Systemic adiponectin levels in colorectal cancer and adenoma: a systematic review and meta-analysis

BACKGROUND: Obesity is a well-established risk factor in the development of colorectal cancer; however, the mechanism mediating this relationship is not well understood. The adipokine, adiponectin, has an inverse relationship with obesity. Experimental studies have shown adiponectin to have dichotomous inflammatory and tumorigenic roles. Its role in the development of colorectal cancer, including the potential effect of its increase following bariatric surgery, is not yet clear. There are conflicting results from studies evaluating this relationship. This study sought to provide a systematic review and meta-analysis to examine the association between systemic adiponectin levels in patients with colorectal cancer and adenoma. METHODS: An electronic literature search was performed using PubMed, EMBASE, Web of Science as well as gray literature. Articles were screened for inclusion criteria and assessed for quality using the Newcastle-Ottawa Scale. Pooled mean differences were calculated using a random effects model. Subgroup and meta-regression analyses were performed to identify potential sources of heterogeneity. RESULTS: Thirty-two observational studies comparing systemic adiponectin in colorectal cancer vs healthy controls were included. Colorectal cancer cases had lower systemic adiponectin levels (overall pooled mean difference = -1.05 mu g/ml [95% CI: -1.99; -0.12] p = 0.03); however, significant heterogeneity was present (I-2 = 95% p < 0.01). Subgroup and meta- regression analyses results could not identify a source of the significant heterogeneity across the studies. CONCLUSIONS: Studies suggest a trend towards lower systemic adiponectin levels in colorectal cancer patients, but the heterogeneity observed showed current evidence is not sufficient to definitively draw any conclusions. These data, however, suggest rising adiponectin is unlikely to account for the reported observation of increased CRC following bariatric surgery. Further studies with prospective age, race, and BMI-matched cohorts, and standardized adiponectin measurements may provide a better understanding of this relationship.