Some folic acid is synthesized by bacteria in the intestines. Cooking destroys some forms of folic acid and it is generally advised that raw fruits and vegetables are included into the diet. Strict vegetarians are prone to be deficient in vitamin B12, because it is obtainable only from animal products but are likely to be well supplied with folic acid because it is abundant in vegetables and fruit. Although it may take years for the new vegan to develop the B12 deficiency, it may be masked by a high folic acid intake. The vegetarian's high intake of folic acid allows red blood cells to develop to normal size and maturity, but does not prevent the nerve damage caused by B12 deficiency.
Pregnancy increases the risk and incidence of folic acid deficiency.
Inadequate dietary intake, intestinal malabsorption, and impaired folate storage in the liver all contribute to folate deficiency. Alcohol ingestion also interferes with vitamin B12 absorption. Thiamine deficiency may occur in long-term alcohol users as a consequence of both inadequate ingestion and malabsorption of the vitamin. The brain dysfunction is global, with disordered thinking, feeling, remembering, and disturbed motor coordination. Thiamine replacement corrects the grosser dysfunctions of the brain and it has been proposed that alcoholic beverages be fortified with thiamine as a means of preventing this syndrome.