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Understanding vitamin D’s role in livestock health

Vitamin D may be described as the sunlight vitamin because it is formed naturally in the skin of animals exposed to ultraviolet light. In most cases, with enough sunlight, animals can cover their full needs in vitamin D only by biosynthesis.

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Vitamin D may be described as the sunlight vitamin because it is formed naturally in the skin of animals exposed to ultraviolet light. In most cases, with enough sunlight, animals can cover their full needs in vitamin D only by biosynthesis. However, this is not always the case because natural biosynthesis is ineffective and variable as it depends on a great number of conditions. It is also a very slow process, which is one of nature’s ways to protect animals living outdoors from vitamin D toxicity.

Biosynthesis of vitamin D is greatly dependent on the duration of exposure to sunlight. Its intensity and quality is also very important as is the age of the animal. For example, in northern climates like Germany, during the winter months there is practically no ultraviolet sunlight. This means there is very little if any biosynthesis of vitamin D, and supplementation is required even for animals raised outdoors. In contrast, it is evident that in Spain, for example, there is little need for vitamin D supplementation for animals raised totally outdoors, especially during the summer months of intense sunlight.

The feed of all animals raised indoors should always be supplemented with enough vitamin D to fully cover their needs.

The picture is quite different for animals raised indoors. Even though some facilities might have windows, these are invariably either too small or too dirty to allow sufficient sunlight to reach all animals housed in the building. This means biosynthesis of vitamin D will be inadequate and highly variable. As such, the feed of all animals raised indoors should always be supplemented with enough vitamin D to fully cover their needs, disregarding natural biosynthesis. This supplementation is usually provided through the vitamin premix.

Biochemistry

In nature, vitamin D is found in two forms. In plants, which contain small quantities of vitamin D, the predominant form is ergocalciferol (D2). In animals, which have higher concentrations of vitamin D, the predominant form is cholecalciferol (D3). Both plants and animals contain both forms of vitamin D, but the majority is only one of the two types above.

Vitamin D is a fat-soluble vitamin. It is absorbed mainly along other lipids in the diet through the lymphatic system, which is a slow process. Following absorption, it then reaches the liver where it is converted (hydroxylated) to 25-OH-D3 (calcidiol). This is the main circulating form of the vitamin because the hydroxylated form is water-friendly and is easier to move around in the organism. Some small quantities of vitamin D are stored in the lipid tissue and may be mobilized when plasma 25-OH-D3 levels drop due to insufficient intake of vitamin D. The functional form of vitamin D, however, is 1,25-(OH)2-D3 (calcitriol). This form is even more water-friendly due to its double degree of hydroxylation. Calcitriol is responsible for the regulation of calcium and phosphorus metabolism.

Bone metabolism

The main function of vitamin D is bone homeostasis, where calcitriol acts like a hormone. When dietary calcium intake is low, blood calcium drops below normal levels. If this is not corrected immediately, the animal develops tetany (lactating cows), goes into a coma and eventually dies. With calcitriol, the organism avoids this unpleasant situation by drawing calcium from reserves in the skeletal tissue. In brief, the effects of calcitriol are increased absorption of calcium from the small intestine and enhanced mobilization of calcium from bones. The opposite mechanism ensures that any excess dietary calcium is diverted towards the skeletal tissue to fill in any gaps caused by previous mobilization of reserves, especially in young animals.

Side roles

Recently, it has been suggested that vitamin D, as calcitriol, may increase the early responses of the immune system. Further developments indicate that vitamin D in the form of calcidiol (25-OH-D3) supplementation may improve meat quality and reproductive performance. It appears the liver is acting as a form of a barrier controlling the levels of circulation calcidiol. As such, supplementation directly with calcidiol bypasses this barrier, increasing the circulating levels of calcidiol, and this has been shown to enhance calcium homeostasis with beneficial results in many productive and reproductive indexes. Nevertheless, commercial application of these findings still lags behind the widespread use of standard vitamin D through vitamin premixes.

Toxicity

Vitamin D toxicity may occur either due to wrong amount of vitamin added in the premix or wrong dosage of the premix in the complete feed. According to the National Research Council (NRC), vitamin D toxicity may rapidly lead to death. For example, pigs between 20 and 25 kg body weight died after four days of consuming nearly 500,000 IU vitamin D daily. On the other hand, feeding 22,000 to 44,000 IU vitamin D to young pigs between 10 and 20 kg body weight severely depressed feed efficiency and weight gain. In chickens, broilers can tolerate up to 40,000 IU/kg vitamin D in the their feed, whereas layers develop toxicity when fed levels of vitamin D in excess of 3,000 IUkg. It appears toxicity is quite rare, although possible, and might only happen due to a gross error in dosage. If it does happen, diagnosis may be done post-mortem based on the calcification observed in the aorta, heart, kidneys and lungs.

Deficiency

A vitamin D deficiency is more likely to occur than toxicity. The main reasons leading to deficiency are either lack of sunlight or wrong dosage of the vitamin or the premix in the final feed. As vitamin D is involved in the metabolism of calcium and phosphorus, its deficiency results in poor bone calcification leading to rickets (young pigs and broiles) and osteomalacia (adult animals). Of course, severe deficiency leads to tetany due to calcium imbalance in the blood, which may result in death.

Supplementation in practice

Recommended levels through premixes vary from 100 IU to over 3,000 IU/kg kg final feed, depending on the age, weight and stage of life cycle of each animal species. Young and breeding animals are usually given higher levels of vitamin D compared to growing-finishing animals towards the end of their market weight. Breeding male animals may also benefit from higher levels of vitamin D. A correct farm-specific assessment of vitamin D requirements, as part of an overall premix-specifications overview by a qualified nutritionist, offers substantial savings in the form of avoiding wastage and(or) maximizing animal performance.

Read more: Stop wasting vitamins, trace minerals in animal feeds

In most premixes, synthetic vitamin D is available as cholecalciferol (straight vitamin D3). One IU of vitamin D3 is equivalent to 0.025 micrograms of cholecalciferol. A new form has recently become available in the form of calcidiol (25-OH-D3 ).  Synthetic calcidiol is formulated such that 1 gram of it replaces 1000 IU of vitamin D3 in premixes.

As little research has focused on vitamin D requirements of modern animal genetics (both pigs and poultry), one has but the minimum requirement specifications set by the NRC as a starting base. Quite often, as with all vitamins, multiples of two or more of the minimum levels are employed under commercial conditions. The exact levels differ among consultants, premix suppliers and vitamin manufacturers.

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