Thursday, September 29, 2011

Does Preoperative Weight Loss Affect LONG-Term Postoperative Weight Loss?

Some surgical weight loss programs require that, in preparation for bariatric surgery, patients lose a certain amount of weight. This might be a fixed amount, such as 25 pounds, a percentage of total body weight, or a percentage of excess body weight. To my knowledge there are no guidelines from the American Society of Metabolic and Bariatric Surgery (ASMBS) in favor of or opposed to this requirement, as it pertains to weight loss. The ASMBS guidelines do recommend presurgical weight loss in order to reduce liver size and thus decrease surgical risk.

In the previous post, we reviewed a study looking at short-term weight loss after Roux-en-Y gastric bypass surgery. Today we will consider a study conducted at a single medical center that recommends that patients lose 10% of their total body weight before surgery. These clinician researchers state that their experience tells them that preoperative weight loss improves weight loss outcome in the long term, and their data support their expectation. However, as we'll see, there are some problems with the study that limit the validity of that claim.

Let's take a look at the results. Some number of patients were recruited for the study and their preoperative weight loss was documented. We are not told how many people initially participated in the study, only that 150 patients were available for follow-up after 3 years and 95 were available after 4 years.

Right off the bat, there is a question. How many people were unavailable after 3 or 4 years, and, more importantly, why? Were there 200 or 500 or 1,000 participants who signed on to the study in the preoperative phase? We don't know, and this means we also don't know why people were unavailable. Did they simply move away? Did they stop coming in for medical care for some other reason relevant to their weight loss? We also cannot consider how - or if - the dropout rate affects the results. Maybe the 150 patients available after 3 years and the 95 available after 4 years had some special characteristics that set them apart from the general population of gastric bypass candidates. We cannot determine this from the article.

The data clearly showed exactly what the researchers expected: a nearly linear, statistically significant correlation between the weight a patient lost before surgery the weight they had lost at 3 and 4 years out. Percentages of both total body weight and excess body weight followed this relationship.

Another problem with the study: no control group. It is a purely correlational study. This wouldn't be a problem if the researchers hadn't used the word "causation." OUCH! Remember from Intro Psych (and many other intro science courses) the 3 requirements for a true experiment: random sampling from the population, random assignment to groups, and the presence of a control group. Based on what is reported in this article, none of these conditions were met.

So, while intuitively it sounds logical that preoperative weight loss will solidify behavioral changes and ultimately influence long-term weight loss, I'm still looking for evidence that this claim can be supported. What is known is that preoperative weight loss has many other benefits for the patient, such as reduced liver size at the time of surgery (reducing the risk of converting a laparoscopic procedure to an open one), shorter operating time, and fewer surgical complications. These factors alone speak to the importance of starting weight loss as early as possible.
Alger-Mayer, S., Polimeni, J., & Malone, M. (2008). Preoperative Weight Loss as a Predictor of Long-term Success Following Roux-en-Y Gastric Bypass. Obesity Surgery, 18 (7), 772-775 DOI: 10.1007/s11695-008-9482-2

1 comment:

  1. I expect that most people setting weight loss objectives do so by just setting a target weight, and maybe within a certain time. However, weight loss may not be the right "target" to achieve weight loss that will last. Your true objectives probably go beyond losing weight, into other areas of your life that drove you to consider losing weight in the first place.

    Kavinace Calm PRT