Vitamin B12 is the only vitamin, which, in its structure, contains the mineral, cobalt, to which it owes its name, Cobalamin. Given its cobalt colour, Vitamin B12 is often referred to as the ‘red vitamin’.

The main forms of Cobalamin are:

• Hydroxocobalamin,
• Cyanocobalamin,
• Methylcobalamin (active coenzymated form)
• Adenosylcobalamin (active coenzymated form).

Adenosylcobalamin is a coenzymatic, natural, mitochondrial form of vitamin B12, as it performs its biological function directly within the mitochondria—the structures responsible for energy production in the cell. It acts as a coenzyme of methylmalonyl-CoA mutase, an enzyme that is essential for incorporating the products of amino acid and fatty acid metabolism into the cell’s energy cycle.

In contrast to methylcobalamin, which is primarily involved in methylation reactions in the cytoplasm, adenosylcobalamin is responsible for the energy-related aspect of cellular metabolism. Its adequate availability is crucial for efficient mitochondrial function, stable ATP production, and the maintenance of high cellular energy efficiency, particularly in tissues with high energy demand, such as the nervous system and muscles.

The demand for adenosylcobalamin increases under conditions of elevated metabolic load, when the body must intensify cellular energy production. This applies in particular to individuals experiencing chronic fatigue, reduced exercise tolerance, or nervous system dysfunction, as well as to those exposed to long-term stress, which significantly increases the energy requirements of cells. Higher needs may also occur in physically active individuals, during periods of intensive training, recovery, as well as in people with malabsorption disorders or inefficient conversion of vitamin B12 into its active forms.

A deficiency of adenosylcobalamin may lead to reduced ATP production, accumulation of abnormal metabolites, and the appearance of neurological symptoms, even when levels of other forms of vitamin B12 are within the normal range. For this reason, supplementation with the adenosyl form may represent an important element of targeted metabolic support, particularly in individuals with high energy demands or increased nervous system burden.

Important sources of Vitamin B12 in the diet are animal foods including meat, dairy products, eggs, as well as fish and shellfish. Therefore, it is common for vegans and vegetarians to ensure the intake of Vitamin B12 from alternative sources. For proper absorption of Vitamin B12, the Castle factor working properly plays a crucial role. In healthy subjects, Vitamin B12 bioavailability from fish, sheepmeat and poultry meat was 42%, 56%-89%, and 61%-66%, respectively. Compared to meat, absorption from eggs is estimated to be less than 9%. Similarly, bioavailability from algae is negligible.