Serum Vitamin D as a Biomarker in Autoimmune, Psychiatric and Neurodegenerative Diseases

Abstract

Vitamin D is a steroid hormone regulating calcium-phosphorus homeostasis, immune response and brain function. In the past thirty years, an increasing number of cohort studies, meta-analyses and randomized controlled trials (RTCs) evaluated the serum levels of 25-hydroxyvitamin D [25(OH)D], which is considered the Vitamin D status biomarker, in patients affected by neurological, psychiatric and autoimmune diseases. Although an association between low 25(OH)D serum levels and the prevalence of these diseases has been found, it is still unclear whether the serum 25(OH)D measurement can be clinically useful as a biomarker for diagnosis, prognosis and predicting treatment response in neurodegeneration, mental illness and immune-mediated disorders. The lack of standardized data, as well as discrepancies among the studies (in the analytical methods, cut-offs, endpoints and study sets), weakened the findings achieved, hindered pooling data, and, consequently, hampered drawing conclusions. This narrative review summarizes the main findings from the studies performed on serum 25(OH)D in neurological, psychiatric and autoimmune diseases, and clarifies whether or not serum 25(OH)D can be used as a reliable biomarker in these diseases.

Keywords: 25(OH)D; Alzheimer’s disease; autoimmune diseases; biomarker; multiple sclerosis; neurodegenerative diseases; psychiatric diseases; standardization; vitamin D.

Figure 1

Vitamin D metabolism pathway. Vitamin D active form is produced in a multi-step process, including the ultraviolet B (UVB) rays’ irradiation of a cutaneous precursor [7-dehydro-cholesterol (7-DHC)], and two hydroxylation steps. In addition to the synthesis of Vitamin D, 7-DHC can be oriented toward the production of cholesterol by Kandutsch–Russell biochemical pathway.

Figure 2

Vitamin D biological functions. Vitamin D’s active form, also known as calcitriol, exerts pleiotropic functions on different cells and organs.