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Iron, B12 and folate
Basic Science
Iron, B12 and folate
The distribution and daily flow of iron in the healthy normal adult
Claire Roddie Bernard Davis
Erythron (30mg/kg)
Myoglobin (4mg/kg)
20–30 mg/day
Macrophages
20–30 mg/day
20–30 mg/day
Abstract Iron, B12 and folate are required for essential metabolic functions in the body. The deficiency states of these nutrients are clinically important because they are common conditions and, untreated, can cause severe morbidity and, previously for pernicious anaemia, mortality. Although disordered haematopoiesis is a frequent finding in iron, B12 and folate deficiency, widespread effects in other organ systems may also occur and may precede the appearance of haematological abnormalities. Investigation of suspected iron, B12 or folate deficiency should be directed at establishing that the deficiency state exists, as well as determining its cause, and should always include a detailed history and physical examination as well as relevant laboratory tests. Correcting the deficiency with supplements is usually straightforward, provided adherence to treatment is ensured. The underlying cause of the nutritional deficiency should also be treated where possible. Blood transfusion should be avoided unless symptoms dictate otherwise.
2–3mg/day Storage iron (6–11mg/kg)
Iron containing enzymes (6–11mg/kg)
1–2mg/day
1–2mg/day
Duodenal enterocytes
Gut, skin, urinary tract
Figure 1
if intake and absorption do not keep pace with requirements or losses. Conversely, as there is no physiological mechanism for excreting iron that is surplus to the body’s requirements, iron accumulates in pathological states of increased gut absorption (e.g. hereditary haemochromatosis) or following repeated blood transfusions (e.g. thalassaemia major).3
Keywords anaemia; deficiency; folate; iron; megaloblastic; vitamin B12
Iron
Absorption and transport of iron Dietary iron is present in the form of haem (red meat, liver) and non-haem iron (cereals, vegetables). An average Western diet contains 10–15 mg per day of iron of which only 5–10% is absorbed, mainly in the duodenum and proximal jejunum. Unlike non-haem iron, haem iron absorption does not require prior solubilization of the iron by gastric acid and ascorbic acid and is not inhibited by gut lumen factors such as phytates. The iron in plasma is bound to transferrin, with only about 30% of its iron binding sites occupied. The majority of transferrinbound iron is used to synthesize haemoglobin. At the end of their life-span, red cells are phagocytosed in macrophages of the reticuloendothelial system, where iron is released from haemoglobin, and either stored as ferritin or exported back to plasma as required for erythropoiesis. Transferrin-bound iron in plasma is also in exchange with storage iron in hepatocytes, myoglobin in muscle and with iron pools within parenchymal cells where iron is utilized for metabolic processes that are essential to life (Figure 1). Daily iron loss occurs mainly through exfoliation of gut mucosal cells, with minor losses from exfoliation of skin and urinary tract cells.
The average body iron content in health is around 40–50 mg/kg body weight, with approximately 75% present in the erythron as haemoglobin and the remainder distributed in various tissues (Figure 1). Between 0 and 2 g of body iron is stored as ferritin and haemosiderin, predominantly in the liver, spleen and bone marrow, the actual amount at any one time depending on the balance between dietary intake and the requirements of the individual1, with hepcidin being the master regulator. Body iron is regulated at both cellular and systemic levels by a complex system of proteins.2 Under normal circumstances the absorption of dietary iron precisely matches obligatory daily losses from the gut, skin and genitourinary tract.1 Menstruation, pregnancy, early infancy and adolescence impose extra demands on daily requirements. Although the absorption of iron can be increased to cope with these demands, deficiency will develop
Claire Roddie MRCP FRCPath is a Specialist Registrar in Haematology at the Whittington Hospital, London, UK. Competing interests: none declared. Bernard Davis FRCP FRCPath is Consultant Haematologist at the Whittington Hospital and Honorary Senior Lecturer at the Royal Free and University College Medical School, London, UK. Competing interests: none declared.
MEDICINE 37:3
Transferrin
Iron deficiency Iron deficiency anaemia (IDA) is the commonest cause of anaemia worldwide with significant economic effects as well as individual 125
© 2008 Published by Elsevier Ltd.
Basic Science
A poor response to oral iron supplementation may be due to poor compliance (commonest), malabsorption, wrong diagnosis (e.g. thalassaemia trait) or uncontrolled blood loss. Where oral iron cannot be tolerated or is ineffective, parenteral replacement is justified.6 Iron stores are rapidly replenished by parenteral preparations, but haemoglobin will rise at the same rate as if oral iron were used. A rare but life-threatening side effect of parenteral iron is anaphylaxis; intravenous preparations should be avoided if a history of hypersensitivity exists. A test dose should always be given prior to infusing the whole dose. Blood transfusion is usually not required for IDA.
morbidity. IDA begins with depletion of tissue iron stores, then proceeds with iron-deficient erythropoiesis and finally anaemia if negative iron balance continues.1,4 Other features of tissue deficiency include koilonychia, angular stomatitis, painful glossitis, pharyngeal webs causing dysphagia (Patterson–Kelly syndrome), atrophic gastritis, pica, and irritability and cognitive impairment in children.5 Causes include: • iron-poor diet • malabsorption – coeliac disease, gastrectomy • increased physiological demands – prematurity/growth/ pregnancy • chronic blood loss – gastrointestinal (GI) losses and menorrhagia are common causes. Neoplastic disease of the GI tract should be excluded in older patients presenting with unexplained iron deficiency and in any patient with a positive family history of colorectal cancer.
Iron overload Excess iron is toxic to parenchymal cells through the formation of highly oxidative free radicals in the presence of non-transferrin bound iron and through accumulation of intracellular haemosiderin. The main organs affected are the liver, skin, endocrine glands and the heart. Iron accumulation occurs in haemochromatosis, chronic transfusion programmes, and some haematological conditions including myelodysplastic syndromes and some congenital haemolytic anaemias. Treatment requires venesections with haemochromatosis and iron chelation therapy for patients with transfusion-dependent anaemia.
Investigation Confirm iron deficiency • Full blood count (FBC) – note that anaemia may not yet have developed. Thrombocytosis is common in iron deficiency, even in the absence of active bleeding. • Blood film (microcytosis, hypochromia, poikilocytosis, target cells, pencil cells). • Serum iron, TIBC and ferritin. Ferritin is decreased in IDA and elevated in iron overload. However, it is also an acute phase protein and levels rise in inflammatory states and acute illness, which may obscure its relevance in iron homeostasis. • Bone marrow aspirate and trephine (Perl’s stain) – rarely needed. Absent stainable iron confirms iron deficiency. • Serum soluble transferrin receptor – values rise in iron deficiency and can help differentiate iron deficiency from anaemia of chronic disease, but the test is not widely available. Determine the underlying cause • Dietary and menstrual history. • Endoscopy/colonoscopy – in individuals with GI symptoms or in asymptomatic individuals with unexplained iron deficiency. • Anti-tissue transglutaminase (anti-tTG) – if coeliac disease is suspected. • Stool for ova, cysts and parasites (in the presence of a positive travel history; hookworm is the commonest cause worldwide).
Megaloblastic anaemia Both B12 and folate deficiencies cause megaloblastic anaemia. Their actions are closely linked but there are some differences in the resulting clinical features.
Vitamin B12 Vitamin B12 (cobalamin) is synthesized by microorganisms and exists in different chemical forms only in foods of animal origin, including milk, cheese and eggs. Daily requirements and losses are 1–4 μg and an average Western diet contains about 5–30 μg/day. Body stores of 3–4 mg (mainly in the liver) are sufficient for about 3 years without further supply. B12 resists cooking. Absorption After its liberation from ingested food by proteolytic enzymes, B12 combines with intrinsic factor (IF), which is secreted by gastric parietal cells (GPC), to form a complex that binds to specific receptor sites on the mucosal surface in the terminal ileum.7 There, the B12 is absorbed, dissociating from the B12:IF complex in the process. On entering the portal circulation it binds to the carrier proteins transcobalamin I, II and III.7 Only transcobalamin II is able to deliver B12 to the bone marrow, therefore its deficiency causes megaloblastic anaemia.7 B12 acts as a coenzyme in the methylation of homocysteine to methionine and in the conversion of l-methylmalonyl coenzyme A to succinyl coenzyme A. The conversion of homocysteine to methionine is essential to the generation of metabolically active forms of folate that are required for DNA synthesis.4
Treatment The cause of the iron deficiency must be found and treated. A history of menorrhagia and/or pregnancy may be acceptable as the cause without extensive further investigation before treatment is started, providing response to treatment is subsequently tested. In all other cases the cause should be assiduously determined, particularly to exclude or identify gastrointestinal (GI) malignancy. Ferrous sulphate given for 4–6 months will correct the anaemia, normalize the morphological abnormalities and replenish tissue stores in that order. It is reasonably well tolerated but GI disturbance can be problematic in a minority of patients. The cause of the iron deficiency must be found and treated. Ferrous sulphate, 200 mg three times a day, is the recommended dose, but many patients, particularly during pregnancy, find this hard to take. A lower dose or alternative preparation may be needed.
MEDICINE 37:3
Vitamin B12 deficiency B12 deficiency may be caused by dietary deficiency, ineffective absorption or metabolic inhibition. 126
© 2008 Published by Elsevier Ltd.
Basic Science
• Dietary deficiency. Strict vegans are at risk of levels low enough to cause megaloblastic anaemia. • Impaired absorption. Pernicious anaemia (PA) is the commonest cause. B12 malabsorption results from the presence of GPC autoantibodies (leading to reduced synthesis of IF) and/or IF antibodies, which either prevent binding of B12 to IF or inhibit binding of the B12:IF complex to its ileal receptor. Other causes are gastrectomy, terminal ileal disease/resection, bacterial overgrowth in small intestine, alcoholism, fish tapeworm and tropical sprue. • Deranged metabolism. Nitrous oxide inhibits the catalytic action of B12 as a co-factor of methionine synthase (α methyltetrahydrofolate-homocysteine S-methyl transferase; MTR) which interferes with the methylation needed for myelin sheath integrity. Repeated exposure, particularly with low B12 level, may lead to subacute combined degeneration of the spinal cord (SACD) even without megaloblatic anaemia. TCII deficiency is an inherited cause of B12 deficiency. The main clinical effects of B12 deficiency are megaloblastic anaemia and neurological complications. • Effects on rapidly dividing tissues. Bone marrow – megaloblastic anaemia, neutropenia, thrombocytopenia. Gut epithelium – atrophic glossitis, angular stomatitis. • Effects on neural tissue. Peripheral neuropathy, SACD of the cord, optic atrophy and neuropsychiatric disturbance, including dementia. The exact mechanism of the neurological complications is obscure.
Investigation of megaloblastic anaemia • FBC and film: anaemia (may not be present), macrocytosis, increased MCH(C), due to over-haemoglobinization, hypersegmented neutrophils and tear drop cells. • Neutropenia and/or thrombocytopenia may be present. • Bone marrow examination is usually not required since the diagnosis is often easily made from the typical blood film features and demonstration of low B12 or folate stores. Megaloblasts in the hypercellular marrow have occasionally been confused with malignant blasts and inappropriate therapy given. • Increased unconjugated bilirubin and LDH (due to ineffective erythropoiesis). • Serum B12 levels – in PA, usually markedly low at diagnosis. Pregnancy, oral contraceptives and HIV infection are common causes of low serum B12 levels in the face of normal tissue stores.9 • Serum and red cell folate (RCF) – both reduced in folate deficiency, but RCF is a better indicator of tissue stores. In B12 deficiency, serum folate levels are normal or high, but RCF is normal or low due to the interruption in intracellular folate metabolism. • GPC and IF antibodies – GPC antibodies are detectable in 90% of patients with PA but are non-specific. IF antibodies occur in some 50% of PA patients but are specific. A Schilling test is not necessary in patients with strongly positive IF antibodies. • Schilling test – a test of B12 absorption. Correction of B12 malabsorption by IF indicates PA, whereas non-correction is seen in terminal ileal disease and small bowel colonization by bacteria. This test can be done even in patients receiving B12 supplementation and therefore treatment should not be delayed whilst awaiting this test. Regulations on the use of radioactive materials have reduced the availability of this test. • Anti-tTG – if coeliac disease is suspected.
Folate Folic acid is found in highest concentration in liver and green vegetables. The daily adult requirement is 100 μg, and body stores (around 10 mg) are usually sufficient for only about 4 months. Folic acid is heat labile and is destroyed by cooking. Lack of folate leads to megaloblastic anaemia as a result of defective DNA synthesis. Other tissue effects of low folate mirror those of B12, with the exception of neuropathy, which appears to be less prevalent. However, the incidence of neural tube defects in foetuses is high in folate deficiency. The risk can be substantially reduced by preconceptional folate supplementation.8 Causes of folate deficiency include: • dietary lack – poverty is a risk factor • malabsorption – e.g. coeliac disease and tropical sprue • increased utilization ○ pregnancy (usually coupled with a poor diet), prematurity ○ chronic haemolytic anaemia, myelofibrosis ○ malignancies – e.g. leukaemia, lymphoma, carcinoma ○ inflammatory conditions – e.g. severe psoriasis, Crohn’s, exfoliative dermatitis. Dietary insufficiency may be a contributory factor • drugs – by a variety of mechanisms. Functional folate deficiency can arise with normal serum folate levels in the presence of folate antagonists (e.g. methotrexate), which inhibit the enzyme dihydrofolate reductase and hence interfere with thymidine and DNA synthesis. This problem can be avoided by the concurrent use of folinic acid that bypasses this enzymatic step.
MEDICINE 37:3
Treatment of megaloblastic anaemia B12 deficiency Loading doses of hydroxocobalamin 1 mg are administered intramuscularly three times per week to a total of six doses in the first 2 weeks. Thereafter, it is given 3-monthly for life in cases of B12 malabsorption.10 However, in dietary deficiency, maintenance oral supplementation with cyanocobalamin may be given once tissues stores have been replenished. Blood transfusion should be avoided even in severe anaemia, unless symptoms or cardiovascular risks dictate otherwise. A blunted response to B12 is usually due to depletion of iron and/or folate stores as massive numbers of new red cells are formed, therefore a short period of iron and folate supplementation may be required. Transient hypokalaemia often occurs during treatment in severely anaemic patients. Neurological complications may develop in the absence of anaemia, therefore it is imperative to commence treatment as soon as the diagnosis is made. Folic acid must not be initiated before B12 as this can precipitate or aggravate neurological complications.8 Folate deficiency Folic acid is given at a dose of 5 mg daily orally for about 4 months to replenish tissue stores.10 Dietetic advice should be given if dietary deficiency is the cause. Prophylactic folate supplements, 127
© 2008 Published by Elsevier Ltd.
Basic Science
400 μg daily, are recommended for potentially pregnant women to reduce fetal neural tube defects. ◆
6 Cook JD. Diagnosis and management of iron-deficiency anaemia. Best Pract Res Clin Haematol 2005; 18: 319–32. 7 Seetharam B, Yammani RR. Cobalamin transport proteins and their cell-surface receptors. Expert Rev Mol Med 2003; 5: 1–8. 8 Reynolds EH. Benefits and risks of folic acid to the nervous system. J Neurol Neurosurg Psychiatry 2002; 72: 567–71. 9 Carmel R, Green R, Rosenblatt DS, et al. Update on cobalamin, folate, and homocyteine. Hematology Am Soc Hematol Educ Program 2003: 62–81. 10 Joint Formulary Committee. British National Formulary, 56th edn. London: British Medical Association and Royal Pharmaceutical Society of Great Britain, 2008.
References 1 Bothwell TH, Charlton RW, Cook JD, Finch CA, eds. Iron metabolism in man. Oxford: Blackwell Scientific Publications, 1979. 2 Mackenzie EL, Iwasaki K, Tsuji Y. Intracellular iron transport and storage: from molecular mechanisms to health implications. Antioxid Redox Signal 2008; 10: 997–1030. 3 Porter JB. Practical management of iron overload. Br J Haematol 2001; 115: 239–52. 4 Koury MJ, Ponka P. New insights into erythropoieisis: the roles of folate, vitamin B12, and iron. Annu Rev Nutr 2004; 24: 105–31. 5 Beard JL, Connor JR. Iron status and neural functioning. Annu Rev Nutr 2003; 23: 41–58.
MEDICINE 37:3
Further reading Nemeth E. Iron regulation and erythropoiesis. Curr Opin Hematol 2008; 15: 169–75.
128
© 2008 Published by Elsevier Ltd.
Iron, B12 and folate
The distribution and daily flow of iron in the healthy normal adult
Claire Roddie Bernard Davis
Erythron (30mg/kg)
Myoglobin (4mg/kg)
20–30 mg/day
Macrophages
20–30 mg/day
20–30 mg/day
Abstract Iron, B12 and folate are required for essential metabolic functions in the body. The deficiency states of these nutrients are clinically important because they are common conditions and, untreated, can cause severe morbidity and, previously for pernicious anaemia, mortality. Although disordered haematopoiesis is a frequent finding in iron, B12 and folate deficiency, widespread effects in other organ systems may also occur and may precede the appearance of haematological abnormalities. Investigation of suspected iron, B12 or folate deficiency should be directed at establishing that the deficiency state exists, as well as determining its cause, and should always include a detailed history and physical examination as well as relevant laboratory tests. Correcting the deficiency with supplements is usually straightforward, provided adherence to treatment is ensured. The underlying cause of the nutritional deficiency should also be treated where possible. Blood transfusion should be avoided unless symptoms dictate otherwise.
2–3mg/day Storage iron (6–11mg/kg)
Iron containing enzymes (6–11mg/kg)
1–2mg/day
1–2mg/day
Duodenal enterocytes
Gut, skin, urinary tract
Figure 1
if intake and absorption do not keep pace with requirements or losses. Conversely, as there is no physiological mechanism for excreting iron that is surplus to the body’s requirements, iron accumulates in pathological states of increased gut absorption (e.g. hereditary haemochromatosis) or following repeated blood transfusions (e.g. thalassaemia major).3
Keywords anaemia; deficiency; folate; iron; megaloblastic; vitamin B12
Iron
Absorption and transport of iron Dietary iron is present in the form of haem (red meat, liver) and non-haem iron (cereals, vegetables). An average Western diet contains 10–15 mg per day of iron of which only 5–10% is absorbed, mainly in the duodenum and proximal jejunum. Unlike non-haem iron, haem iron absorption does not require prior solubilization of the iron by gastric acid and ascorbic acid and is not inhibited by gut lumen factors such as phytates. The iron in plasma is bound to transferrin, with only about 30% of its iron binding sites occupied. The majority of transferrinbound iron is used to synthesize haemoglobin. At the end of their life-span, red cells are phagocytosed in macrophages of the reticuloendothelial system, where iron is released from haemoglobin, and either stored as ferritin or exported back to plasma as required for erythropoiesis. Transferrin-bound iron in plasma is also in exchange with storage iron in hepatocytes, myoglobin in muscle and with iron pools within parenchymal cells where iron is utilized for metabolic processes that are essential to life (Figure 1). Daily iron loss occurs mainly through exfoliation of gut mucosal cells, with minor losses from exfoliation of skin and urinary tract cells.
The average body iron content in health is around 40–50 mg/kg body weight, with approximately 75% present in the erythron as haemoglobin and the remainder distributed in various tissues (Figure 1). Between 0 and 2 g of body iron is stored as ferritin and haemosiderin, predominantly in the liver, spleen and bone marrow, the actual amount at any one time depending on the balance between dietary intake and the requirements of the individual1, with hepcidin being the master regulator. Body iron is regulated at both cellular and systemic levels by a complex system of proteins.2 Under normal circumstances the absorption of dietary iron precisely matches obligatory daily losses from the gut, skin and genitourinary tract.1 Menstruation, pregnancy, early infancy and adolescence impose extra demands on daily requirements. Although the absorption of iron can be increased to cope with these demands, deficiency will develop
Claire Roddie MRCP FRCPath is a Specialist Registrar in Haematology at the Whittington Hospital, London, UK. Competing interests: none declared. Bernard Davis FRCP FRCPath is Consultant Haematologist at the Whittington Hospital and Honorary Senior Lecturer at the Royal Free and University College Medical School, London, UK. Competing interests: none declared.
MEDICINE 37:3
Transferrin
Iron deficiency Iron deficiency anaemia (IDA) is the commonest cause of anaemia worldwide with significant economic effects as well as individual 125
© 2008 Published by Elsevier Ltd.
Basic Science
A poor response to oral iron supplementation may be due to poor compliance (commonest), malabsorption, wrong diagnosis (e.g. thalassaemia trait) or uncontrolled blood loss. Where oral iron cannot be tolerated or is ineffective, parenteral replacement is justified.6 Iron stores are rapidly replenished by parenteral preparations, but haemoglobin will rise at the same rate as if oral iron were used. A rare but life-threatening side effect of parenteral iron is anaphylaxis; intravenous preparations should be avoided if a history of hypersensitivity exists. A test dose should always be given prior to infusing the whole dose. Blood transfusion is usually not required for IDA.
morbidity. IDA begins with depletion of tissue iron stores, then proceeds with iron-deficient erythropoiesis and finally anaemia if negative iron balance continues.1,4 Other features of tissue deficiency include koilonychia, angular stomatitis, painful glossitis, pharyngeal webs causing dysphagia (Patterson–Kelly syndrome), atrophic gastritis, pica, and irritability and cognitive impairment in children.5 Causes include: • iron-poor diet • malabsorption – coeliac disease, gastrectomy • increased physiological demands – prematurity/growth/ pregnancy • chronic blood loss – gastrointestinal (GI) losses and menorrhagia are common causes. Neoplastic disease of the GI tract should be excluded in older patients presenting with unexplained iron deficiency and in any patient with a positive family history of colorectal cancer.
Iron overload Excess iron is toxic to parenchymal cells through the formation of highly oxidative free radicals in the presence of non-transferrin bound iron and through accumulation of intracellular haemosiderin. The main organs affected are the liver, skin, endocrine glands and the heart. Iron accumulation occurs in haemochromatosis, chronic transfusion programmes, and some haematological conditions including myelodysplastic syndromes and some congenital haemolytic anaemias. Treatment requires venesections with haemochromatosis and iron chelation therapy for patients with transfusion-dependent anaemia.
Investigation Confirm iron deficiency • Full blood count (FBC) – note that anaemia may not yet have developed. Thrombocytosis is common in iron deficiency, even in the absence of active bleeding. • Blood film (microcytosis, hypochromia, poikilocytosis, target cells, pencil cells). • Serum iron, TIBC and ferritin. Ferritin is decreased in IDA and elevated in iron overload. However, it is also an acute phase protein and levels rise in inflammatory states and acute illness, which may obscure its relevance in iron homeostasis. • Bone marrow aspirate and trephine (Perl’s stain) – rarely needed. Absent stainable iron confirms iron deficiency. • Serum soluble transferrin receptor – values rise in iron deficiency and can help differentiate iron deficiency from anaemia of chronic disease, but the test is not widely available. Determine the underlying cause • Dietary and menstrual history. • Endoscopy/colonoscopy – in individuals with GI symptoms or in asymptomatic individuals with unexplained iron deficiency. • Anti-tissue transglutaminase (anti-tTG) – if coeliac disease is suspected. • Stool for ova, cysts and parasites (in the presence of a positive travel history; hookworm is the commonest cause worldwide).
Megaloblastic anaemia Both B12 and folate deficiencies cause megaloblastic anaemia. Their actions are closely linked but there are some differences in the resulting clinical features.
Vitamin B12 Vitamin B12 (cobalamin) is synthesized by microorganisms and exists in different chemical forms only in foods of animal origin, including milk, cheese and eggs. Daily requirements and losses are 1–4 μg and an average Western diet contains about 5–30 μg/day. Body stores of 3–4 mg (mainly in the liver) are sufficient for about 3 years without further supply. B12 resists cooking. Absorption After its liberation from ingested food by proteolytic enzymes, B12 combines with intrinsic factor (IF), which is secreted by gastric parietal cells (GPC), to form a complex that binds to specific receptor sites on the mucosal surface in the terminal ileum.7 There, the B12 is absorbed, dissociating from the B12:IF complex in the process. On entering the portal circulation it binds to the carrier proteins transcobalamin I, II and III.7 Only transcobalamin II is able to deliver B12 to the bone marrow, therefore its deficiency causes megaloblastic anaemia.7 B12 acts as a coenzyme in the methylation of homocysteine to methionine and in the conversion of l-methylmalonyl coenzyme A to succinyl coenzyme A. The conversion of homocysteine to methionine is essential to the generation of metabolically active forms of folate that are required for DNA synthesis.4
Treatment The cause of the iron deficiency must be found and treated. A history of menorrhagia and/or pregnancy may be acceptable as the cause without extensive further investigation before treatment is started, providing response to treatment is subsequently tested. In all other cases the cause should be assiduously determined, particularly to exclude or identify gastrointestinal (GI) malignancy. Ferrous sulphate given for 4–6 months will correct the anaemia, normalize the morphological abnormalities and replenish tissue stores in that order. It is reasonably well tolerated but GI disturbance can be problematic in a minority of patients. The cause of the iron deficiency must be found and treated. Ferrous sulphate, 200 mg three times a day, is the recommended dose, but many patients, particularly during pregnancy, find this hard to take. A lower dose or alternative preparation may be needed.
MEDICINE 37:3
Vitamin B12 deficiency B12 deficiency may be caused by dietary deficiency, ineffective absorption or metabolic inhibition. 126
© 2008 Published by Elsevier Ltd.
Basic Science
• Dietary deficiency. Strict vegans are at risk of levels low enough to cause megaloblastic anaemia. • Impaired absorption. Pernicious anaemia (PA) is the commonest cause. B12 malabsorption results from the presence of GPC autoantibodies (leading to reduced synthesis of IF) and/or IF antibodies, which either prevent binding of B12 to IF or inhibit binding of the B12:IF complex to its ileal receptor. Other causes are gastrectomy, terminal ileal disease/resection, bacterial overgrowth in small intestine, alcoholism, fish tapeworm and tropical sprue. • Deranged metabolism. Nitrous oxide inhibits the catalytic action of B12 as a co-factor of methionine synthase (α methyltetrahydrofolate-homocysteine S-methyl transferase; MTR) which interferes with the methylation needed for myelin sheath integrity. Repeated exposure, particularly with low B12 level, may lead to subacute combined degeneration of the spinal cord (SACD) even without megaloblatic anaemia. TCII deficiency is an inherited cause of B12 deficiency. The main clinical effects of B12 deficiency are megaloblastic anaemia and neurological complications. • Effects on rapidly dividing tissues. Bone marrow – megaloblastic anaemia, neutropenia, thrombocytopenia. Gut epithelium – atrophic glossitis, angular stomatitis. • Effects on neural tissue. Peripheral neuropathy, SACD of the cord, optic atrophy and neuropsychiatric disturbance, including dementia. The exact mechanism of the neurological complications is obscure.
Investigation of megaloblastic anaemia • FBC and film: anaemia (may not be present), macrocytosis, increased MCH(C), due to over-haemoglobinization, hypersegmented neutrophils and tear drop cells. • Neutropenia and/or thrombocytopenia may be present. • Bone marrow examination is usually not required since the diagnosis is often easily made from the typical blood film features and demonstration of low B12 or folate stores. Megaloblasts in the hypercellular marrow have occasionally been confused with malignant blasts and inappropriate therapy given. • Increased unconjugated bilirubin and LDH (due to ineffective erythropoiesis). • Serum B12 levels – in PA, usually markedly low at diagnosis. Pregnancy, oral contraceptives and HIV infection are common causes of low serum B12 levels in the face of normal tissue stores.9 • Serum and red cell folate (RCF) – both reduced in folate deficiency, but RCF is a better indicator of tissue stores. In B12 deficiency, serum folate levels are normal or high, but RCF is normal or low due to the interruption in intracellular folate metabolism. • GPC and IF antibodies – GPC antibodies are detectable in 90% of patients with PA but are non-specific. IF antibodies occur in some 50% of PA patients but are specific. A Schilling test is not necessary in patients with strongly positive IF antibodies. • Schilling test – a test of B12 absorption. Correction of B12 malabsorption by IF indicates PA, whereas non-correction is seen in terminal ileal disease and small bowel colonization by bacteria. This test can be done even in patients receiving B12 supplementation and therefore treatment should not be delayed whilst awaiting this test. Regulations on the use of radioactive materials have reduced the availability of this test. • Anti-tTG – if coeliac disease is suspected.
Folate Folic acid is found in highest concentration in liver and green vegetables. The daily adult requirement is 100 μg, and body stores (around 10 mg) are usually sufficient for only about 4 months. Folic acid is heat labile and is destroyed by cooking. Lack of folate leads to megaloblastic anaemia as a result of defective DNA synthesis. Other tissue effects of low folate mirror those of B12, with the exception of neuropathy, which appears to be less prevalent. However, the incidence of neural tube defects in foetuses is high in folate deficiency. The risk can be substantially reduced by preconceptional folate supplementation.8 Causes of folate deficiency include: • dietary lack – poverty is a risk factor • malabsorption – e.g. coeliac disease and tropical sprue • increased utilization ○ pregnancy (usually coupled with a poor diet), prematurity ○ chronic haemolytic anaemia, myelofibrosis ○ malignancies – e.g. leukaemia, lymphoma, carcinoma ○ inflammatory conditions – e.g. severe psoriasis, Crohn’s, exfoliative dermatitis. Dietary insufficiency may be a contributory factor • drugs – by a variety of mechanisms. Functional folate deficiency can arise with normal serum folate levels in the presence of folate antagonists (e.g. methotrexate), which inhibit the enzyme dihydrofolate reductase and hence interfere with thymidine and DNA synthesis. This problem can be avoided by the concurrent use of folinic acid that bypasses this enzymatic step.
MEDICINE 37:3
Treatment of megaloblastic anaemia B12 deficiency Loading doses of hydroxocobalamin 1 mg are administered intramuscularly three times per week to a total of six doses in the first 2 weeks. Thereafter, it is given 3-monthly for life in cases of B12 malabsorption.10 However, in dietary deficiency, maintenance oral supplementation with cyanocobalamin may be given once tissues stores have been replenished. Blood transfusion should be avoided even in severe anaemia, unless symptoms or cardiovascular risks dictate otherwise. A blunted response to B12 is usually due to depletion of iron and/or folate stores as massive numbers of new red cells are formed, therefore a short period of iron and folate supplementation may be required. Transient hypokalaemia often occurs during treatment in severely anaemic patients. Neurological complications may develop in the absence of anaemia, therefore it is imperative to commence treatment as soon as the diagnosis is made. Folic acid must not be initiated before B12 as this can precipitate or aggravate neurological complications.8 Folate deficiency Folic acid is given at a dose of 5 mg daily orally for about 4 months to replenish tissue stores.10 Dietetic advice should be given if dietary deficiency is the cause. Prophylactic folate supplements, 127
© 2008 Published by Elsevier Ltd.
Basic Science
400 μg daily, are recommended for potentially pregnant women to reduce fetal neural tube defects. ◆
6 Cook JD. Diagnosis and management of iron-deficiency anaemia. Best Pract Res Clin Haematol 2005; 18: 319–32. 7 Seetharam B, Yammani RR. Cobalamin transport proteins and their cell-surface receptors. Expert Rev Mol Med 2003; 5: 1–8. 8 Reynolds EH. Benefits and risks of folic acid to the nervous system. J Neurol Neurosurg Psychiatry 2002; 72: 567–71. 9 Carmel R, Green R, Rosenblatt DS, et al. Update on cobalamin, folate, and homocyteine. Hematology Am Soc Hematol Educ Program 2003: 62–81. 10 Joint Formulary Committee. British National Formulary, 56th edn. London: British Medical Association and Royal Pharmaceutical Society of Great Britain, 2008.
References 1 Bothwell TH, Charlton RW, Cook JD, Finch CA, eds. Iron metabolism in man. Oxford: Blackwell Scientific Publications, 1979. 2 Mackenzie EL, Iwasaki K, Tsuji Y. Intracellular iron transport and storage: from molecular mechanisms to health implications. Antioxid Redox Signal 2008; 10: 997–1030. 3 Porter JB. Practical management of iron overload. Br J Haematol 2001; 115: 239–52. 4 Koury MJ, Ponka P. New insights into erythropoieisis: the roles of folate, vitamin B12, and iron. Annu Rev Nutr 2004; 24: 105–31. 5 Beard JL, Connor JR. Iron status and neural functioning. Annu Rev Nutr 2003; 23: 41–58.
MEDICINE 37:3
Further reading Nemeth E. Iron regulation and erythropoiesis. Curr Opin Hematol 2008; 15: 169–75.
128
© 2008 Published by Elsevier Ltd.