The Suitability of Various Vitamin D3 Metabolites in Animal Nutrition

The adequate supplementation of vitamin D3 to food-producing animals is of crucial importance due to its multiple benefits in bone and muscle development as well as in immune response modulation. This 3-part series provides an overview on the complex metabolism and functions of vitamin D3 and reviews the suitability and safety of other sources of vitamin D3 such as calcifediol (25-hydroxy-cholecalciferol) and calcitriol (1,25-dihydroxy-cholecalciferol) for animal nutrition purposes since over- or under-supply of this essential micronutrient can result in unintended clinical problems.

According to its molecular structure and its metabolic function, calcitriol (or 1,25-[OH]2-D3) is considered as a secosteroid hormone (DeLuca, 1974; Norman and Henry, 1979), which regulates the expression of genes and cellular elements involved in the absorption and homeostasis of Ca and P and other growth factor genes, regulating cellular proliferation, differentiation and hypertrophy. Calcitriol first binds to the vitamin D receptor (VDR) molecule that in turn binds to response elements on the genes, representing a behavior which is typical for a hormone. 

The two-step activation cascade of vitamin D3 is physiologically important in order to secure that 1,25-(OH)2-D3 is only released when it is needed, meaning when the plasma Ca level is low.

Under normo-calcemic conditions, a continuous supply of calcitriol via the feed could permanently activate Ca absorption in the intestine and reduce Ca excretion in the kidneys with the risk that the Ca-concentration in the blood would exceed the maximum tolerable level, resulting in hypercalcemia. In such a condition, Ca can be deposited in soft tissues and thereby form calcinotic lesions in the kidneys, the heart and in other organs, impairing their physiologic function. 

Contrary to that, under moderate hypo-calcemic conditions, this oversupply of 1,25-(OH)2-D3 could result in the re-mobilization of Ca from the bones by stimulation of osteoclast differentiation, which creates an acidic microenvironment that allows the mineral component of the bone to dissolve (Soares, 1984), resulting in an eventual weakening of the skeletal structure. 

Calcitriol is a very potent compound, exerting metabolic functions, with its therapeutic window being rather narrow, in the range of 2 to 3 mcg, which means that the difference between a beneficial and a toxic dose is small (Rennie et al., 1995). Under physiologic conditions, the transformation of 25-OH-D3 into 1,25-(OH)2-D3 represents a strictly controlled mechanism in order to secure that the maximum tolerable level of calcitriol can never be exceeded.

If 1,25-(OH)2-D3 is supplied directly via the feed, its concentration could go beyond the acceptable threshold and induce subclinical or even clinical toxicity, such as inhibition of growth, loss of weight, reduced appetite or, eventually, high mortality.

Calcitriol and the embryonic development in poultry

Research from Henry and Norman (1978) has shown that breeder hens, fed for 13 weeks with 1,25-(OH)2-D3 exhibited poor hatchability of fertilized eggs. Also, Sunde et al. (1978) found that hatchability of chicken eggs was abnormally low and that there was a high incidence of abnormal embryos with defective upper mandibles after 28 weeks of feeding 1,25-(OH)2-D3. Affected embryos showed serious hypocalcemia and low tibial ash weight, indicating a defect of Ca mobilization from the shell (Narbaitz et al., 1987). Similar findings were reported by other authors as well (Abdulrahim et al., 1979; Soares et al., 1979). In all cases, normal hatchability could be restored by feeding vitamin D3 or 25-OH-D3. The true underlying reason for this metabolic failure is not clear, although it was discussed whether 1,25-(OH)2-D3 was not adequately transported into the fertilized eggs. 

The occurrence of embryonic abnormalities such as the retarded growth of the upper mandible and pipping tooth development when feeding 1,25-(OH)2-D3 raises questions about a direct toxic effect of this compound to embryos and potentially to hens as well. In this respect, it is remarkable that Rings et al. (2011) have reported intoxication by feeding 1a-OH-D3, which is a synthetic analog of calcitriol, to broiler breeder hens. These authors reported a marked drop in hatchability and a decrease in egg production as well. But there was also an increase in mortality and a surprising range of clinical signs such as severely swollen and pale kidneys and vents stained with urates and feces in the hens. Histopathology showed moderate renal nephrosis. In view of all these potential risks, it seems safer not to expose breeding hens to calcitriol. 

This situation can be even more challenging if these molecules are not properly registered as feed additives. In most countries, feed additive registration requires the submission and approval of a full dossier to prove the efficacy and safety of the molecule as well as the recommended and maximum levels of use. For vitamin D sources this is essential to know, particularly in countries where there is a maximum legal level of D3 in feed, alone or in combination with other approved D3 metabolites such as 25-OH-D3. 

Feed materials are normally used based on fiber, protein, or another nutrient content. When adding feed materials containing calcitriol to the animal diet, the contribution of calcitriol in terms of vitamin D activity should be carefully considered and regulated to avoid any potential risk of toxicity. 

Conclusion

Vitamin D3 is an essential nutrient primarily used for ensuring a healthy status of bones and skeleton. Recent research indicates a direct effect of some vitamin D3 sources, such as 25-OH-D3, on muscle cells formation and it represents, together with the activity on immune system, promising areas for further investigation. 

25-OH-D3 is considered the circulating form of vitamin D3 being produced in the liver and 1-25(OH)2 D3 is the active hormonal form of vitamin D3 produced in the kidney. Under physiologic conditions, the transformation of 25-OH-D3 into 1,25-(OH)2-D3 in the kidney represents a safe and strictly controlled mechanism which ensures that calcitriol is only released when it is needed by the animal, such as when the plasma Ca level is low, and the maximum tolerable level of 1,25-(OH)2-D3 should not be exceeded. 

An optimum and safe use of vitamin D3 sources in feed seems to be a balanced strategy to achieve the full potential from this essential micronutrient. The transformation of 25-OH-D3 into 1,25-(OH)2-D3 in the kidney represents a safe and strictly controlled mechanism which will ensure that calcitriol is only released when it is needed by the animal and that the maximum tolerable level cannot be exceeded.