Wild rabbits living in Finland located at 60 ºN have very low serum 25-hydroxyvitamin D concentrations, as mean concentration was only 3.3 ng/mL. Season of sample collection did not affect concentrations despite UVB radiation being higher during summer months. By contrast, using the same analysis method, a mean 25(OH)D concentration of 26.0 ng/mL was reported in Finnish pet rabbits [12]. Outdoor access during summer months was not associated with 25(OH)D concentration, and diet was determined to be the main source of vitamin D in pet rabbits [12]. The diet of wild rabbits consists of fresh grass and plants during summer and is very restricted during winter when snow covers the ground. As ergocalciferol is synthesized after sun exposure in the cell membrane of fungi- contaminated plant material, fresh grass and plants have low vitamin D content. The diet of Finnish wild rabbits during winter comprises mainly twigs, roots, bark, and conifer needles [13]. In urban areas, wild rabbits also eat flowers from cemeteries and seeds from bird feeders [13]. These are all poor sources for vitamin D and cannot compensate for the lack of endogenous vitamin D synthesis during winter months. In pet rabbits’ diet, good-quality dry hay in Finland contains vitamin D precursors at approximately 1000 IU/kg and commercial rabbit, horse, and cattle food fed to Finnish pet rabbits at 1000− 3000 IU/kg [12]. The differences in dietary vitamin D levels therefore likely explain the differences in 25(OH)D concentrations between wild and pet rabbits.
Living conditions for wild rabbits in Finland are challenging and differ from those of the Iberian Peninsula, where rabbits originate [14]. Endogenous vitamin D synthesis in the skin is possible in Southern Finland for people with light skin types (I–III) only between mid-March and mid-October if the exposure time is at least 30 min [15]. For people with darker skin types (IV–VI), synthesis occurs only during one or two summer months [15]. Rabbits are crepuscular animals and might therefore spend mid-day in underground burrows. Mid-day is the best time for endogenous vitamin D synthesis, and wild rabbits may therefore miss the opportunity for endogenous vitamin D synthesis, as was suspected to happen in pet rabbits with limited outdoor access [12]. In the Iberian Peninsula, located at latitude 40 ºN, where UVB radiation is higher than in Finland, endogenous vitamin D synthesis is possible throughout the year [15]. People with light skin types (I–III) can receive one standard vitamin D dose (SDD) when 1/4 of the body surface is exposed for 15 min during 9.5 months of the year [15]. This corresponds to an oral dose of about 1000 IU (25 µg) of vitamin D [15]. If exposure time is elongated to 60 min, endogenous vitamin D synthesis is possible at latitude 40 ºN throughout the year [15].
Previously, 25(OH)D concentration of 17 ng/mL was suggested as a threshold for vitamin D deficiency in rabbits [11]. Using this benchmark, all wild rabbits in our study appeared to have severe vitamin D deficiency. In humans, the threshold for severe vitamin D deficiency is 12 ng/mL, but concentrations of up to 40 ng/ml are recommended for optimal cellular health [16, 17]. The lifespan of wild rabbits is short relative to pet rabbits, with rabbits in the wild rarely reaching the age of 3 years [18]. Life expectancy of a newborn rabbit is 70 days, and 40% of adult rabbits die before beginning the second reproductive season [18]. The most common reasons for mortality are diarrhoea due to coccidiosis [18] and rabbit haemorrhagic disease, other diseases, predators, and accidents [19]. The possible association of vitamin D deficiency in these diseases has not been investigated. In humans, optimal vitamin D status has been recognized as a front-line factor in prophylaxis for musculoskeletal disorders, infections and autoimmune diseases, cardiovascular disease, type 1 and 2 diabetes mellitus, several types of cancers, neurocognitive dysfunction and mental illness, and reproductive diseases [20].
Calcium metabolism in rabbits differs from that of many other mammals. Calcium absorption from the intestines is passive [2]. Vitamin D-dependent absorption is needed only if dietary calcium content is low. Almost all dietary calcium is absorbed and blood calcium concentration in rabbits is therefore higher than in other mammals. Calcium metabolism has similarities with that of horses. In horses, low serum 25(OH)D concentrations have also been reported [21]. When 25(OH)D concentrations were compared in pasturing horses with or without full-covering blankets, no differences were observed [21]. Serum 25(OH)D2 concentrations were, however, higher than 25(OH)D3 concentrations during the summer months in both groups [21]. The authors postulated that horses were dependent on dietary sources of vitamin D instead of endogenous synthesis [21]. In rabbits, higher 25(OH)D concentrations were observed after artificial UVB light exposure [7, 8]. These studies measured the total 25(OH)D concentration, as did ours. Measuring 25(OH)D2 and 25(OH)D3 separately would likely yield more information regarding the source of the vitamin D. In an artificial UVB light study, rabbits were fed commercial rabbit food and timothy hay [7, 8]. Vitamin D in hay originates from high amount of ergosterol in cell membranes of endophytic fungi that is synthesised to D2 after UVB exposure. As the 25(OH)D2 and 25(OH)D3 concentrations were not measured separately in the previously mentioned artificial UVB study, it is possible that ergocalciferol level in the hay increased during the trial, explaining the increase in serum total 25(OH)D concentrations.
Our study has some limitations. The lack of separate analyses of 25(OH)D2 and 25(OH)D3 concentrations is a clear drawback. The need for separate analyses was noted only after receiving the unexpectedly low results. In addition, blood sample collection was spread over several years due to difficulties in sample collection. The rabbits were mainly hunted at night in different locations, making sample collection logistically challenging. Also, rabbit haemorrhagic disease and myxomatosis spread in Finland during this period, reducing the wild rabbit population radically. Some samples were thus frozen for several years. However, 25(OH)D has been shown to be very stable, up to 24 years when stored at − 24 ºC [22]. A decrease of only 2.3% in 25(OH)D concentration was observed after human whole-blood samples were stored for 72 h at room temperature [23]. In our study, samples were collected as soon after death as possible. As capture was done mainly during colder months, temperature was unlikely to be a significant factor. Some of the samples were haemolytic and some lipaemic. According to the manufacturer, haemoglobin up to 1470 mg/dL, bilirubin up to 513 µmol/L, and triglyceride up to 5.6 mmol/L do not interfere with the assay used, and thus, haemolysis and lipaemia were unlikely to affect the results. While no apparent diseases were observed among the wild rabbits caught in this study, it is possible that our sample may have been biased if rabbits with exceptionally low 25(OH)D concentrations were more likely to be caught.
Add Comment