Physiological limit of the daily endogenous cholecalciferol synthesis from UV light in cattle

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The link between UV light (sunlight) and what was eventually to be recognised as the effect of endogenous cholecalciferol (vitamin D3, D3) synthesis in the skin in humans has been known for more than a 100 years, since doctors for the first time successfully cured rickets in children by exposing them to UV light (Rajakumar, 2003). However, it was not until many years later that a connexion between exposure of dairy goats and cattle to UV light and an increase in the antirachitic properties of their milk was shown by Steenbock et al. (1925); thus, for the first time, providing evidence that cattle also express a significant D3 synthesis in the skin during UV‐light exposure.
Cholecalciferol status in the body is assessed by measuring the plasma concentration of 25‐hydroxycholecalciferol (25OHD3), which in cattle, as well as in humans, is the major circulating metabolite of D3 derived from the liver (Olds et al., 2008). Whether a given species is physiologically adapted for obtaining D3 through UV‐light exposure, or for obtaining vitamin D through dietary sources either as D3 or as ergocalciferol (vitamin D2, D2), depends on the nature of the species. In humans, pigs and cattle, UV light is a significant source of endogenous D3 (Horst and Littledike, 1982; Webb et al., 1989), whereas species evolved and living in habitats lacking in, or void of, sunlight obtain vitamin D solely from dietary sources as either D2 or D3. For example, polar bears have very low capacity for D3 synthesis in the skin, probably due to adaptation to heavy fur coverage at far northern latitudes and a marine diet high in D3 (Kenny et al., 1998) and in mole rats, which live under ground, plasma concentrations of 25OHD3 can only be increased through oral supplementation with D3 and not by exposure to UV light (Pitcher et al., 1994), probably because their natural vitamin D source is D2 from moulds in their diet. Isolated skin from fur‐covered animals such as rats, rabbits and guinea pigs can synthesise D3 (Bekemeier, 1959), and the plasma concentration of 25OHD3 in rats does increase during UV‐light exposure (Lawson et al., 1986) but it has been speculated that D3 in rat skin is synthesised in the sebum on the fur and subsequently ingested by the rat during grooming (Carpenter and Zhao, 1999).
Sheep and goats also respond to UV‐light exposure by increasing their plasma concentration of 25OHD3 (Kohler et al., 2013; Kovács et al., 2015). Sheep, however, show higher plasma 25OHD3 concentrations after being exposed to UV light when shorn than when unshorn (Hidiroglou and Karpinski, 1989; Kohler et al., 2013) and can therefore utilise D3 from sunlight during summer when their wool is thin (Kohler et al., 2013). Goats seem to respond similar to cattle to UV light, as shown by Kohler et al. (2013) in a study on goats and sheep grassing at high or low altitude in alpine regions of Switzerland, probably due to goats and cattle being covered by hair rather than fur or wool, which efficiently prevents UV light from reaching the skin surface. Hence, the evolutionary ancestors of goats may have evolved at high‐altitude habitats, where UV light is a significant source of endogenous D3. Horses on the other hand respond very little to both UV‐light exposure and oral supplementation with synthetic D3 with respect to plasma 25OHD3 concentration and appear to have a D3 physiology very different from cattle and other large herbivores (Hymøller and Jensen, 2015).
Cattle can utilise both synthetic D3 from vitamin supplements absorbed from the gastrointestinal tract (Maislos et al., 1981; Dueland et al.
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