Vitamin B12

Physiological Role

Total vitamin B12, or cobalamin, covers all the B12 circulating in the blood. It sums two fractions: the active fraction, bound to transcobalamin and internalized by cells, and the storage fraction, bound to haptocorrin and metabolically inert. The active fraction represents about 10 to 30% of the total.

Once inside the cell, active B12 drives two major enzymatic pathways. It serves as a cofactor for methionine synthase, the enzyme that converts homocysteine into methionine. This reaction fuels the methylation cycle, the biochemical process supporting DNA synthesis, neurotransmitter production, and S-adenosylmethionine, the body's principal methyl donor.

B12 also operates in the mitochondrion, assisting methylmalonyl-CoA mutase in the metabolism of specific fatty acids and amino acids. This dual localization explains why a suboptimal B12 status manifests through hematological, neurological, and energy-related signals at once.

Reference Ranges

These reference ranges are derived from scientific literature and may differ from your laboratory's reference values.

Source : NIH ODS (2024)

Biological Significance

A total vitamin B12 value in the optimal zone reflects satisfactory status across all tissues. The methylation cycle and cellular energy production have access to the cofactor they require.

Low values signal insufficient intake or absorption. At-risk profiles include vegan diets and individuals over 65. Metformin and proton pump inhibitors are also contexts to monitor. Total B12 is the universal reference measure for this status: it is measured on serum, widely available, and reimbursed.

When total B12 is low, homocysteine offers a complementary functional reading also measured by Singular: it rises in case of methylation impairment. The active fraction alone, Holo-TC, is more specific to tissue status, but it remains a niche assay, poorly available and not reimbursed in France. Singular therefore favors total B12, cross-referenced with homocysteine, to read functional status. In clinical settings, methylmalonic acid may complement the exploration of the mitochondrial pathway. Singular also measures vitamin B9, enabling an integrated reading of the methylation crossroads.

Very high total B12 values typically reflect active oral supplementation. They do not pose an overload risk, as excess B12 is eliminated through the kidneys.

Influencing Factors

Diet. Vitamin B12 comes exclusively from animal sources: meat, fish, eggs, and dairy. Strict vegan diets rapidly lead to declining B12 without adapted supplementation. Long-term vegetarian diets also carry a risk of progressive decline.

Gastrointestinal absorption. B12 absorption depends on intrinsic factor, a protein secreted by the parietal cells of the stomach. Age-related gastric atrophy reduces this secretion. This explains why B12 status weakens beyond 65, even when dietary intake is adequate.

Medications. Some medications modulate B12 absorption or availability. Metformin, used in the context of glucose metabolism, interferes with ileal absorption of vitamin B12. The DPPOS cohort documented increased risk after several years of use. Proton pump inhibitors and antacids reduce the gastric acidity needed to release B12 from food.

Genetics. Genetic variations in absorption and transport, such as polymorphisms in the FUT2 and TCN2 genes, modulate measured total B12 at equivalent intake. These variants do not alter the biological function of B12 but influence observed values.

Supplementation. Oral supplementation with methylcobalamin or cyanocobalamin rapidly raises total B12. Singular uses methylcobalamin, directly usable by methionine synthase without an enzymatic conversion step.

Age. B12 needs increase with age due to reduced absorption. Regular measurement of total B12 after 50 allows intake to be adjusted before functional signals such as elevated homocysteine appear.

Methylation cofactors. Vitamins B9 and B6, along with trimethylglycine, participate in the same biochemical cycle as B12. An imbalanced intake can mask or amplify the reading of B12 status, justifying an integrated approach to the methylation crossroads.

In the Singular Formula

The Singular formulation engine uses total vitamin B12 as a marker of B12 status. Several concrete rules adjust the formula based on the observed zone.

When total B12 sits in the low zone, vitamin B12 is calibrated to a sustained dose to restore cellular availability. In the very low zone, the dose is reinforced further. Singular formulates with methylcobalamin, directly usable by methylation enzymes. When the marker returns to the optimal zone, the formula moves to a maintenance dose.

A specific safety logic frames vitamin B9. High folate intake can mask the hematological picture without changing actual B12 availability, blurring the reading of B12 status. When total B12 is low, vitamin B9 dosing is deliberately moderated to avoid masking B12 dynamics.

The engine systematically cross-references total B12 with two other markers of the methylation crossroads. When homocysteine is elevated and total B12 remains low, vitamin B12 takes priority. When homocysteine is elevated but total B12 is already optimal, the formula reinforces folates or methyl support instead. This cross-reading avoids generic corrections and targets the truly limiting link.

Beyond B12 itself, trimethylglycine and vitamin B6 in its P5P form are selected for their documented contribution to the methylation cycle. Iron, whose metabolism intersects with red blood cell formation, completes this integrated reading of hematological and energy status.

Scientific Studies

Frequently Asked Questions