I live at a Northern latitude and my doctor recently diagnosed me as vitamin D deficient. She recommended I take an oral vitamin D supplement. My doctor explained that vitamin D deficiency in people is being more commonly recognized in geographic regions like mine and is due to diminished contact with sunlight for extended periods of the year. This got me wondering, is the same also true for horses? Do horses become vitamin D deficient in winter and should I be supplementing in some way?
A: A review article written by Danish researchers and published in the Journal of Equine Veterinary Science in 2015 summarized our current knowledge about vitamin D and horses. The authors’ take-home message: We “know next to nothing about vitamin D in horses.” This makes it hard to give you a concise answer, but from what we do know I do think that provision of supplemental sources of dietary vitamin D may be of benefit to some horses.
What is Vitamin D?
Vitamin D is available to your horse in two forms: vitamin D2 (ergocalciferol) found in plants and vitamin D3 (cholecalciferol), which is synthesized in the skin on exposure to sunlight or provided orally in synthetic form. Neither of these forms of vitamin D are active and both must go through a number of activation steps before they can influence physiologic functions in the body. The first step occurs in the liver where they are enzymatically hydroxylated to 25-hydroxyvitamin D2 (25OHD2) and 25-hydroxyvitamin D3 (25OHD3). These metabolites then circulate throughout the body in blood plasma.
Almost all organs of the body can further activate these two metabolites to their active form however most of the final conversion occurs in the kidneys. Here 25OHD2 and 25OHD3 are converted to 1,α25-dihydroxyvitamin D2 (1,25(OH)2D2) and 1,α25-dihydroxyvitamin D3 (1,25(OH)2D3) the hormonally active forms of the vitamin that can bind to the vitamin D receptors throughout the body, where they influence physiological functions.
While we think of vitamin D as a vitamin it’s actually a hormone because it’s transported in blood and has regulatory actions on various cells in the body.
Typically, vitamin D status is assessed by measuring 25OHD2and 25OHD3 in plasma, and results indicate how much influence exposure to sunlight versus plant-based vitamin D are having on vitamin D status. However, not all labs are able to distinguish between these two forms and instead report combined levels as 25OHDx. Additionally, there’s little research in this area in horses, so it’s hard to know how much horses rely on vitamin D3 generated from sunlight and how much this availability changes throughout the year or at different latitudes.
Latitude, Location, and Vitamin D
As you recently found out, time of year and latitude can affect 25OHD2and 25OHD3 status in people. This is because how efficiently the D3 form is synthesized depends on the intensity of the sunlight. Specifically light in the UV-B spectrum is needed for synthesis of vitamin D3. In the winter the sun appears lower in the sky resulting in a larger zenith angle (the angle created between a vertical line and the angle at which the sun’s rays hit the earth). This causes wavelengths in the UV-B spectrum to hit the Earth’s atmosphere in such a way that they are reflected. Thus, activation of the precursors to vitamin D3 in the skin isn’t possible between September and April above a latitude of 51oN, which equates to much of Canada, Great Britain, northern Germany, Scandinavia, etc.
As it turns out, the levels of plasma 25OHD2and 25OHD3 in horses are generally very low regardless of time of year or latitude. In fact, there’s debate as to whether animals with substantial haircoat are able to synthesize vitamin D3 at all. The limited data that we do have suggests that, in fact, horses do generate vitamin D3 in skin but that it may have limited impact on plasma 25OHD3. However, until more research is conducted, don’t assume that endogenous sources of vitamin D3 aren’t important to the horse.
Vitamin D Sources for Horses
Sun-cured forages have historically been considered good sources of vitamin D2, because levels increase during the curing process. However, some researchers believe that this source might not be as abundant today as it was historically, because farming practices have changed dramatically since this early research was done. Plus, much of the data collected on the availability of vitamin D2was conducted using rat models and methods that might not extrapolate well to horses.
It’s clear that our knowledge of vitamin D synthesis and metabolism in the horse is lacking. It does appear that vitamin D3 has a limited impact on 25OHD3 status and forages might not provide as much vitamin D2 as we previously thought. A number of new studies have taken place since the National Research Council guidelines were published in 2007.
While these recommendations might not be completely accurate based on the knowledge we’re gaining about vitamin D’s physiological impacts (they might, in fact, need more than we currently recommend), the current recommendations are as follows:
- For mature horses, aim for 6.6 IU per kilogram of body weight; and
- For growing horses, the recommended daily needs decrease with age: Horses at 0-6, 7-12, 13-18, and 19-24 months of age should receive 22.2, 17.4, 15.9, and 13.7 IU per kilogram of body weight, respectively.
Based on the large gaps in our current knowledge of vitamin D requirements in horses, and the fact that the presumed safe upper limit for vitamin D in the horse is currently 44 IU/kg body weight per day, I recommend ensuring that horses at northern latitudes and those with limited sunlight exposure to sunlight get fed levels of vitamin D that meet the current NRC guidelines. You can achieve this by selecting a fortified commercial feed or supplement that provides information about the guaranteed levels of vitamin D, and then feeding in the correct amount.