Cynacobalamin - B12

What it is

Vitamin B₁₂ is an essential part of several enzyme systems. Most of these involve the transfer or synthesis of single-carbon units. Thus vitamin B₁₂ is responsible for a number of basic metabolic functions in association with other vitamins such as folic acid. The most important tasks relate to the metabolism of proteins but it also features in the metabolism of fats and of carbohydrates.

Under normal feed conditions, vitamin B₁₂ is probably linked to peptides or even to protein. This linkage is broken during digestion. The released vitamin B₁₂ molecule cannot be absorbed through the intestinal wall without a carrier. Various products, collectively known as the intrinsic factor, have been shown to carry vitamin B₁₂. These vary between species but most appear to be glycoproteins. Only cats appear able to absorb vitamin B₁₂ without the intervention of the intrinsic factor.

The physiological activities of vitamin B₁₂ are very closely inter-related with those of folic acid, but the actual mechanisms are poorly understood. It is known that one activity is the formation of labile methyl groups that play a significant part in the biosynthesis of methionine that, in turn, affects the synthesis of body proteins.

There is good reason to believe that the impairment of protein synthesis is the principal cause of the growth depression that is frequently observed in animals deficient of vitamin B₁₂. The cobalt atom appears to be responsible for the transmethylating capacity of cobalamin because the methyl-cobalt derivative is formed.

One interesting and important function of vitamin B₁₂ is the metabolism of propionate products of dietary or metallic origin. Propionate is converted into succinate in the Krebs cycle. Propionate contains three, and succinate four, carbon atoms. The extra methyl group is supplied through methylmalonyl-CoA that is activated by methylmalonyl-CoA isomerase, a vitamin B₁₂-dependent enzyme. This reaction is particularly important in ruminants where propionic acid is the main product of rumen breakdown of starchy feeds.

Deficiency symptoms

Animals given purely vegetable-based rations or semi-purified diets have slower growth rates than those given feeds with animal protein ingredients or supplemented with vitamin B₁₂. Nervous disorders followed by uncoordinated movements and increased irritability appear in animals with moderate vitamin B₁₂ deficiency, accompanied by rough coats and internal changes such as microcytic anaemia and atrophy of the thymus, spleen and suprarenals. Hatchability of fertile poultry eggs is severely reduced with dead-in-shell showing embryonic malformation and dying in the last few days before hatch.

Vitamin B₁₂ improves the uptake and utilisation of carotenes from the intestine and the function of vitamin A in maintaining the integrity of mucosal and epithelial cells. Calcium, copper and ferrous iron also act with vitamin B₁₂, improving absorption from the intestine and the efficiency of its metabolic functions.

Requirements and allowances

Most of the research on the vitamin B₁₂ requirements of various species arrives at a requirement of 10 µg/kg feed dry matter. In the absence of other information, 10 µg/kg should be regarded as the minimum to be supplied in the feed, particularly of non-ruminant species.

There is evidence of beneficial effects such as the optimisation of growth rate or hatchability when vitamin B₁₂ supplies are supplemented to give a total in the ration of 30 µg/kg. This is also the recommended level of supply to ruminants unable to synthesise sufficient vitamin B₁₂ in the rumen.

Stability and solubility

The very large molecule of vitamin B₁₂ results in general instability. Heat, light, acids, alkalis and oxidising agents affect it. It is not seriously affected by moisture. During normal feed mixing and pelleting operations losses of vitamin B₁₂ are of the order of 10%. If the feed is extruded, losses may be much greater and can exceed 30%.

The pure crystalline cobalamin and powder dilutions are relatively stable to air and heat but are attacked by light, particularly in the UV wavelengths. It is essential, therefore, to keep vitamin B₁₂ products in light-tight containers, preferably in a cool, dry place.

Suggested levels of supplementation for optimum performance

In preparing the list on the previous page, it has been assumed that only minimal amounts of animal protein products are likely to be included in feed formulations. Should the animal protein proportion exceed 5% of the mix, (with the exception of calf milk replacers which are assumed to be based on dry skimmed milk or dried whey), the recommendations can be reduced by 5 µg/kg.

Recommendations for vitamin B₁₂ addition to feeds for various ages and species

Species and age		Vitamin B12 supplement (µg/kg)
Fish	trout, salmon	30-50
	carp	20-50
	eels	100-200
Shrimp	Penaeus monodon
	Macrobrachium rosenbergii

In many cases, vitamin B₁₂ deficiencies produce very generalised, non-specific symptoms. Any animal that is not growing as it should, particularly if its coat or plumage is harsh, might benefit from additional dietary supplies of vitamin B₁₂.