To our knowledge, this is the largest FEP cohort with a schizophrenia-spectrum disorder having received CSF diagnostics and vitamin B12 and folate serum assessments. The aim of this investigation was to explore the relationship between BBB dysfunction represented by altered age-adjusted Qalb and vitamin B12 and folate serum levels in FEP patients. The cohort presented here was part of a larger clinical population, that was previously described by our research group [12]. Corresponding to the aforementioned cohort, we report signs of BBB dysfunction (increased Qalb) in 17.1% of the included population. Moreover, a relatively small percentage of the cohort showed decreased vitamin B12 or folate serum levels (10.4% and 11.1% respectively). Put in perspective with data from healthy controls of similar age [33], only 1.0% of omnivores (5.7% of vegetarians and 7.5% of vegans) presented decreased vitamin B12 levels. Nevertheless, in the healthy control cohort a lower vitamin B12 cut-off was applied (150 pg/ml) than in our FEP analyses (250 pg/ml), which could explain the large difference in the prevalence of vitamin B12 deficiency. In the aforementioned study among healthy subjects [33], decreased folate serum levels were reported in 58% of omnivores (30.2% of vegetarians and 13.2% of vegans), a greatly higher level than the 11.1% reported in our cohort, although the cut-off used in the aforementioned publication was set at 6.6 ng/ml and thus higher than the one used in our approach (4.6 ng/ml). As mentioned before, definitions for vitamin B12 and folate deficiency are highly heterogeneous and might explain the heterogeneity in the prevalence of hypovitaminoses across the literature. In this context, it must be noted that confounding factors such as nutritional and lifestyle habits or body-mass-index were not recorded, so that it remains difficult to pinpoint a cause for the higher prevalence of vitamin B12 deficiency and the reduced prevalence of decreased folate levels in our cohort compared to the healthy control cohort. When assessing the relation between BBB dysfunction and vitamin B12 or folate deficiency, no statistically significant association could be found. Nevertheless, a potential causal relationship between BBB disruption and vitamin deficiencies cannot be excluded based on our cross-sectional retrospective data.

In our cohort, the mean duration of illness was 13.4 months, which could imply that not all patients with a vitamin B12 deficiency already had low vitamin B12 serum levels at an early disease stage. It is known that a vitamin B12 deficiency can be preclinical for more than 3–5 years [34], thus the results have to be interpreted with caution. Further investigation is needed to evaluate the course of especially FEP patients and vitamin B12 levels, starting with the prodromal stage of psychosis and following the course of the disease.

When assessing brain MRI data, we found alterations in 40.1% of patients. A much lower rate than the approximately 70% reported in another large cohort which was not restricted to FEP patients [11]. In our cohort we report WMLs in 29.3% of brain MRI scans. As a reference for comparison: WMLs occurred in around 5.3% of healthy controls with a similar mean age [35]. We found no statistically significant association between WMLs occurrence and increased Qalb levels as the pathogenesis of WMLs remains controversial and in need of more appropriate investigations [36].

A major limitation of the present study is its retrospective design. As a consequence, several patients did not receive all assessments included in this analysis. Thus, from a cohort of initially 687 FEP patients, only 222 underwent a lumbar puncture as part of the clinical routine and received an assessment of vitamin B12 and folate levels within 60 days prior or following a lumbar puncture. Hypotheses for the lack of CSF diagnostics in a substantial part of the cohort are difficult to test retrospectively, whereas vitamin B12 and folate deficiencies are rarely examined in general and rather have the status of optional examinations. This is astonishing, since there is evidence that supplementation of certain vitamins may reduce psychiatric symptoms in people with schizophrenia [26]. Of note, the assessment of vitamin B12 and folate serum levels among FEP patients is currently suggested as facultative in some guidelines [37] or completely omitted in others [38]. Moreover, in a few cases absolute values for vitamin B12 and folate were missing in our analyses, since these values were not specified when minimal and maximal cut-offs were exceeded. Since different cut-offs for vitamin B12 and folate levels were used in our tertiary care hospital during the period of interest, we used a pragmatic approach for our analyses. Of note, applying different reference values with lower cut-offs would not have a substantial impact on the results of our comparison analyses. Furthermore, a vitamin B12 deficiency syndrome can be associated with normal vitamin B12 serum levels. Around 80% of the circulating biomarker vitamin B12, determined in the serum is protein-bound to haptocorrin and therefore not bioavailable for cellular uptake [39]. In addition, the serum and cellular levels of vitamin B12 differ, so that the diagnostic value of a serum sample is reduced [39]. Nevertheless, serum B12 remains a valuable biomarker to better characterize the prognosis and status of several diseases [40], but it has a limited diagnostic value as a stand-alone biomarker. Complementing the diagnostics of a vitamin B12 deficiency through assessment of serum holotranscobalamin and MMA can increase the sensitivity in assessing a sub-clinical functional vitamin B12 deficiency [41] and should be considered in future studies. This could be particularly important for FEP cohorts, since a functional B12 deficiency may be present even if it cannot be detected by measuring vitamin B12 serum levels alone. Given the association between reduced vitamin D and illness severity [21], a more thorough assessment of the vitamin status might be recommended for future clinical practice. However, among the selected population only data regarding vitamin B12 and folate were available. For this reason, determining the status of other key vitamins was not possible. Given the retrospective design of this study, it was not possible to determine whether patients presenting a B12 or folate deficiency received any vitamin supplementation. Although lumbar punctures were performed within days of the vitamin assessment, a possible role of supplementation in offsetting a potential BBB dysfunction cannot be ruled out. It is important to mention the relative inhomogeneity regarding characteristics, e.g. age and comorbidities in our cohort. This is due to the retrospective study design with a large sample size, hence a broader and less fine selected patient’s population is assessed which ultimately better represents real world settings. Finally, no information were available regarding symptom severity as measured by standardized questionnaires, limiting the investigation of possible correlations between biological markers and impairments in different symptom domains.

In summary, our study contributes to an increased knowledge about vitamin deficiencies in FEP patients. To draw potential associations between vitamin deficiencies and symptoms severity, prospective studies are needed. We recommend that these studies should employ CSF diagnostics beyond the clinical routine, standardized (follow-up) measurements of vitamin levels as well as assessments of cognitive functioning and symptom severity to further foster the evidence with regard to the prevalence and clinical implications of vitamin deficiency syndromes in FEP. Overall, this study provides epidemiological evidence regarding vitamin deficiencies and BBB dysfunctions in first-episode schizophrenia-spectrum psychosis. We believe this work could raise awareness on this important topic and pave the way for prospective implementation of vitamin assessments in the clinical practice and especially in the diagnostic algorithms of psychosis.