PRISMA 2020 checklist.

ObjectiveThere is widespread agreement about the key role of hemoglobin for oxygen transport. Both observational and interventional studies have examined the relationship between hemoglobin levels and maximal oxygen uptake () in humans. However, there exists considerable variability in the scientific literature regarding the potential relationship between hemoglobin and . Thus, we aimed to provide a comprehensive analysis of the diverse literature and examine the relationship between hemoglobin levels (hemoglobin concentration and mass) and (absolute and relative ) among both observational and interventional studies.MethodsA systematic search was performed on December 6th, 2021. The study procedures and reporting of findings followed Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Article selection and data abstraction were performed in duplicate by two independent reviewers. Primary outcomes were hemoglobin levels and values (absolute and relative). For observational studies, meta-regression models were performed to examine the relationship between hemoglobin levels and values. For interventional studies, meta-analysis models were performed to determine the change in values (standard paired difference) associated with interventions designed to modify hemoglobin levels or . Meta-regression models were then performed to determine the relationship between a change in hemoglobin levels and the change in values.ResultsData from 384 studies (226 observational studies and 158 interventional studies) were examined. For observational data, there was a positive association between absolute and hemoglobin levels (hemoglobin concentration, hemoglobin mass, and hematocrit (PPP = 0.006)).ConclusionThese findings suggest that values are closely associated with hemoglobin levels among both observational and interventional studies. Although our findings suggest a lack of sex differences in these relationships, there were limited studies incorporating females or stratifying results by biological sex.