age-related changes in brain metabolites were reported but considerable heterogeneity of metabolite concentrations across studies were found (Kadota et al. 2001; Baker et al. 2008; Maudsley et al. 2009; Chang et al. 1996). Note also that metabolite ratios are difficult to interpret since it is unclear what proportion of the age-dependent increase in NAA/Cho reflects age-dependent increase in NAA, because Cho concentration also increases with age during adolescence (Cloak et al. 2011). Therefore, future MRS studies investigating metabolite changes related to age and sex, or the additional influence of genetic factors, should measure metabolite concentrations rather than ratios.
