Aspects of vitamin B12 and folate metabolism in malabsorption syndromes

Subclinical vitamin B12 deficiency in pregnant women attending an antenatal clinic in Nigeria

SUMMARY

Inadequate vitamin B12 status in a pregnant woman increases the risk for adverse maternal and fetal outcomes. The use of serum vitamin B12 concentration alone to assess vitamin B12 status in pregnant women is unreliable because of the decrease in serum vitamin B12 levels in normal pregnancy. The combination of serum vitamin B12 and methylmalonic acid (MMA) concentrations may provide a better estimate of vitamin B12 status. We obtained blood samples from 98 pregnant women in the third trimester at an antenatal clinic in Jos, Nigeria. All subjects were taking iron and folate supplements. Twelve of the subjects had a serum vitamin B12 concentration <148 pmol/l and 18 subjects had a serum MMA level >271 nmol/l. Using a combination of low serum vitamin B12 and elevated MMA concentrations, eight subjects were classified as having subclinical vitamin B12 deficiency. Because of the potential harmful consequences of vitamin B12 deficiency in pregnant women, it would be advisable to add vitamin B12 supplements to the existing regimen of folate and iron supplements currently provided to pregnant women in Nigeria.

Efficacy and safety of fortification and supplementation with vitamin B12: biochemical and physiological effects

Long known as an uncommon but serious medical disorder requiring medical management, vitamin B12 deficiency is now seen to be common worldwide, but it is in a quite different form than traditionally envisioned. Most of the newly recognized deficiency is subclinical in nature, its health impact and natural history are uncertain, and its prevalence has been greatly inflated by also including persons with "low-normal" vitamin B12 levels, few of whom are deficient. The spread of folic acid fortification has also introduced concerns about folate's potentially adverse neurologic consequences in persons with undetected vitamin B12 deficiency. Fortification with vitamin B12 may prove more complicated than fortification with folic acid, however, because the bioavailability of vitamin B12 is limited. Bioavailability for those who need the vitamin B12 the most is especially poor, because they often have malabsorption affecting either classical intrinsic factor-mediated absorption or food-vitamin B12 absorption. Moreover, new evidence shows that many elderly persons respond poorly to daily oral doses under 500 microg (1 microg = 0.74 nmol), even if they do not have classical malabsorption, which suggests that proposed fortification with 1 to 10 microg may be ineffective. Those least in need of vitamin B12 usually have normal absorption and are thus at greatest risk for whatever unknown adverse effects of high-dose fortification might emerge, such as the effects of excess accumulation of cyanocobalamin. Studies are needed to define the still unproven health benefits of vitamin B12 fortification, the optimal levels of fortification, the stability of such fortification, interactions with other nutrients, and any possible adverse effects on healthy persons. The answers will permit formulation of appropriately informed decisions about mandatory fortification or (because fortification may prove a poor choice) about targeted supplementation in subpopulations with special needs for additional vitamin B12, such as vegetarians, nursing mothers, and the elderly.

Very low oral doses of vitamin B-12 increase serum concentrations in elderly subjects with food-bound vitamin B-12 malabsorption

The BOSSANOVA study, a randomized double-blind trial, was designed to test the ability of very low oral doses of vitamin B-12 to increase the serum vitamin B-12 concentration in elderly subjects with food-bound vitamin B-12 malabsorption, and to determine whether there was a dose response. We also aimed to quantitatively assess the most efficient dose to be added to flour in addition to folic acid (flour cofortification with vitamin B-12 and folic acid). Sixty-seven patients were randomly assigned to 1 of 6 groups receiving various daily oral doses of vitamin B-12 (i.e., 2.5, 5, 10, 20, 40, or 80 microg/d) for 30 d. The dose-response was tested for different biological variables using a mixed model, taking into account the variable's initial value (between-subject effect), a linear log-dose effect, and a linear log (dosextime) interaction, where time was d 15 or d 30. We planned to determine the amount of oral vitamin B-12 that would increase the serum vitamin B-12 concentration by 37 pmol/L (50 ng/L). Significant between-subject effects were found for serum vitamin B-12, plasma homocysteine, and methylmalonic acid concentrations, but a log-dose effect was found only for vitamin B-12 (P<0.001). The slope of the line tended to be higher (P=0.07) at d 30 than at d 15. For a mean serum vitamin B-12 increase of 37 pmol/L, a dose of 5.9 (95% CI, 0.9-12.1) microg/d was needed. We concluded that very low oral doses of vitamin B-12 increased serum vitamin B-12 concentrations in elderly subjects with subclinical vitamin B-12 deficiency, following a log-dose pattern. Our results could be beneficial in the design of a public health program for safe flour cofortification with folic acid.

Vitamin deficiency and hyperhomocysteinemia in pseudoexfoliation glaucoma

Pseudoexfoliation syndrome (PEX) is a systemic disorder characterized by the deposition of an abnormal fibrillar material in ocular and various extraocular tissues. It represents the most common identifiable cause of glaucoma (PEX glaucoma = PEXG). Due to similar pathogenetic mechanisms, glaucoma has been called "ocular Alzheimer's disease". PEXG and Alzheimer's disease share common associations such as the higher prevalence of hyperhomocysteinemia in both disorders. In order to investigate the cause of hyperhomocysteinemia in PEXG, we evaluated B-vitamin levels (folate, B12, B6) and their associations with homocysteine (Hcy) in plasma of 70 PEXG patients and 70 control subjects. Folate, vitamin B12 and B6 levels were significantly decreased and associated with elevated Hcy levels in PEXG. Low B-vitamin levels in PEX might also help explain, at least in part, the higher prevalence of B-vitamin deficiency in disorders associated with PEX such as Alzhemier's disease.

Update on Cobalamin, Folate, and Homocysteine

Three topics affecting cobalamin, folate, and homocysteine that have generated interest, activity, and advances in recent years are discussed. These are: (I) the application of an expanded variety of tools to the diagnosis of cobalamin deficiency, and how these affect and are affected by our current understanding of deficiency; (II) the nature of the interaction between homocysteine and vascular disease, and how the relationship is affected by vitamins; and (III) the improved understanding of relevant genetic disorders and common genetic polymorphisms, and how these interact with environmental influences.

The diagnostic approach to cobalamin deficiency now allows better diagnosis of difficult and atypical cases and more confident rejection of the diagnosis when deficiency does not exist. However, the process has also become a complex and sometimes vexing undertaking. Part of the difficulty derives from the lack of a diagnostic gold standard among the many available tests, part from the overwhelming numerical preponderance of patients with subclinical deficiency (in which isolated biochemical findings exist without clinical signs or symptoms) among the cobalamin deficiency states, and part from the decreased availability of reliable tests to identify the causes of a patient’s cobalamin deficiency and thus a growing deemphasis of that important part of the diagnostic process. In Section I, Dr. Carmel discusses the tests, the diagnostic issues, and possible approaches to the clinical evaluation. It is suggested no single algorithm fits all cases, some of which require more biochemical proof than others, and that differentiating between subclinical and clinical deficiency, despite their overlap, may be a helpful and practical point of departure in the evaluation of patients encountered in clinical practice. The arguments for and against a suggested expansion of the cobalamin reference range are also weighed.

The epidemiologic data suggest that homocysteine elevation is a risk factor for vascular and thrombotic disease. In Section II, Dr. Green notes that the interactions of metabolism and clinical risk are not well understood and a causative relationship remains unproven despite new reports that lowering homocysteine levels may reduce vascular complications. Genetic and acquired influences may interact in important ways that are still being sorted out. The use of vitamins, especially folate, often reduces homocysteine levels but also carries potential disadvantages and even risks. Folate fortification of the diet and supplement use have also markedly reduced the frequency of folate deficiency, and cobalamin deficiency is now the more common deficiency state, especially among the elderly.

Although genetic disorders are rare, they illuminate important metabolic mechanisms and pose diagnostic challenges, especially when clinical presentation occurs later in life. In Section III, Drs. Rosenblatt and Watkins use selected disorders to illustrate the subject. Imerslund-Gräsbeck syndrome, a hereditary disorder of cobalamin absorption at the ileal level, demonstrates genetic heterogeneity. Finnish patients show mutation of the gene for cubilin, the multiligand receptor for intrinsic factor. Surprisingly, Norwegian and other patients have been found recently to have mutations of the AMN (amnionless) gene, mutations that are lethal in mice at the embryonic stage. Two disorders of cobalamin metabolism, cblG and cblE, are now known to arise from mutations of the methionine synthase and methionine synthase reductase genes, respectively. These disorders feature megaloblastic anemia and neurologic manifestations. The folate disorder selected for illustration, methylenetetrahydrofolate reductase (MTHFR) deficiency, paradoxically causes neurological problems but no megaloblastic anemia. This rare deficiency is the most common inborn error of folate metabolism. It is distinct from the very common MTHFR gene polymorphisms, mutations that cause mild to moderate reductions in MTHFR activity but no direct clinical manifestations. The MTHFR polymorphisms, especially the 677C→T mutation, may contribute to vascular and birth defect risks, while reducing the risk of certain malignancies, such as colon cancer. These polymorphisms and those of genes for other enzymes and proteins related to cobalamin, folate, and homocysteine metabolism may be important role players in frequent interactions between genes and the environment.

Tropical sprue and subclinical enteropathy: a vision for the nineties

Aside from infectious intestinal diseases with known etiology, there is a group of gastrointestinal disorders mainly affecting the small intestine of individuals predominantly living in and less often visiting or returning from the Third World, usually the tropics, and ranging from asymptomatic structural and/or functional abnormalities of the gastrointestinal mucosa (subclinical enteropathy, SE) to a fully symptomatic condition highlighted by malabsorption of nutrients with associated nutritional deficiencies responsive to folate and broad spectrum antibiotic treatment (tropical sprue, TS). Mounting evidence supports an infectious cause in many instances. The exact nature of the infection, whether initiated and/or perpetuated by enterotoxigenic coliform bacteria, virus(es) or a combination of these, is not clear. Further studies, including those using molecular techniques, are needed in order to clarify various aspects of these widely prevalent disorders.

Subtle and atypical cobalamin deficiency states

Evidence for cobalamin (vitamin B12) deficiency usually involves some combination of low serum cobalamin levels, clinical abnormalities (classically, megaloblastic anemia and neurologic defects), metabolic abnormalities, and response to therapy. However, cobalamin deficiency may often display few of the expected clinical findings. Identification of the underlying cause is also important in the diagnosis of deficiency, and its value may be particularly great when the expression of deficiency is subtle. The cause of cobalamin deficiency is usually malabsorptive, but may sometimes be limited to malabsorption of food cobalamin while free cobalamin is absorbed normally. Nongastroenterologic entities may sometimes also be found. All of these considerations allow the proposal of four patterns of cobalamin deficiency. The first type is classical deficiency; typical megaloblastic anemia with or without neurologic dysfunction occurs because of classical cobalamin malabsorption such as lack of intrinsic factor (pernicious anemia). The second type consists of classical cobalamin malabsorption in which the cobalamin deficiency is expressed subtly rather than in classical fashion. There is no megaloblastic anemia and sometimes the only evidence of deficiency may be metabolic. In the third type, cobalamin deficiency is expressed classically but is attributable to a subtle or atypical cause, such as food-cobalamin malabsorption. In the fourth type, deficiency is both expressed subtly and arises from subtle or atypical causes. Such presentations require further investigation but are a challenging expansion of our understanding and recognition of cobalamin deficiency.

Inhibitory effect of unconjugated bile acids on the intestinal transport of 5-methyltetrahydrofolate in rat jejunum in vitro

The effect of the unconjugated bile acids, cholic, deoxycholic, chenodeoxycholic, and ursodeoxycholic acids, and of the conjugated bile acid taurocholic acid on the mucosal-to-serosal transport and tissue uptake of the naturally occurring folate derivative, 5-methyltetrahydrofolate (5-CH3H4PteGlu) was examined in everted sacs of rat jejunum. Each of the unconjugated bile acids examined inhibited the transport and tissue uptake of 5-CH3H4PteGlu in a concentration dependent manner. At low concentrations (0.01-0.1 mM) of cholic and deoxycholic acids, no structural or functional damage to the intestinal mucosa occurred and the transport of 5-CH3H4PteGlu was inhibited competitively with Ki values of 0.114 mM and 0.055 mM for cholic and deoxycholic acids, respectively. The greater inhibition of 5-CH3H4PteGlu transport by unconjugated bile acids at 1 mM can be attributed to observed structural and functional damage to the intestinal mucosa. The addition of 2 mM lecithin to the mucosal medium failed to prevent the inhibitory effect of 0.1 mM deoxycholic acid on the transport of 0.5 microM 5-CH3H4PteGlu. Compared with the effect of unconjugated bile acids, the conjugated bile acid taurocholic acid (0.01-5 mM) showed no effect on the transport and tissue uptake of 5-CH3H4PteGlu. The results of this study show that intestinal transport and tissue uptake of 5-CH3H4PteGlu are inhibited by unconjugated bile acids in a dose-dependent fashion. The clinical and physiological implications of these observations are discussed.

Effect of cimetidine on the absorption of vitamin B12

The effect of cimetidine on the absorption of orally administered crystalline or food (egg yolk-bound) vitamin B12 (B12) was studied in 13 patients. Absorption or crystalline B12 was normal and not significantly changed by cimetidine. In contrast, the uptake of food-bound B12 decreased in all patients, from a mean of 5.3% without the drug to 2.5% after it, a fall of 53% (p less than 0.0001). This impairment of B12 absorption raises the possibility that long-term, full-dose therapy with cimetidine may produce B12 deficiency similar to that seen in other hypochlorhydric states. Our data indicate that cimetidine-induced B12 malabsorption would not be detected by the standard Schilling test.