The American Journal of Cardiology just published a new meta-analysis (a study looking at a collection of other studies) that evaluates the effectiveness of bariatric surgery for cardiovascular risk reduction. As we’ve discussed before, meta-analyses are often used to sort through large numbers of studies to look for trends and weed out aberrant results or poorly designed studies. (This is particularly helpful when a poorly designed study gives conflicting results in comparison to the rest of the existing studies.) So, we are going to talk a bit more about the meta-analysis.
In this case, the authors started with 637 studies to evaluate, but ended up using the data from only 52 studies involving almost 17,000 patients. The first step of a meta-analysis is to find every single study even remotely related to your topic. So the authors pulled out, printed and looked at every single study they could find talking about bariatric surgery.
Then the authors start eliminating studies that aren’t relevant to their topic because once you take a closer look; a lot of the studies initially gathered aren’t really related to your topic at all. (For example: If the authors gathered all studies talking about Bariatric surgery outcomes – on closer examination – a study about the rate of depression in bariatric surgery patients wouldn’t have any information usable to evaluate cardiac risk in these patients.) Otherwise it would be like comparing apples to oranges.
Once authors have narrowed the pool to studies that are only looking at relevant topics, with measurable results – the authors then examine the studies themselves. The authors evaluate all aspects of the studies: what is the study design, what does it measure, (is it designed to measure what it is supposed to measure?), what are the results? (were the results calculated correctly?) what are the conclusions? what are the limitations of the study?
Then the authors summarize all of the findings, and draw conclusions based on the results. (if 50 studies involving 16,900 people show one thing – and 2 studies involving 100 people show something completely different – the authors will discuss that.)
The strengths of meta-analyses are that they summarize all of the existing studies out there – and provide readers with fairly powerful results because they involve large numbers of people.
For researchers, meta-analyses are cheap – particularly in comparison to designing, conducting a large-scale study with hundreds or thousands of subjects. A meta-analysis doesn’t require federal grants or institutional permissions. It just requires a computer and journal access (along with a good knowledge of study design, statistics).
As you can imagine, the downside of meta-analyses is that they don’t generate NEW knowledge, since they are summaries of other studies. Meta-analyses are also limited by the AMOUNT of data already published. If few researchers have written about a topic, then a meta-analysis isn’t very effective or powerful. (A meta-analysis on three studies involving only 25 total patients, for example).
Now that we’ve discussed the purpose and function of the meta-analysis, let’s discuss the results of Heneghan’s reported results.
Now, readers need to be very careful when reading blogs, and other articles like mine reporting results such as this – because this is filtered, third-hand information by the time it’s published on blogs, or newspaper articles. (First source is the meta-analysis itself – which as we’ve discussed is actually a summary evaluation of other work). Secondary is the Medscape article which summarizes and discusses the results of Heneghan’s study.
Now, that means that anything you read here is essentially third-hand information – if it’s based on the Medscape article. That’s why we provide links to our sources here at Cartagena Surgery – so readers can read it all first-hand. This is important because just like the children’s game of telephone, as information is passed from source to source, it is edited, filtered and subtly changed (for reasons of space, editorial preference etc.)
heneghan’s meta-analysis results showed significant reductions in weight, blood pressure, cholesterol and hemoglobinA1c (blood glucose levels) after bariatric surgery. The Framingham risk score (a score developed based on the landmark Framingham study) which predicts the risk of cardiovascular events (heart attacks, strokes) also showed a significant reduction (which would be expected if all the risk factors such as hypertension were improved).
Now, a lot of readers might say, “Wait a minute – isn’t this self-evident? If you lose weight – shouldn’t all of these things (glucose, blood pressure, cholesterol) improve?”
Yes .- logical reasoning suggests that they should – but in medicine we require hard data, in addition to logical reasoning (ie. A should lead to B versus a study with ten thouand patients proving A does lead to B.)
We need to be particularly careful when suggesting or assuming causality from treatments (surgery) for conditions. A good example of this is liposuction. Since liposuction involves the removal of subcutaneous fat – and may result a (a small amount) of weight loss – many consumers assumed that this limited weight loss conferred additional health benefits associated with traditional weight loss. Wrong!
Sucking fat out of your behind (liposuction) will not lower your blood pressure, cholesterol, or blood pressure and does not replace the health benefits of weight loss or exercise. I can hear readers snickering now – but that’s because of my phrasing. For years – many people, some health care providers themselves thought that weight loss, any weight loss lead to the above mentioned health benefits, and that included liposuction related weight loss. It took several studies to disprove this. So, in medicine – nothing is obvious – until we prove it is obvious! (Remember: much of what was “obvious” in 1950’s medicine – is now considered absurd.)
Original Research Article Citation:
Heneghan HD, et al “Effect of bariatric surgery on cardiovascular risk profile” Am J Cardiol 2011; DOI:10.1016/j.amjcard.2011.06.076. (abstract only – article for purchase).
Medpage Summary Article:
Bankhead, C. (2011). Medical News: Bariatric Surgery gets high marks for CVD risk reduction. Medpage Today.