Functional vitamin B12 deficiency and determination of holotranscobalamin in populations at risk

Background: The prevalence of a subclinical functional vitamin B-12 deficiency in the general population is higher than previously expected. Total serum vitamin B-12 may not reliably indicate vitamin B-12 status. To get more specificity and sensitivity in diagnosing vitamin B-12 deficiency, the concept of measuring holotranscobalamin II (holoTC), a subfraction of vitamin B-12, has aroused great interest. HoloTC as a biologically active vitamin B-12 fraction promotes a specific uptake of its vitamin B-12 by all cells. In this study we investigated the diagnostic value of storage (holoTC) of vitamin B-12 and functional markers (methylmalonic acid (MMA)) of vitamin B-12 metabolism in populations who are at risk of vitamin B-12 deficiency. Subjects and Methods: Our study included 93 omnivorous German controls, 111 German and Dutch vegetarian subjects, 122 Syrian apparently healthy subjects, 127 elderly Germans and finally 92 German predialysis renal patients. Serum concentrations of homocysteine (Hcy) and MMA were measured by gas chromatographymass spectrometry, folate and vitamin B-12 by chemiluminescence immunoassay, and holoTC by utilizing a RIA test. Results: High Hcy (>12 mol/l), high MMA (>271 nmol/l) resp. low holoTC (vitamin B-12) in serum were detected in 15%, 8% resp. 13% (1%) of German controls, 36%, 60%, resp. 72% (30%) of vegetarians, 42%, 48% resp. 50% (6%) of Syrians, 75%, 42%, resp. 21% (7%) of elderly subjects and 75%, 67% resp. 4% (2%) of renal patients. The lowest median levels of holoTC were observed in vegetarians, followed by the Syrian subjects (23 and 35 pmol/l, respectively). Renal patients had significantly higher levels of holoTC compared to the German controls (74 vs. 54 pmol/l). In the vitamin B-12 range between 156 pmol/l (conventional cutoff level) and 241 pmol/l, both mean concentrations of holoTC and MMA were in the pathological range. HoloTC was the earliest marker for vitamin B-12 deficiency followed by MMA. Vitamin B-12 deficiency causes folate trapping. A higher folate level is required to keep Hcy normal. The relationship between MMA and holoTC seemed dependent on renal function. In renal patients with a glomerular filtration rate below 36 ml/min, a significantly lower mean level of MMA was detected within the highest tertile of holoTC concentration, compared to the lowest tertile. Thus, in renal patients, a higher serum concentration of circulating holoTC is required to deliver sufficient amounts of holoTC into the cells. Conclusion: Our data support the concept that the measurement of holoTC and MMA provides a better index of cobalamin status than the measurement of total vitamin B-12. HoloTC is the most sensitive marker, followed by MMA. The use of holoTC and MMA enables us to differentiate between storage depletion and functional vitamin B-12 deficiency. Renal patients have a higher requirement of circulating holoTC. In renal dysfunction, holoTC cannot be used as a marker of vitamin B-12 status.