Sleeve gastrectomy was associated with marked longitudinal changes in BIA-derived estimates of body composition during the first 24 postoperative months, including reductions in fat mass, fat-free mass, and skeletal muscle mass, as well as transient changes in phase angle.
Key Findings
Results
Substantial reductions in body weight and BMI occurred following sleeve gastrectomy, with the largest changes observed during the first postoperative year.
Study included 49 patients who underwent laparoscopic sleeve gastrectomy at a tertiary bariatric center.
Body composition was assessed preoperatively and at regular postoperative follow-up visits over 24 months.
Changes in body weight and BMI subsequently stabilized after the first postoperative year.
Patients had class III obesity at baseline.
Results
Fat mass and visceral adipose tissue estimates decreased substantially following sleeve gastrectomy.
Reductions in fat mass were observed throughout the 24-month follow-up period.
Visceral adipose tissue estimates also showed substantial reductions during follow-up.
The largest changes in fat mass occurred during the first postoperative year.
Body composition was assessed using standardized multifrequency bioelectrical impedance analysis (BIA).
Results
Fat-free mass and skeletal muscle mass decreased following sleeve gastrectomy alongside fat mass reductions.
Decreases in fat-free mass were observed throughout the 24-month observation period.
Skeletal muscle mass also declined postoperatively.
Total body water decreased during follow-up.
These findings were derived from multifrequency BIA assessments and represent impedance-derived estimates rather than direct tissue measurements.
Results
Phase angle decreased during the early postoperative period before showing partial recovery over time.
Phase angle (PhA) is a BIA-derived parameter reflecting cellular integrity and hydration status.
The initial decrease in phase angle indicated early postoperative physiological changes.
Partial recovery of phase angle was observed over the subsequent follow-up period.
The extracellular water-to-total body water ratio (ECW/TBW) was among the parameters tracked longitudinally.
Results
Estimated resting energy expenditure decreased during the 24-month follow-up after sleeve gastrectomy.
Resting energy expenditure was estimated via BIA at each follow-up visit.
Reductions in resting energy expenditure paralleled reductions in body weight, fat-free mass, and skeletal muscle mass.
The largest changes occurred during the first postoperative year and subsequently stabilized.
This finding was part of the comprehensive BIA-derived body composition assessment.
Results
Exploratory subgroup analyses suggested differences in absolute body composition trajectories between men and women and between patients with baseline BMI below and above 50 kg/m².
Subgroup analyses were performed according to sex and baseline BMI.
Patients were stratified by baseline BMI below versus above 50 kg/m².
Authors cautioned that these findings should be interpreted within the context of the observational study design and limited statistical power due to the sample size of 49 patients.
These subgroup findings are described as exploratory and hypothesis-generating.
Methods
The study was a retrospective observational study using BIA-derived estimates, which the authors note should not be interpreted as direct measurements of individual tissue compartments.
Body composition was assessed exclusively by bioelectrical impedance analysis (BIA), not by gold-standard methods such as DXA or CT.
The study included 49 patients at a single tertiary bariatric center.
The observational design limits causal inference.
Authors noted that subgroup analyses should be interpreted with caution because of the limited sample size.
Results are described as 'impedance-derived estimates rather than direct measurements of individual tissue compartments.'
What This Means
This research examined how body composition changes over two years following sleeve gastrectomy — a type of weight-loss surgery — in 49 patients with severe (class III) obesity. Rather than simply tracking weight and BMI, the researchers used a technique called bioelectrical impedance analysis (BIA) to separately estimate fat mass, muscle mass, body water, and other components at regular intervals before and after surgery. This allowed them to paint a more detailed picture of how the body physically transforms after this procedure.
The study found that while patients lost substantial amounts of fat and visceral (abdominal) fat — which was expected — they also lost meaningful amounts of muscle mass and fat-free mass. Most of these changes happened within the first year after surgery and then leveled off. A measure called 'phase angle,' which reflects cell health and hydration, initially dropped after surgery but partially recovered over time. Estimated resting energy expenditure also decreased, which makes sense as the body becomes smaller and lighter. Preliminary comparisons suggested men and women, and patients with very high versus moderately high BMIs, may follow somewhat different trajectories, though the small sample size means these differences should be treated cautiously.
This research suggests that tracking body composition — not just weight — after bariatric surgery can reveal important physiological changes that standard measures miss, particularly the loss of muscle alongside fat. The authors highlight that the muscle loss observed warrants attention, and they call for future studies to investigate whether structured nutrition and exercise programs after surgery could help patients preserve more muscle while still losing fat. Because the study relied on BIA rather than more precise imaging methods, and had a relatively small sample from a single center, the findings should be considered estimates that need confirmation in larger, more rigorous studies.
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Lippmann L, Kissler H, Settmacher U, Rauchfuss F, Dondorf F, Rohland O, et al.. (2026). Longitudinal changes in bioelectrical impedance-derived body composition after sleeve gastrectomy in patients with class III obesity.. Langenbeck's archives of surgery. https://doi.org/10.1007/s00423-026-04232-x