This is not the full story. Whilst it has been stated that the 35 parallel is the point where winter sunlight is insufficient to generate sufficient cholecalciferol – across species, this does not explain why most of North America & Australia systems require D3 supplementation. Native alpacas are reared in upland areas of Southern Peru, Western Bolivia, Ecuador, and Northern Chile (>3500m); it has been stated that due to this, and the tropical daylengths, alpacas are subjected to high levels of uv radiation and so have a reduce efficiency of conversion. That is, lower levels of cholecalciferol are generated.
Vitamin D3 is generated from cholesterol via 7-dehydrocholesterol through UV radiation under the skin:
Vitamin D from the skin diffuses into the blood, where it is transported by vitamin binding protein (DPB) to the liver, whereas vitamin D3 from the diet is absorbed in the small intestine and transported to the liver via chylomicrons and DPB. Whilst UK alpaca diets are supplemented with synthetic D3, those reared, and wild vicuna, in S. America appear not to sufficiency from serious deficiencies – possibly more common in cria. It would appear that either they can generate sufficient D3 through the skin or they have diets rich in D3.
With temperate climates, alpacas appear to need extra supplemental D3; in Australia, most of which is within the 35o Southern Latitude and lower than 1000m, it is suggested that for alpacas in southern Australia a subcutaneous dose of 1000 IU D3/kg body weight to crias in late autumn and again in mid winter and to adult females in mid winter should prevent vitamin D inadequacy.
Extrapolating this to the UK, at 54o Parallel, and <1000m, it could be argued that there is insufficient UV radiation to generate sufficient D3 even during summer months, although with daylength up to 18hours, there is sufficient time to synthesise D3 sub-dermally.
Having said that, in an earlier article (Vitamin D and Alpacas, Injection, paste or something else?), it was concluded that increasing dietary intake of vitamin D3 should be a more cost effective. For example doubling the intake of Camelibra would cover the nutritional shortfall of 900IU per day that winter brings. One or two injections across the season would need to have an overage to take into account the half life of cholecalciferol.
It has generally been accepted that plant sources of vitamin D is as D2 or ergocalciferol, and that levels are low; high levels are found in algae & fungi and it is assumed that ingestion alongside forage accounts for some improvement in vitamin D status in herbivores.
Ergocalciferol is thought to have poorer absorbability and lower plasma transport systems than cholecalciferol and so background D levels are not usually accounted for when supplementing diets for alpacas. However, its biochemical activity is the same as cholecalciferol.
It has now been found that plant metabolism is capable of synthesising D3 & its metabolites, via lanesterol being a precursor of cholesterol in plants; it is known to be a precursor of cholesterol in animals and ergocalciferol in fungi.
This illustrates the pathway of lanosterol to 7-dehydroxycholesterol, which is Provitamin D3. That is, Plants are capable of generating vitamin D3, and its use of UVB can explain high levels of D3 in sun dried hays.
Angiosperms (flowering plants) have this apparent biochemical pathway, with some families having particularly high levels – the Solanaceae. Included in this family are nightshades, potato, tomato, whilst other groups include legumes, peas, beans and grasses & cereals. Field dried hay can have vitamin D3 levels up to 2300 IU/kg, and fresh grass 900. This would suggest that there would be situations where D3 deficiencies may not occur.
Obviously, there will be situations where deficiency does occur; Australian research does recommend winter injections; however it is possible that, having a shorter summer daylength, there is a lower D level in preserved forage, and possibly forage per se.
Vitamin D toxicity is unlikely when administering winter injections. Although administration of ~40x recommended level of 2000IU per kg liveweight has been proven lethal (based on an mathematical misunderstanding - 1µg D = 40IU), there is no danger in supplemental injections.
However, it can be argued, there is no benefit either.
References:
- Hymøller and Jensen. We Know Next to Nothing About Vitamin D in Horses! Journal of Equine Veterinary Science 35 (2015) 785–792.
- Hymøller and Jensen, Plasma transport of ergocalciferol and cholecalciferol and their 25-hydroxylated metabolites in dairy cows, Domestic Animal Endocrinology, Volume 59, April 2017, Pages 44-52
- Janoušek et al. Vitamin D: sources, physiological role, biokinetics, deficiency, therapeutic use, toxicity, and overview of analytical methods for detection of vitamin D and its metabolites. 2022. Critical Reviews in Clinical Laboratory Sciences, 59:8, 517-554
- Jäpelt and Jakobsen. Vitamin D in plants: a review of occurrence ,analysis, and biosynthesis. Frontiers in Plant Science. May2013|Volume4|Article136 |
- Judson and Feakes. Vitamin D doses for alpacas (Lama pacos). Aust Vet J Vol 77, No 5, May 1999. 310
- Wagener et al. Calcinosis in Alpaca Crias (Vicugna pacos) Due to Vitamin D Intoxication—Clinical, Laboratory and Pathological Findings with a Focus on Kidney Function. Animals 2021, 11, 2332.
