From Top to Bottom, or Cause and Effect

Pre-Op Weight Loss and Mortality, Cause and Effect

It’s fascinating how readily people agree with the truth that correlation does not prove causation. Talk is cheap. But throw a study of correlation at them and poof! Caution evaporates. People leap from correlation to causality. So it is with a new study of pre-op weight loss and mortality after bariatric surgery.

In JAMA Network Open, Yangbo Sun and colleagues report findings of an association. They clearly state:

Despite all the advantages of MBSAQIP, our study could not establish causality between preoperative weight loss and postoperative mortality because of the observational nature of the study design.

Yet every single report on this study falsely states that pre-op weight loss reduces mortality.

Thoughtfully Considering the Evidence

To its credit, the journal invited a commentary on this subject. Micaela Esquivel and Dan Azagury present a concise yet complete review of what the evidence tells us about this question. Bottom line, we can learn a lot from looking at the weight history of people who have bariatric surgery. Getting good care early in the course of this chronic disease helps.

But there’s no evidence whatsoever that blanket requirements to lose weight before surgery will produce better outcomes. Indeed, a study of data from the same source (MBSAQIP) just last year came to precisely the opposite conclusion. Colleen Tewksbury et al wrote:

Weight loss prior to metabolic and bariatric surgery may not be necessary or safe for all patients. Unsafe weight loss prior to surgery may compromise nutrition status and lead to increased infection rates.

An Excuse for Limiting Access to Care

This kind of faulty thinking is pernicious. It provides a convenient excuse for erecting barriers to obesity care. Those barriers “feel right” because weight bias is so prevalent. The implicit thinking is that people with obesity did this to themselves. So if they’re not ready to change their “errant” ways, then spending money on obesity care is a waste.

Though the bias is false, it’s common. It makes effective care for obesity hard to get and serves to make people sicker. Thus, the leap to suggest that pre-op weight loss cuts mortality from bariatric surgery is not merely false. It’s harmful to patients who deserve better care – uncompromised by bias.

Click here for the study by Sun, here for the commentary by Esquivel, and here for the earlier study by Tewksbury.

From Top to Bottom or Cause and Effect, hand-colored etching by Elie / Wikimedia Commons

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May 15, 2020

4 Responses to “Pre-Op Weight Loss and Mortality, Cause and Effect”

  1. May 15, 2020 at 9:58 am, Allison Harrington said:

    I’ve been advised by a bariatric surgeon that the weight loss is to bring down the BMI to under 50%, to make the liver smaller, and to change the fat texture to one that is less firm, more “squishy.”

    • May 15, 2020 at 11:28 am, Ted said:

      That makes sense, Allison. It’s the kind of clinical decision-making that patients and doctors need to make together, based on your medical needs. Not something that an insurance company should mandate across the board for financial reasons.

  2. May 15, 2020 at 11:05 am, Danielle Kappele said:

    The link for the study is not working.

    • May 15, 2020 at 11:46 am, Ted said:

      Thanks for the heads up Daniele! The link is working now.