Until clinicians can effectively manipulate the microbiome to alter the metabolic profile of severely obese patients, Dr Laferrère advised, “the best way to do this is to stay away from an unhealthy processed-food, low-fiber, and sweetened-beverages-based diet.”

Bariatric Surgery Improves Weight, Not the Microbiome

While evidence points to changes in the metabolic signature in patients with moderate obesity, little is known about microbial richness and metabolic risk in patients with severe obesity. In a 2018 study, researchers sought to determine whether bariatric surgery altered microbial gene richness in patients with severe obesity as well as improved their metabolic health.⁴

The study assessed the microbiomes of 61 women with severe or morbid obesity who had undergone adjustable gastric banding or Roux-en-Y gastric bypass at 1, 3, and 12 months postsurgery.⁴ Despite improvements in their weight and inflammation postsurgery, 75% of patients had low microbial gene richness, which was associated with increased trunk fat mass, type 2 diabetes, and hypertension.⁴ In a subset of study patients, the researchers found that microbial gene richness did not improve, even at 5 years postprocedure.⁴

Severe Obesity Affects Gene Richness

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“Clinicians should consider that there are biological differences between patients with obesity that mediate their weight and metabolic responses to bariatric surgery, not all of which are characterized by the currently available tools,” said endocrinologist Rinki Murphy, MBChB, FRACP, PhD, associate professor at the University of Auckland in New Zealand.

Such was the case in a systematic review that Dr Murphy and colleagues conducted to determine the metabolic effects of bariatric surgery.⁵ The review of 14 studies with 222 patients included the gamut of bariatric surgeries: 146 patients who underwent Roux-en-Y gastric bypass, 25 who had sleeve gastrectomy, 30 who had biliointestinal bypass, 7 who underwent vertical banded gastroplasty, and 14 who received an adjustable gastric band.

Overall, surgery resulted in a richer biodiversity of gut microbiota, although it was unclear how these changes affected diabetes.⁵ Most of the studies assessed the microbiome between 3 months and 1 year postsurgery.⁵ Regarding microbial changes, the majority of studies showed that the most prominent shifts after surgery included a decrease in relative abundance of Firmicutes and an increase in Bacteroidetes and Proteobacteria.⁵

“While gut microbiota differences are one likely factor, we don’t yet have a clear understanding of which key microbial species signal responsiveness (ie, weight loss or diabetes remission) to bariatric surgery,” said Dr Murphy. “The therapeutic implications of identifying robust gut microbial changes among patients [who undergo] bariatric surgery are both in the development of novel probiotics and fecal biomarkers. Novel probiotics could improve the metabolic health of obese patients before and after surgery, while fecal biomarkers preoperatively could distinguish who would benefit most from which type of bariatric surgery.”

Summary and Clinical Applicability

Researchers are still elucidating how weight loss after bariatric surgery affects the gut microbiome and overall health a year or more after surgery. Many have suggested that obesity has a lasting effect on intestinal microbiota even after surgical weight loss.

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