When science seems fickle.
Every so often in Focus on Healthy Aging, we highlight studies that cast doubt on established medical advice. In this month's issue, it's our page 4 article featuring two studies that question the benefits of calcium for bone density.
While it may seem that medical science is fickle, looking at the studies that provided the evidence for these recommendations gives a clue as to the real reason why your doctor might seem to give you contradictory advice from year to year. Headlines are often generated from studies that are "not yet ready for prime time"--they show associations, but not cause and effect. Oftentimes these associations are just that--for example, studies will show that people with yellow fingertips are more likely to get lung cancer, but lung cancer isn't caused by the yellow fingertips!
To determine causation, you need to have two groups of subjects who have similar risks for the outcome (here, a decrease in fractures or bone density) at the start of the study--thus, any significant differences between the groups at the end can reasonably be attributed to the treatment (calcium) and not to some other unidentified factor. The best way to do this is through a randomized controlled trial (RCT), one in which subjects are randomly assigned to take calcium or not, and all the risk factors that could cause low bone density or a fracture are the same in both groups. Unfortunately, this doesn't always happen. Additionally people may vary in the amount of dietary calcium and vitamin D (needed for calcium absorption) they take during the course of the study, particularly given that these are easily available, compared with a new drug that may be tested in the same way.
The studies we report on this month are meta-analyses, studies that combine the results of multiple other studies. The best of these would be the meta-analysis of RCTs, like the study looking at bone density. Of course we're not really interested in bone density alone, but rather whether fractures are prevented. Unfortunately only two RCTs (with a total of262 patients) studied fractures as an outcome. In all the other studies, patients decided whether they would take calcium or not--they weren't randomized. So the lack of an association could have been due to differences in their overall health, how many actually had osteoporosis, whether they did weight-bearing exercise, smoked, etc. This is what happened in the case of hormone replacement therapy (HRT). HRT was seen as a way to prevent cardiac disease--but this was based on an association that wasn't found in follow-up RCTs, because those women who took HRT also ate better, exercised more, smoked less, etc.
This isn't to say that the calcium metaanalyses are wrong--rather its to show you how complicated it can be to get to a real answer. The major take-home for me is that most of the participants in the calcium studies were healthy older adults. It's frail elders who are most likely to be deficient in calcium and vitamin D, and who benefit. So if you fit this description, continuing to supplement your dietary intake is likely to help protect your bones.
By Rosanne M. Leipzig, MD, PhD
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|Title Annotation:||FROM THE EDITOR|
|Author:||Leipzig, Rosanne M.|
|Publication:||Focus on Healthy Aging|
|Date:||Jan 1, 2016|
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