Prevalence and correlates of anemia following Roux-en-Y gastric bypass: a systematic review and meta-analysis

To estimate the prevalence and associated factors of anemia after Roux-en-Y gastric bypass (RYGB).

A comprehensive literature search was conducted across PubMed, EMBASE, and CENTRAL from inception through 18 January 2025, utilizing search terms including anemia, anaemia, hemoglobin, haemoglobin, Hb, gastric bypass, and RYGB. Eligible studies reported anemia prevalence post-RYGB with data on either follow-up duration or Roux limb length.

From 2787 screened records, 74 studies involving 12,262 patients met inclusion criteria. Pooled analysis revealed a 26% overall postoperative anemia prevalence. Temporal stratification revealed a progressive increase in prevalence: 15% at ≤1 year, 27% at >1-5 years, and 35% at >5 years post-surgery. Meta-regression confirmed a significant positive correlation between anemia prevalence and follow-up duration. Studies with inadequate supplementation of iron, vitamin B12, and folic acid reported higher anemia rates compared to those with adequate supplementation. Etiological analyses identified strong associations of anemia with vitamin B12 and iron deficiencies, but not with folic acid deficiency. Demographic risk factors included preoperative anemia, female sex, and baseline BMI >45 kg/m 2 . RYGB with longer Roux limbs showed a trend toward higher anemia risk.

This study quantifies a substantial burden of anemia following RYGB, demonstrating its temporal progression. The findings underscore the necessity for protocolized long-term hematologic surveillance, optimized nutrient repletion strategies targeting iron and vitamin B12, and personalized risk mitigation approaches for high-risk cohorts, particularly women and those with long Roux limbs, preoperative anemia, or severe obesity.