( Originally Published 1956 )
The folic acid group of substances has been shown to be essential for blood formation in man; presumably it is necessary for other important functions as demonstrated in several animal species. Macrocytic anemia which responds to folic acid has been observed in infancy, during pregnancy, in sprue and other malabsorption syndromes, in association with poor diets (nutritional macrocytic anemia) and in other less common conditions such as intestinal stricture or following operative procedures in which the intestinal tract was short circuited.
Folic acid is present in foods in both free and conjugated forms, namely, as pteroylglutamic acid, pteroyltriglutamic acid and pteroylheptaglutamic acid. Another member of the folic acid group of substances, folinic acid or citrovorum factor, a formyltetrahydro derivative of folic acid, is also found in natural materials in free and combined form. This compound is also formed in the body from folic acid and is excreted in the urine. Information concerning the amounts of the various folic acid compounds in foods and their biological availability is meagre. The best sources appear to be liver, deep green, leafy vegetables, other green vegetables, kidney, muscle meat and whole wheat cereals. Darby (111) has reported that diets adequate in other respects furnish about 0.1 to 0.4 mg. of folic acid compounds daily, poor diets less than 0.1 mg.
Folic acid is synthesized by intestinal bacteria and this source of supply may be important in man, since experimental deficiency has not been induced by diets low in folic acid. Evidence from studies in animals, and from therapeutic tests in human macrocytic anemia, suggest that the dietary requirement of folic acid is less than 0.2 mg. daily.
Folic acid participates in the synthesis of compounds which are used in the formation of nucleoproteins and in transmethylation processes. It is concerned with incorporation of the "single-carbon unit" into the 2 and 8 positions of the purine ring and the 5-methyl group of thy-mine and with some of the reactions relating to the formation of choline, creatine and histidine (112). Folic acid is essential for maintenance of normal hematopoiesis, exerting a hematologic effect in practically all types of human macrocytic anemia, presumably by its function in the formation of thymine and other purines and pyrimidines. Although folic acid stimulates blood regeneration in pernicious anemia, neurologic lesions are unaffected and hematologic relapse occurs unless vitamin 1312 is ad-ministered.
Folic Acid Deficiency
The syndrome of folic acid deficiency is exemplified by the toxic symptoms which develop during administration of aminopterin, a folic acid antagonist (110). These symptoms include glossitis, diarrhea, gastrointestinal lesions and anemia. Toxic manifestations of aminopterin may be reversed by citrovorum factor, if administered promptly, but not by folic acid. Presumably aminopterin interferes with the conversion of folic acid to its metabolically active form.
Several types of human macrocytic anemia with meg aloblastic bone marrow appear to be primarily syndromes of folic acid deficiency, namely, the macrocytic anemia of pregnancy and the megaloblastic anemia of infancy. Nutritional macrocytic anemia and sprue also appear to be due to deficiency of folic acid in some in-stances while in others, deficiency of Vitamin B12 or both factors may be present. The megaloblastic anemia associated with intestinal stricture or anastomosis and, at times, the macrocytic anemia which occurs in primary liver disease responds to folic acid therapy.
Clinical findings common to both sprue and nutritional macrocytic anemia are the history of an inadequate diet, glossitis, diarrhea, loss of weight and impaired absorption from the gastrointestional tract. In both conditions, a flat glucose intolerance test is a frequent finding. Steatorrhea is charactertistic of sprue, the stools containing large amounts of fatty acids but little neutral fat. Absorption of fat and of fat soluble vitamins is impaired. This may be demonstrated by administration of fat or of oily preparations of vitamin A and measuring the respective blood levels at intervals thereafter (p. 59). Tetany due to hypocalcemia may occur in sprue due to the combination of fatty acids and calcium in the intestinal tract to form insoluble soaps. Recent studies in our laboratory indicate that so-called nutritional macrocytic anemia is, in many instances, a malabsorption syndrome; glucose, fat and vitamin B12 (even when combined with intrinsic factor) are poorly absorbed from the intestinal tract.
Administration of folic acid or citrovorum factor will be followed by hematopoietic response and relief of most of the abnormal findings in sprue and nutrional macrocytic anemia in the majority of subjects. In sprue, steatorrhea may persist and in both conditions, it is at times necessary to continue therapy indefinitely to prevent relapse. In some instances, vitamin B12 is as effective as folic acid in the therapy of these diseases and occasion-ally the two vitamins are more beneficial when given together than is either one alone.
It is interesting to speculate as to the mechanism which leads to folic acid deficiency in The diverse syndromes mentioned above. In several animal species, folic acid has been shown to have a role in the reproductive process (113). It was reported recently that administration of a folic acid antagonist, 4-amino pteroylglutamic acid, to 12 women who were less than three months pregnant was followed by abortion in ten instances (114). Perhaps there is an increased need of folic acid during pregnancy which is not always met by the dietary intake. In such instances, macrocytic anemia might ensue.
Megaloblastic anemia of infancy presents a complex problem; it appears to be related to deficiency of ascorbic acid as well as of folic acid (112). In the United States, this anemia was observed chiefly in infants who received a powdered milk preparation low in vitamin C and has been encountered only rarely since ascorbic acid was added to the formula of this product. The anemia occurs in the same age group in which the incidence of scurvy is high. Megaloblastic anemia of infancy responds to folic acid or citrovorum factor but not to ascorbic acid. May and associates (117) produced an anemia in monkeys, comparable to that seen in infants, by administration of a diet deficient in both ascorbic and folic acid. The anemia could be prevented by ascorbic acid alone but responded only partially to treatment with this vitamin. Therapy with folic acid was completely satisfactory.
In South Africa, megaloblastic anemia has been en-countered in adults with severe scurvy although it is rare . This anemia responds to ascorbic acid. Interrelationships of folic acid, vitamin B12 and ascorbic acid in patients with megaloblastic anemia have been discussed in detail by Mueller and Will (116).
Other metabolic relationships between folic acid and ascorbic acid have been reported. Abnormal excretion of tyrosine metabolites occurs in infants with scurvy and in premature infants given a diet which furnishes large amounts of tyrosine. This abnormal excretion may be prevented or relieved by administration of ascorbic acid or by large doses of folic acid. There is some evidence that ascorbic acid assists in the conversion of folic acid to citrovorum factor or in the release of citrovorum factor from conjugated form .
The pathogenesis of nutritional macrocytic anemia and sprue is not clear. Neither appears to be due solely to dietary deficiency of folic acid, of vitamin 13,2 or of both factors. In some instances, inflammatory disease of the intestinal tract precedes the development of the sprue syndrome and could be responsible for defective absorption but this is not always the case. In some subjects with so-called nutritional macrocytic anemia, relapse occurs when therapy with folic acid or vitamin B12 is discontinued even though an adequate diet is ingested. As noted previously, some of these subjects have defects in absorption which may be permanent; others may have increased requirement and/or abnormal utilization of the anti-anemic vitamins. Some subjects may be deficient in intrinsic factor as are patients with pernicious anemia.
Laboratory tests have been developed which permit measurement of folic acid concentration in blood and excretion of folic acid and citrovorum factor in the urine. In normal subjects receiving ordinary diets, average excretion was reported to be 4.1 and 2.6 micrograms daily for folic acid and citrovorum factor, respectively.
Some reports indicate that patients with pernicious anemia and sprue excrete smaller amounts of folic acid and citrovorum factor after a test dose of folic acid than do normal subiects. Girdwood (120b) has suggested that an excretion of less than 1.5 mg. in 24 hours after an oral test dose of 5.0 mg. of folic acid is indicative of either severe tissue depletion or malabsorption. If tissues have been loaded with folic acid prior to testing, a small excretion after an oral, as compared to a par-enteral, test dose is indicative of malabsorption. Few studies of levels of folic acid in blood have been carried out. None of these procedures has been studied extensively in the evaluation of folic acid nutrition.