Table 4.

Main role of selected B vitamins in the nervous system.

Vitamin B	Effects on the Nervous System and Risk of Depression
Vitamin B1 (thiamine)	Thiamine is an important coenzyme during the synthesis of neurotransmitters, such as acetylcholine and serotonin, for example. The most important function of thiamine is considered to be that it makes a major contribution to cellular energy metabolism and, as an essential cofactor in carbohydrate metabolism, it helps to supply energy to nerve cells [84]. An inverse relationship has been shown between thiamine levels and depressive symptoms in adults [85].
Vitamin B6 (pyridoxine)	Pyridoxine functions as a cofactor in the pathways involved in myelin synthesis and enzymatic reactions, including the synthesis of neurotransmitters such as gamma-aminobutyric acid (GABA), serotonin, and dopamine [86]. Furthermore, it controls glutamate excitability and neuronal metabolism. Vitamin B6 and magnesium both modulate neurobiological mechanisms, leading to speculation that they may exert synergistic effects [87]. Pyridoxal-5-phosphate concentrations, the active form of vitamin B6, were measured in Hispanic adults in years 2 and 5 of the study, and it was observed that depressive symptoms were higher in those with low values [88].
Vitamin B9 (folic acid)	Folic acid is involved in the synthesis and metabolism of neurotransmitters associated with depression (serotonin, dopamine, norepinephrine). In addition, it plays a vital role in the regeneration of tetrahydrobiopterin (BH4), a cofactor essential for the formation of neurotransmitters [89]. An association has been shown between lower serum folic acid levels during pregnancy and prenatal depression [90].
Vitamin B12 (cobalamin)	A specific function of vitamin B12 is to participate in the DNA synthesis of myelin-producing oligodendrocytes and in the synthesis of myelin [83,85]. Cobalamin is a cofactor of the methionine synthase enzyme, which catalyses the reaction to transfer a methyl group to a homocysteine molecule. Methionine is formed, which is the precursor of S-adenosylmethionine (SAM). SAM plays an important role in the methylation processes necessary for the normal synthesis and/or metabolism of membrane phospholipids, DNA, RNA, neurotransmitters and for the normal function of the myelin sheaths of nerve fibres [91]. Vitamin B12 deficiency may be associated with impaired glutathione peroxidase activity elevated levels of free radicals. Furthermore, the prevalence of depression tends to be higher among vegetarians due to insufficient intake of vitamin B12 [92].