been shown, conclusively, | ||
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+ | Controlled trials showed that HRT caused a small and significant increase in risk of CHD. Re-analysis of the data showed that women undertaking HRT were more likely to be from [[http:// | ||
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+ | According to [[http:// | ||
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+ | One retrospective commentary, full of all the requisite browbeating, | ||
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+ | ===== People who supplement with vitamin D are demonstrably different ===== | ||
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+ | A number of epidemiological studies examining the efficacy of vitamin D are vulnerable in the same way the hormone replacement therapy studies are. | ||
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+ | Studies in which a cohort is self-selecting can be fraught with error, especially when the groups are substantively different. The people who take vitamin D are different than those who don't. They tend to have more money, better education, and better health care, a well-documented pattern. | ||
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+ | One recent British study is typical.(({{pubmed> | ||
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+ | According to the results, subjects in the highest socioeconomic class were almost twice as likely to take cod liver, fish oil, or other supplements containing vitamin D as those in a lower class (p < 0.0001). | ||
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+ | ^Socioeconomic status ^Percent taking vitamin D ^ | ||
+ | |**I and II**, professional and managerial | ||
+ | |**III**, nonmanual | ||
+ | |**III**, manual |12.2% | | ||
+ | |**IV and V**, partially skilled and unskilled | ||
+ | |**Unclassified** |12.2% | | ||
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+ | Even studies looking at rates of supplementation in countries where the practice is more widespread have noted this effect. In a study of the 1966 Northern Finland birth cohort, which consisted of 7,648 people, it's apparent that socioeconomic status correlates with the choice to give supplements containing vitamin D.(({{pubmed> | ||
+ | * 86% of mothers who follow health education gave their children vitamin D supplements as opposed to 79% who rarely or never did. | ||
+ | * 88% of professional mothers gave their sons and daughters vitamin D supplements as opposed to 74% of farmers' | ||
+ | * 87% of those with more than a basic education gave the supplement as opposed to 80% who had a basic or no education. | ||
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+ | ===== Notes and comments ===== | ||
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+ | *Legacy content | ||
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+ | ===== References ===== |