Bariatric Surgery and Cardiovascular Deaths – Does Size Matter?

Earlier this week, I posted on an analysis of the SCOUT trial, in which we found a clear and significant ‘dose-response’ relationship between moderate weight loss (3-10 Kg) and reduced cardiovascular outcomes (including death) in the over 10,000 high-risk participants in this trial.

So far, the only data showing a reduction in overall mortality with weight loss comes from studies looking at the effects of bariatric surgery, such as the ongoing Swedish Obese Subjects (SOS) study, which just published its latest findings in the Journal of the American Medical Association (JAMA).

In this paper, Lars Sjöström and colleagues from the University of Gothenburg, Sweden, specifically examine the relationship between bariatric surgery, weight loss, and cardiovascular events in the SOS study.

Some readers may recall that the SOS study is an ongoing, nonrandomized, prospective, controlled study conducted at 25 public surgical departments and 480 primary health care centers in Sweden of 2010 obese participants who underwent bariatric surgery and 2037 contemporaneously matched obese controls who received usual care. Inclusion criteria were age 37 to 60 years and a body mass index of at least 34 in men and at least 38 in women. Surgery patients underwent gastric bypass (13.2%), banding (18.7%), or vertical banded gastroplasty (68.1%), and controls received usual care in the Swedish primary health care system.

In the present analysis, bariatric surgery was associated with an almost 50% reduction in the number of cardiovascular deaths (28 events among 2010 patients in the surgery group vs 49 events among 2037 patients in the control group) over the almost 15 years of follow-up. Similarly, there was an almost 35% reduction in the number of total first time (fatal or non-fatal) cardiovascular events (myocardial infarction or stroke, whichever came first) in the surgery group (199 vs. 234 events).

Notably, however, the investigators found no significant relationship between cardiovascular outcomes and baseline BMI or the magnitude of weight loss. This is in contrast to the SCOUT study, where we found a clear ‘dose-response’ relationship between the amount of weight lost and the reduction in cardiovascular outcomes.

Several factors may explain this lack of ‘dose-response’ relationship in the SOS study.

For one, the SOS population, despite being far more obese than the participants in the SCOUT trial, were at a much lower risk for cardiovascular complications. Thus, the annualised event rate per 1000 participants in the SCOUT trial population was more than four times that of the SOS trial (30 vs. 7). Thus, the SOS study, despite its size, duration, and heavier BMI of participants, may simply be underpowered to demonstrate a ‘dose-response’ relationship between the magnitude of weight loss and reduction in cardiovascular outcomes.

Another explanation for the lack of ‘dose response’ in the SOS vs. SCOUT trial could be related to the overall magnitude of weight loss. Thus, while average weight loss in the SCOUT trial was just under 5%, the average weight loss in SOS was about 20%. if, as many suspect, a moderate 5-10% weight loss is all it takes to significantly reduce cardiovascular risk, the average weight loss of 20% in SOS would obscure any ‘dose-response’ relationship, as all participants in SOS, so to say, were already on the highest effective ‘dose’ of weight loss.

Thus, by no means is it clear from the SOS study that losing 20% of your body weight through bariatric surgery is any better than losing just 5% of your body weight as far as cardiovascular outcomes are concerned (this is not denying any other potential benefits of bariatric surgery on diabetes, cancer, arthritis, sleep apnea, or quality of life).

Thus, in light of the recent SCOUT findings, the superiority of bariatric surgery to medical obesity treatment in reducing cardiovascular outcomes would indeed need to be demonstrated in a ‘head-to-head’ trial  – a study that is unlikely to be done anytime soon.

interestingly, the lack of relationship between baseline BMI and cardiovascular outcomes and the remarkably low overall incidence of cardiovascular complications in the SOS patients raises a number of other important issues regarding indications for surgery – a topic that I will save for tomorrow’s post.

AMS
Berlin, Germany

Sjöström L, Peltonen M, Jacobson P, Sjöström CD, Karason K, Wedel H, Ahlin S, Anveden Å, Bengtsson C, Bergmark G, Bouchard C, Carlsson B, Dahlgren S, Karlsson J, Lindroos AK, Lönroth H, Narbro K, Näslund I, Olbers T, Svensson PA, & Carlsson LM (2012). Bariatric surgery and long-term cardiovascular events. JAMA : the journal of the American Medical Association, 307 (1), 56-65 PMID: 22215166