Modeling a methylmalonic acid-derived change point for serum vitamin B-12 for adults in NHANES.

BACKGROUND: No consensus exists about which cutoff point should be applied for serum vitamin B-12 (SB-12) concentrations to define vitamin B-12 status in population-based research. OBJECTIVE: The study's aim was to identify whether a change point exists at which the relation between plasma methylmalonic acid (MMA) and SB-12 changes slope to differentiate between inadequate and adequate vitamin B-12 status by using various statistical models. DESIGN: We used data on adults (≥19 y; n = 12,683) from NHANES 1999-2004-a nationally representative, cross-sectional survey. We evaluated 6 piece-wise polynomial and exponential decay models that used different control levels for known covariates. RESULTS: The MMA-defined change point for SB-12 varied depending on the statistical model used. A linear-splines model was determined to best fit the data, as determined by the approximate permutation test; 3 slopes relating SB-12 and MMA and resulting in 2 change points and 3 subgroups were shown. The first group (SB-12 <126 pmol/L) was small and had the highest MMA concentration (median: 281 nmol/L; 95% CI: 245, 366 nmol/L; n = 157, 1.2%); many in this group could be considered at high risk of severe deficiency because combined abnormalities of MMA and homocysteine were very frequent and the concentrations themselves were significantly higher. The highest SB-12 group (SB-12 >287 pmol/L; n = 8569, 67.6%) likely had adequate vitamin B-12 status (median MMA: 120 nmol/L; 95% CI: 119, 125 nmol/L). The vitamin B-12 status of the sizable intermediate group (n = 3957, 33%) was difficult to interpret. CONCLUSIONS: The 3 distinct slopes for the relation between SB-12 and MMA challenges the conventional use of one cutoff point for classifying vitamin B-12 status. In epidemiologic research, the use of one cutoff point would fail to separate the small, severely deficient group from the intermediate group that has neither normal nor clearly deficient vitamin B-12 concentrations (ie, unknown vitamin B-12 status). This intermediate group requires further characterization.

Diagnosis and management of clinical and subclinical cobalamin deficiencies: why controversies persist in the age of sensitive metabolic testing.

In the past two decades, sensitive biochemical tests have uncovered cobalamin deficiency much more frequently than ever before. Almost all cases involve mild, biochemical changes without clinical manifestations (subclinical cobalamin deficiency; SCCD), whose health impact is unclear. Because the causes of SCCD are most often unknown, nonmalabsorptive, and seldom documented, controversy and confusion surround the diagnostic criteria and, inevitably, consequences and management of SCCD. To complicate matters, our grasp of the rarer clinical deficiency, usually a serious, progressive medical disease rooted in severe malabsorption, has receded as absorption testing has disappeared. Reexamining the accumulation of assumptions and misperceptions about cobalamin deficiency and distinguishing SCCD from clinical deficiency is long overdue. The biology of cobalamin provides an important starting point: cobalamin stores exceed daily losses so greatly and binding proteins regulate absorption so effectively that deficiency typically achieves clinical expression only after years of severe, relentless malabsorption. Dietary insufficiency, mild, partial malabsorption, and other incomplete, intermittent causes can usually produce only SCCD. Thus, the most fundamental difference between the two deficiencies is the relentlessness of the underlying cause, which determines prognosis and health impact. Inattention to absorptive status has exacerbated the limitations of biochemical testing. All the biochemical tests are highly sensitive but specificity is poor, no diagnostic gold standard exists, and diagnostic cutpoints fluctuate excessively. To limit the adverse diagnostic consequences, the diagnosis of SCCD, whose need for treatment is unclear, should be deferred unless at least two tests are abnormal. Indeed, cobalamin biology indicates that the absorption system, while enhancing cobalamin delivery, also sets a strict upper limit on it, which suggests that cobalamin excess is undesirable. Solving cobalamin deficiency requires balanced assessment of the different imperatives of clinical and public health concerns, better rationalization of diagnostic testing, consistent definitions of normality in relation to SCCD, and rational cutpoint selection.

Subclinical cobalamin deficiency.

PURPOSE OF REVIEW: This review focuses on recent developments and controversies in the diagnosis, consequences, and management of subclinical cobalamin deficiency (SCCD), which affects many elderly persons. RECENT FINDINGS: Diagnosis of SCCD depends exclusively on biochemical tests whose individual limitations suggest that combinations of tests are needed, especially in epidemiologic research. The causes of SCCD are unknown in more than 60% of cases, which limits prognostic predictions and identification of health consequences. After years of varying, often inconclusive associations, new clinical trials suggest that homocysteine reduction by high doses of folic acid, cobalamin, and pyridoxine may reduce progression of structural brain changes and cognitive impairment, especially in predisposed individuals. The causative or contributory roles, if any, of SCCD itself in cognitive dysfunction require direct study. If the findings are confirmed, high-dose supplementation with three vitamins will probably be more effective than fortification of the diet. SUMMARY: The story of SCCD, which is severalfold times more common in the elderly than clinical cobalamin deficiency but also differs from it in arising only infrequently from severe malabsorption and thus being less likely to progress, continues to evolve. Preventive benefits need to be confirmed and expanded, and will require fuller understanding of SCCD pathophysiology, natural history, and health consequences.

Monitoring of vitamin B-12 nutritional status in the United States by using plasma methylmalonic acid and serum vitamin B-12.

BACKGROUND: Various definitions, criteria, tests, and cutoffs have been used to define vitamin B-12 status; however, a need exists for the systematic study of vitamin B-12 status in the United States because of concerns about high folic acid intakes and the potential for associated adverse effects. OBJECTIVE: The objective was to determine the effect of different cutoff choices on outcomes and of the different degrees of serum vitamin B-12 status, definable by the concurrent use of a functional and circulating marker as the first steps to developing a data-based consensus on the biochemical diagnosis of vitamin B-12 deficiency. DESIGN: Data from NHANES, a nationally representative cross-sectional survey, were examined for adults aged >19 y (mean ± SD age: 45 ± 1 y) from 1999 to 2004 (n = 12,612). RESULTS: Commonly used cutoffs had a greater effect on prevalence estimates of low vitamin B-12 status with the use of vitamin B-12 than with the use of methylmalonic acid (MMA; 3-26% and 2-6%, respectively). A cutoff of >148 pmol/L for vitamin B-12 and of ≤210 nmol/L for MMA resulted in significant misclassifications. Approximately 1% of adults had a clear vitamin B-12 deficiency (low vitamin B-12 and elevated MMA); 92% of adults had adequate vitamin B-12 status. A high percentage of younger women characterized the group with low vitamin B-12 and normal MMA (2% of adults) and may have falsely reflected low vitamin B-12. Adults with elevated MMA (5%) only were demographically similar (ie, by age and race) to the deficient group and may have included some individuals with early vitamin B-12 deficiency. CONCLUSIONS: These analyses indicate the challenges of assessing vitamin B-12 status when uncertainties exist about the appropriate cutoffs. Future studies should determine definable endpoints to achieve this goal.

Biomarkers of folate status in NHANES: a roundtable summary.

A roundtable to discuss the measurement of folate status biomarkers in NHANES took place in July 2010. NHANES has measured serum folate since 1974 and red blood cell (RBC) folate since 1978 with the use of several different measurement procedures. Data on serum 5-methyltetrahydrofolate (5MTHF) and folic acid (FA) concentrations in persons aged ≥60 y are available in NHANES 1999-2002. The roundtable reviewed data that showed that folate concentrations from the Bio-Rad Quantaphase II procedure (Bio-Rad Laboratories, Hercules, CA; used in NHANES 1991-1994 and NHANES 1999-2006) were, on average, 29% lower for serum and 45% lower for RBC than were those from the microbiological assay (MA), which was used in NHANES 2007-2010. Roundtable experts agreed that these differences required a data adjustment for time-trend analyses. The roundtable reviewed the possible use of an isotope-dilution liquid chromatography-tandem mass spectrometry (LC-MS/MS) measurement procedure for future NHANES and agreed that the close agreement between the MA and LC-MS/MS results for serum folate supported conversion to the LC-MS/MS procedure. However, for RBC folate, the MA gave 25% higher concentrations than did the LC-MS/MS procedure. The roundtable agreed that the use of the LC-MS/MS procedure to measure RBC folate is premature at this time. The roundtable reviewed the reference materials available or under development at the National Institute of Standards and Technology and recognized the challenges related to, and the scientific need for, these materials. They noted the need for a commutability study for the available reference materials for serum 5MTHF and FA.

Biomarkers of cobalamin (vitamin B-12) status in the epidemiologic setting: a critical overview of context, applications, and performance characteristics of cobalamin, methylmalonic acid, and holotranscobalamin II.

Cobalamin deficiency is relatively common, but the great majority of cases in epidemiologic surveys have subclinical cobalamin deficiency (SCCD), not classical clinical deficiency. Because SCCD has no known clinical expression, its diagnosis depends solely on biochemical biomarkers, whose optimal application becomes crucial yet remains unsettled. This review critically examines the current diagnostic concepts, tools, and interpretations. Their exploration begins with understanding that SCCD differs from clinical deficiency not just in degree of deficiency but in fundamental pathophysiology, causes, likelihood and rate of progression, and known health risks (the causation of which by SCCD awaits proof by randomized clinical trials). Conclusions from SCCD data, therefore, often may not apply to clinical deficiency and vice versa. Although many investigators view cobalamin testing as unreliable, cobalamin, like all diagnostic biomarkers, performs satisfactorily in clinical deficiency but less well in SCCD. The lack of a diagnostic gold standard limits the ability to weigh the performance characteristics of metabolic biomarkers such as methylmalonic acid (MMA) and holotranscobalamin II, whose specificities remain incompletely defined outside their relations to each other. Variable cutoff selections affect diagnostic conclusions heavily and need to be much better rationalized. The maximization of reliability and specificity of diagnosis is far more important today than the identification of ever-earlier stages of SCCD. The limitations of all current biomarkers make the combination of ≥2 test result abnormalities, such as cobalamin and MMA, the most reliable approach to diagnosing deficiency in the research setting; reliance on one test alone courts frequent misdiagnosis. Much work remains to be done.

Biomarkers of vitamin B-12 status in NHANES: a roundtable summary.

A roundtable to discuss the measurement of vitamin B-12 (cobalamin) status biomarkers in NHANES took place in July 2010. NHANES stopped measuring vitamin B-12-related biomarkers after 2006. The roundtable reviewed 3 biomarkers of vitamin B-12 status used in past NHANES--serum vitamin B-12, methylmalonic acid (MMA), and total homocysteine (tHcy)--and discussed the potential utility of measuring holotranscobalamin (holoTC) for future NHANES. The roundtable focused on public health considerations and the quality of the measurement procedures and reference methods and materials that past NHANES used or that are available for future NHANES. Roundtable members supported reinstating vitamin B-12 status measures in NHANES. They noted evolving concerns and uncertainties regarding whether subclinical (mild, asymptomatic) vitamin B-12 deficiency is a public health concern. They identified the need for evidence from clinical trials to address causal relations between subclinical vitamin B-12 deficiency and adverse health outcomes as well as appropriate cutoffs for interpreting vitamin B-12-related biomarkers. They agreed that problems with sensitivity and specificity of individual biomarkers underscore the need for including at least one biomarker of circulating vitamin B-12 (serum vitamin B-12 or holoTC) and one functional biomarker (MMA or tHcy) in NHANES. The inclusion of both serum vitamin B-12 and plasma MMA, which have been associated with cognitive dysfunction and anemia in NHANES and in other population-based studies, was preferable to provide continuity with past NHANES. Reliable measurement procedures are available, and National Institute of Standards and Technology reference materials are available or in development for serum vitamin B-12 and MMA.

Mandatory fortification of the food supply with cobalamin: an idea whose time has not yet come.

The success of folic acid fortification has generated consideration of similar fortification with cobalamin for its own sake but more so to mitigate possible neurologic risks from increased folate intake by cobalamin-deficient persons. However, the folate model itself, the success of which was predicted by successful clinical trials and the known favorable facts of high folic acid bioavailability and the infrequency of folate malabsorption, may not apply to cobalamin fortification. Cobalamin bioavailability is more restricted than folic acid and is unfortunately poorest in persons deficient in cobalamin. Moreover, clinical trials to demonstrate actual health benefits of relevant oral doses have not yet been done in persons with mild subclinical deficiency, who are the only practical targets of cobalamin fortification because >94% of persons with clinically overt cobalamin deficiency have severe malabsorption and therefore cannot respond to normal fortification doses. However, it is only in the severely malabsorptive disorders, such as pernicious anemia, not subclinical deficiency, that neurologic deterioration following folic acid therapy has been described to date. It is still unknown whether mild deficiency states, which usually arise from normal absorption or only food-bound cobalamin malabsorption, have real health consequences or how often they progress to overt clinical cobalamin deficiency. Reports of cognitive or other risks in the common subclinical deficiency state, although worrisome, have been inconsistent. Moreover, their observational nature proved neither causative connections nor documented health benefits. Extensive work, especially randomized clinical trials, must be done before mandatory dietary intervention on a national scale can be justified.

Genomic mutations associated with mild and severe deficiencies of transcobalamin I (haptocorrin) that cause mildly and severely low serum cobalamin levels.

Transcobalamin (TC) I deficiency, like the function of TC I itself, is incompletely understood. It produces low serum cobalamin levels indistinguishable from those of true cobalamin deficiency. Diagnosis is especially elusive when TC I deficiency is mild. To provide new, more substantive definition, the TCN1 gene was examined in two well-characterised families that included members with both severe and mild TC I deficiencies. A severely deficient proposita with undetectable TC I levels displayed compound heterozygosity for two mutations, each causing a premature stop codon. Relatives in both families who had mildly low or low-normal plasma levels of TC I and cobalamin were heterozygous for one or the other of these mutations. An unrelated patient with mild TC I deficiency and unknown familial TC I and cobalamin status was then tested and found to be similarly heterozygous for one of the mutations. The two nonprivate mutations identify a genetic basis for TC I deficiency for the first time. They also add new approaches to studying mild and severe TC I deficiency and to reducing confusion of its low cobalamin levels with those of cobalamin deficiency and its often dramatically different prognosis and management.

Nutritional anemias and the elderly.

Nutritional anemias are important because they are easily reversed and because their underlying causes, most often unrelated to dietary intake, require individualized assessment. Iron-deficiency anemia (IDA) usually results from iron losses accompanying chronic bleeding, including loss to intestinal parasites, or from gastric disorders or malabsorption in the elderly. Cobalamin-deficiency anemia, the only nutritional anemia with predilection for the elderly, nearly always stems from failure of intrinsic factor (IF)-related absorption. Folate-deficiency anemia, the only nutritional anemia usually caused by poor intake, has nearly disappeared in countries that fortify food with folic acid. Copper-deficiency anemia, which usually results from malabsorptive disorders or from medical or nutritional interventions that provide inadequate copper or excess zinc, is uncommon but increasingly recognized. The prevalences of nutritional anemias, which are not always distinguished from non-anemic deficiency, are uncertain. The mean corpuscular volume (MCV) provides an essential diagnostic tool leading to judicious matching of relevant biochemical changes with relevant anemia. Nutritional anemias usually feature abnormal MCV, whereas the predominant anemias in the aged, especially the anemias of chronic disease/chronic inflammation (ACD/ACI), of renal failure, and of unknown causes, are typically normocytic.

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.

How I treat cobalamin (vitamin B12) deficiency.

The challenges in medical management of cobalamin deficiency lie in attention to the unique pathophysiology that underlies cobalamin deficiency, more than in the mechanics of therapy. The central physiologic principles are that clinically important deficiency is more likely to occur (and progress) when intrinsic factor-driven absorption fails than when diet is poor and that most causes take years to produce clinically obvious deficiency. Transient defects have little clinical impact. The key management principle is the importance of follow-up, which also requires knowing how the deficiency arose. The virtues of these principles are not always fully appreciated. Recent developments have made diagnosis and management more difficult by diminishing the ability to determine cobalamin absorption status. Clinicians must also grapple with premature medicalization of isolated, mild biochemical changes that added many asymptomatic cases of still undetermined medical relevance to their caseload, often expanded by inflated cobalamin level criteria. The potential for misattribution of cobalamin-unrelated presentations to nongermane cobalamin and metabolite abnormalities has grown. Pathophysiologically based management requires systematic attention to each of its individual components: correctly diagnosing cobalamin deficiency, reversing it, defining its underlying cause, preventing relapse, managing the underlying disorder and its complications, and educating the patient.