Overall, d-galactose administration in rodents impaired cognition-related outcomes by 1.79 SMD (95% confidence interval (CI), 1.49 to 2.08, 78 comparisons; Fig 2). There was substantial heterogeneity between studies (χ2 = 382.37, I2 = 79.9%, degree of freedom (d.f.) = 77, p <0.0005). D-galactose administration also resulted in neurochemical changes consistent with ageing (effect size 3.20 SMD, 95% confidence interval (CI) 2.95 to 3.44, 250 comparisons, S2 File). Again, there was substantial heterogeneity between studies (χ2 = 1517.45, I2 = 83.6%, d.f. = 249, p <0.0005).