Circulating Muscle-specific microRNAs and Renal, Nutritional and Inflammatory Biomarkers of Skeletal Muscle Loss During the First 18 Months After Laparoscopic Sleeve Gastrectomy

Metabolic and bariatric surgery is the most effective durable treatment for severe obesity, but the weight lost is not confined to adipose tissue. A systematic review and meta-analysis of 59 longitudinal studies comprising 2,270 individuals reported pooled twelve-month losses of 8.13 kg (95% CI -9.01 to -7.26) of lean body mass, 8.23 kg (-10.74 to -5.73) of fat-free mass and 3.18 kg (-5.64 to -0.71) of skeletal muscle mass, and established that the loss is markedly front-loaded, with approximately 55% of the twelve-month lean-body-mass loss occurring within the first three post-operative months. A 2026 meta-analysis of 21 randomised trials confirmed the magnitude in contemporary practice, with bariatric surgery producing 9.14 kg of fat-free mass loss representing 32.9% of total weight lost, compared with 31.5% for incretin-based therapy and 14.3% for diet and exercise.

Longitudinal imaging confirms both the magnitude and the timing. Whole-body magnetic resonance imaging demonstrated an 11.1% reduction in skeletal muscle volume at twelve months, with most loss occurring by three months, while serial dual-energy X-ray absorptiometry after sleeve gastrectomy showed lean tissue mass falling approximately 20% by twelve months, after which a new steady state was established. In a 2026 cohort of 179 patients, fat-free mass at six months was the only independent predictor of fat-free mass at twelve months, and 41-46% of patients exceeded the 25% threshold for excessive relative fat-free mass loss.

The clinical consequences are substantial: sarcopenia rose from 8% before surgery to 32% at one year in a prospective cohort of 184 patients, and a recent scoping review concluded that approximately one in four patients undergoing metabolic and bariatric surgery meets criteria for sarcopenic obesity, while noting the absence of standardised diagnostic approaches. Critically, mass and function do not move together: lean mass fell by 7.4 kg in meta-analysis while pooled handgrip change was -0.46 kg and not statistically significant.

Direct body-composition measurement remains the reference standard but is intermittent and cannot signal accelerating catabolism between visits. No validated circulating biomarker of skeletal muscle loss exists for this population. The clinical value of such a marker has risen sharply now that lean-mass preservation is pharmacologically achievable: in a phase 2 trial, bimagrumab plus semaglutide produced lean mass loss of 2.3% versus 6.9% for semaglutide alone.

Muscle-specific microRNAs are plausible candidates. Expression of miR-133a and miR-206 rises during human skeletal muscle development and differentiation, miR-206 promotes myoblast differentiation, and miR-1 and miR-206 regulate satellite-cell proliferation through repression of Pax7. Circulating myomiRs have been proposed as a distinct biomarker class for monitoring skeletal muscle. Human evidence is not uniform: serum hsa-miR-133a-3p was significantly lower in sarcopenic older adults in the SarcoPhAge cohort, and lower circulating miR-133b and miR-206 were associated with sarcopenia and nutritional status in a case-control study, whereas a plasma study found miR-133a and miR-133b unchanged in sarcopenia. Directly relevant to the present population, intraoperative sampling during bariatric surgery demonstrated that miR-206 is approximately 2,000-fold enriched in skeletal muscle relative to visceral adipose tissue, establishing tissue specificity in exactly the patients under study.

Two recent studies temper expectations and are addressed prospectively. An unbiased screen of 352 circulating microRNA targets in sarcopenic obesity identified miR-486-5p among discriminators but did not identify miR-133a-3p or miR-206, and a 2026 trial of twelve-week high-intensity interval training found muscle myomiRs unchanged. Both examined either an established steady-state phenotype or an anabolic training adaptation. The early post-operative window is neither: it is a period of rapid negative energy and protein balance with active muscle catabolism, in which release of muscle-derived microRNAs plausibly reflects ongoing tissue remodelling rather than a stable phenotypic difference. This distinction is testable, and a null result is informative. To guard against target mis-selection, hsa-miR-486-5p is included as a comparator.

Renal markers warrant parallel attention. Creatinine is muscle-derived, and estimation error correlates with appendicular skeletal muscle index, while the difference between cystatin C-based and creatinine-based estimates carries information about muscle quantity. This is operationalised in the sarcopenia index (creatinine ÷ cystatin C × 100), validated in 2026 with an optimal cut-off of 77.3, but never applied in a bariatric surgical cohort. Albumin and prealbumin are inflammation-sensitive and are explicitly not valid proxies for total body protein or muscle mass, although transthyretin tracks lean body mass in other settings; composite inflammation ratios show only modest discrimination. These measures are therefore included as contextual and exploratory markers with a pre-specified realistic performance ceiling.

Three gaps are addressed. First, no study has measured circulating miR-133a-3p or miR-206 in patients undergoing metabolic and bariatric surgery. Second, the creatinine-to-cystatin C sarcopenia index has not been evaluated in this population. Third, the prealbumin/CRP ratio has not been examined against directly measured muscle loss after surgery.