When evaluating options in terms of supplementing the vitamin, there are many different commercial metabolites available. Care should be taken when making the final selection though: metabolites matter, but not all are created equal.
To be able to choose the right vitamin D supplementation, understanding the vitamin's metabolic pathway in the animal itself and how supplementation can link into this is fundamental. Vitamin D3 is usually taken as the starting point of the metabolic pathway and there are two ways an animal can source this particular metabolite: either from a precursor in the skin or from the diet.
The precursor route requires direct sunlight to be converted into vitamin D3, but access to direct sunlight is usually limited for most production animals. At the same time, standard raw materials in the diet often do not contain sufficient amounts of vitamin D3 on their own. This makes supplementation vital, but vitamin D3 is not the only option for this.
Returning to the metabolic pathway itself, there are two distinct steps to arrive at the active form of vitamin D. First, vitamin D3 is transported to the liver where the first hydroxylation process turns it into 25-hydroxyvitamin D3. A second hydroxylation in the kidneys then transforms 25-hydroxyvitamin D3 into the 1,25-dihydroxyvitamin D3 hormone, considered to be the active form of vitamin D.
It is important to understand that vitamin D supplementation can link in at every step of the pathway, giving producers a certain degree of flexibility. Briefly, there are three possible metabolites that can be supplemented:
- vitamin D3
- 1,25-dihydroxyvitamin D3
- 25-hydroxyvitamin D3
It is not easy to choose between these: each option has its own benefits and drawbacks, while there are also practical and regulatory aspects to keep in mind.
Active form of vitamin D
As a first option, vitamin D3 is often considered the easiest and most economic metabolite to fulfil vitamin D requirements. However, vitamin D3 depends on proper liver function to reach the active form of vitamin D. This should not be underestimated as the liver faces plenty of challenges in modern animal production: mycotoxins, age, diet type, fatty liver syndrome and heat stress are all common examples of parameters influencing good liver function.
The same applies to 1-α-hydroxyvitamin D3, a synthetic precursor to the active hormone that still requires hydroxylation in the liver. Proper liver function is not a concern for supplementation with 1,25-dihydroxyvitamin D3 itself, as the hormone is already the active form of vitamin D. Although it can have a benefit, this also means that the natural feedback mechanism of the parathyroid glands in terms of the amount of 1,25-dihydroxyvitamin D3 is by-passed. This mechanism is regulated via the production of parathyroid hormone (PTH) in relation to the presence of 1,25-dihydroxyvitamin D3, and plays a pivotal role in the animal's vitamin D homeostasis.
Under normal circumstances, the levels of active 1,25-dihydroxyvitamin D3 are regulated quite well to prevent overproduction of the active 1,25-dihydroxyvitamin D3 and the associated toxicity risk. From a practical point of view it is important to remember 1,25-dihydroxyvitamin D3 itself has a very short half-life. This makes it a difficult parameter to assess the animal's overall vitamin D status, especially in relation to dietary vitamin D supplementation.
As a third option, 25-hydroxyvitamin D3 combines the benefits of the other two without the risks. It joins the vitamin D metabolic pathway after hydroxylation in the liver takes place, hence it does not rely on the organ to achieve effective vitamin D supplementation. Although still one step removed from the main active form of vitamin D - this minimises the potential risks. Evaluating the metabolite's supplementation is also fairly straightforward because serum 25-hydroxyvitamin D3 can be easily detected and responds rapidly to changes in supplementation.
Keeping the above in mind, it is clear that the underlying physiology of vitamin D supplementation should be considered when evaluating which metabolite to add to the animals' diet. Selecting 25-hydroxyvitamin D3 as the preferred option thus makes sense. When looking at the current commerical market, it is important to realize that not all 25-hydroxyvitamin D3s are the same: depending on the nature of the production process, there are distinct differences in terms of bioavailability and stability.
Bio D®, a novel 25-hydroxyvitamin D3 product, has been designed with precisely this in mind. It is produced via fermentation, resulting in great 25-hydroxyvitamin D3 bioavailability and stability. As such, Bio D® helps producers to meet their animals' vitamin D requirements, effectively supporting them from start to finish.