Effects of exercise training on blood rheology: a meta-analysis

Regular exercise has been reported to improve blood rheology, but all the studies investigating this issue included a reduced number of subjects, and thus it was logic to perform a meta-analysis of them in order to better characterize this physiological effect. Research was handed on Medline from 1950 to 2010. Studies were selected if they were in English and if they had one or several of these following outcomes: lactate, blood viscosity, RBC rigidity, hematocrit (%), RBC aggregation, fibrinogen and plasma viscosity. They were also kept if they included exercise in their protocol. Results were computed with the fixed effect model and the weighting method was the inverse variance. 11 studies with 175 people were found and included in this meta-analysis. None of these studies had the whole mentioned outcomes. The meta-analysis shows significant effects on on RBC aggregation (-0.59 CI 95% [-0.72; -0.46]), whole blood viscosity (-0.30 [-0.31; -0.28] p < 0.001) and hematocrit (-0.296%; CI 95% [-0.57; -0.01]). Aggregation, hematocrit and viscosity data showed significant heterogeneity, aggregation I(2) = 94.95%, hematocrit I(2) = 96.46%, viscosity I(2) = 99.25%. RBC aggregation (0.53 CI 95% [0.40; 0.66]). Three studies could be included for an intervention versus control analysis which shows significant effects on hematocrit (-1.06% CI 95% [-1.43; -0.68]) but not on aggregation, with again a significant heterogeneity (hematocrit I(2) = 96.46%). These results confirm that regular exercise decreases hematocrit and RBC aggregation, but the heterogeneity which is evidenced should be pointed out. This heterogeneity will require a new computation taking into account a "random effect" by using a pooling method. In addition, this heterogeneity leads to conclude that more studies are needed to further analyze these effects which are described by a low number of articles, which could explain some of the non-significant results.