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Background: The prevalence of a sub-clinical 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 sub-fraction 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 pre-dialysis 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 (&gt;12 μmol/l), high MMA (&gt;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 cut-off 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.

Functional Vitamin B12 Deficiency and Determination of Holotranscobalamin in Populations at Risk