Vitamin D metabolism in a frugivorous nocturnal mammal, the Egyptian fruit bat (Rousettus aegyptiacus)

doi:10.1016/S0016-6480(03)00150-3

General and Comparative Endocrinology

Volume 133, Issue 1, August 2003, Pages 109-117

https://doi.org/10.1016/S0016-6480(03)00150-3 Get rights and content

Abstract

The nocturnal, frugivorous Egyptian fruit bat (Rousettus aegyptiacus) has no obvious access to either endogenous or dietary sources of vitamin D. We hypothesized that this species under natural conditions would be vitamin D deficient and that both serum mineral concentrations and vitamin D metabolite concentrations would be low. Both wild populations and captive populations appear to have an impoverished vitamin D status, as concentrations of the principle circulating metabolite, 25-hydroxyvitamin D [25(OH)D] are undetectable (<4 ng/mL) and those of the active metabolite, 1,25-dihydroxyvitamin D [1,25(OH)₂D] are low. Intraperitoneal administration of labelled 25(OH)D revealed enhanced 1 α-hydroxylase activity confirming a natural state of vitamin D deficiency. This may account for the undetectable levels of 25(OH)D; for limited amounts of the prohormone substrate are rapidly converted to the active hormone. Both vitamin D₂ and D₃ metabolites were detected in bat serum, albeit in very small amounts, inferring that in their natural habitat fruit bats may have limited access to both exogenous dietary sources and endogenous sources. Despite the low levels of vitamin D metabolites in wild-caught and captive D-unsupplemented individuals, serum mineral concentrations were well regulated and similar to those of bats receiving D-supplements, with no pathological problems associated with vitamin D deficiency evident.

Introduction

The vitamin D endocrine system is unique amongst endocrines in that its precursor molecule (7-dehydrocholesterol) is dependent upon sunlight for its endogenous synthesis (Holick and Clark, 1978). Furthermore, this endocrine system also requires multi-organ participation in the activation and biological expression of the metabolically active hormone 1,25-dihydroxyvitamin D [1,25(OH)₂D] (DeLuca, 1984; Holick, 1995; Norman and Silva, 2001). Despite this complexity, the vitamin D endocrine system is evolutionarily conserved and is present in both invertebrates and vertebrates (Bidmon and Stumpf, 1991; Carman-Elliott et al., 2000; Holick et al., 1995; Horst et al., 1981; Kobayashi et al., 1991; Laing and Fraser, 1999; Reichel et al., 1987). Not surprisingly, across all phyla of the animal kingdom, vitamin D is considered essential for life. This pleiotropic hormone regulates cell proliferation and differentiation, mineral metabolism, immune responses, and brain function as well as influencing many intracellular processes (DeLuca and Cantorna, 2001; Garcion et al., 2002; Hewison et al., 2001; Norman and Silva, 2001). Vitamin D status is of considerable importance in calcium metabolism because the active hormone regulates active calcium uptake in the gastrointestinal tract, renal calcium excretion, and skeletal calcium turnover (DeLuca, 1984; Holick, 1989; Norman and Silva, 2001). Because of its ubiquitous role in mineral metabolism, it is generally accepted that in states of chronic vitamin D deficiency, skeletal integrity, and cellular functions are compromised.

In most vertebrates the metabolite produced in the liver, 25-hydroxyvitamin D [25(OH)D], is the principal circulating form and its concentration is used clinically to assess vitamin D status (Audran and Kumar, 1985; Holick, 1989). In blood, vitamin D and its metabolites are transported bound to a specific carrier, vitamin D binding protein [DBP] that is found in all vertebrates studied to date (Bidmon and Stumpf, 1991; Haddad, 1984).

The photo-biosynthetic prohormone, vitamin D, is stored within lipids but primarily in the liver. Vitamin D₃ (D₃) is commonly acquired by consumption of animal fats while vitamin D₂ (D₂) is mainly obtained in the irradiated portions of plants (Aburjai et al., 1998; Boland, 1986; Horst et al., 1984). Generally, D₃ concentrations are considerably higher than those of D₂ and contribute substantially more to the maintenance of normal vitamin D status (Horst et al., 1981; Reichel et al., 1987). The ratio of D₂ concentration to that of D₃ reflects primarily the habitat and diet of animals as well as the rate of metabolism of the two vitamin D analogues, with most animals discriminating against D₂ in favour of D₃ (Horst et al., 1981).

The Egyptian fruit bat (Rousettus aegyptiacus) is a gregarious cave dweller. At night it leaves its roost to forage for fruit (Skinner and Smithers, 1990), eating the pulp but discarding the peel, some insoluble fiber and the seeds. The pulp is not exposed to ultra violet irradiation and, like most plant material with the exception of two Argentinian plants (Breslau and Horst, 1997; Wasserman et al., 1976), does not contain measurable quantities of either vitamin D₃ or D₂ (Gouws and Langenhoven, 1986). Being nocturnal and specialized frugivores, these bats thus have no obvious access to vitamin D. Previous studies have shown that these fruit bats make vitamin D dependent calcium binding proteins (Opperman and Ross, 1990), yet gastrointestinal mineral absorption does not rely upon active transport but rather uses a passive process (Keegan et al., 1980). We questioned if these mammals were naturally in a state of vitamin D deficiency, and whether or not metabolism of vitamin D to more active metabolites as well as the mode of transport of this hormone differed from that of animals regularly exposed to sunlight and able to manufacture endogenously this vital hormone. In addition we assessed the effects of vitamin D supplementation and prolonged captivity on concentrations of vitamin D metabolites and calcium.

Section snippets

Bat capture

Eighty-one wild bats were caught on the farm “Haffned Heights” at Matlapitsi, Limpopo, South Africa. Nets were placed across the entrance of the roosting cave and the animals were caught on their return to the cave just before sunrise. Wild bats (n=10) were bled from the wing vein within 6 h of capture.

Care of captive bats

Captive fruit bats were cared for in the animal unit of the South African Institute for Medical Research (S.A.I.M.R.) Johannesburg, South Africa. They were housed in a large dark room in which a

Concentrations of vitamin D metabolites in bat serum

Irrespective of both time in captivity and vitamin D₃ supplementation, bats had undetectable circulating levels (<4 ng/ml) of 25(OH)D. The 1,25(OH)₂D serum concentrations in both captive groups and the wild caught animals were significantly different; captive animals receiving a vitamin D-supplement showed the highest serum 1,25(OH)₂D concentrations while those not supplemented with vitamin D showed the lowest concentrations (Table 1). Indeed six of the captive bats maintained on a diet not

Discussion

We questioned if a nocturnal frugivorous mammal with no obvious access to either photo-biosynthetic sources or dietary sources of vitamin D would (a) have a fully functional vitamin D endocrine system and (b) be naturally in a vitamin D depleted state. We noted that fruit bats have vitamin D binding proteins and metabolise tritium-labelled vitamin D₃ to both 25(OH)D₃ and other metabolites. They, therefore, possess the full complement of proteins needed for a fully functional vitamin D endocrine

Acknowledgements

Sincere thanks to Dr J. van der Westhuizen for kindly catching the wild bats and to the Animal Unit of the SAIMR, Johannesburg for housing and caring for the animals used in this study. Financial support from the South African Medical Research Council and the University of the Witwatersrand is gratefully acknowledged.

References (52)

T. Aburjai et al.
Vitamin D₃ and its metabolites in tomato, potato, egg plant and zuchini leaves
Phytochemistry
(1998)
M. Audran et al.
The physiology and pathophysiology of vitamin D
Mayo Clin. Proc.
(1985)
E. Garcion et al.
New clues about vitamin D functions in the nervous system
Trends Endocrinol. Metab.
(2002)
M.F. Holick
The use and interpretation of assays for vitamin D and its metabolites
J. Nutr.
(1990)
M.F. Holick
Environmental factors that influence the cutaneous production of vitamin D
Am. J. Clin. Nutr.
(1995)
R.L. Horst et al.
Quantitation of vitamin D and its metabolites and their plasma concentrations in five species of animals
Anal. Biochem.
(1981)
R.L. Horst et al.
The isolation and identification of vitamin D2 and vitamin D3 from Medicago sativa (alfalfa plant)
Arch. Biochem. Biophys.
(1984)
G.G. Kwiecinski et al.
Observations on serum 25-hydroxyvitamin D and calcium concentrations from wild-caught and captive neotropical bats, Artibeus jamaicensis
Gen. Comp. Endocrinol.
(2001)
C.J. Laing et al.
The vitamin D endocrine system in iguanian lizards
Comp. Biochem. Physiol. B
(1999)
O.H. Lowry et al.
Protein measurements with the folin phenol reagent
J. Biol. Chem.
(1951)

L.A. Opperman et al.

The adult fruit bat expresses only calbindin 9 K (vitamin D dependent calcium binding protein)

Comp. Biochem. Physiol.

(1990)

H.J. Bidmon et al.

Phylogeny of receptors for 1,25-dihydroxyvitamin D₃. (1,25-D₃, soltriol) in vertebrates and invertebrates

R.L. Boland

Plants as a source of vitamin D₃ metabolites

Nutr. Rev.

(1986)

A. Breidenbach et al.

Peculiarities of vitamin D and of the calcium and phosphate homeostatic system in horses

Vet. Res.

(1998)

N.A. Breslau et al.

Pharmacology of vitamin D preparations

R. Buffenstein et al.

Subterranean mole-rats naturally have an impoverished calciol status, yet synthesize calciol metabolites and calbindins

Eur. J. Endocrinol.

(1994)

R. Buffenstein et al.

Vitamin D hydroxylases and their regulation in a naturally vitamin D-deficient subterranean mammal, the naked mole rat (Heterocephalus glaber)

J. Endocrinol.

(1993)

N. Carman-Elliott et al.

Photobiosynthetic opportunity and ability for UV-B generated vitamin D synthesis in free-living House Geckos (Hemidactylus turcicus) and Texas spiny lizards (Sceloporus olivaceous)

Copeia

(2000)

J.J.L. Cilliers et al.

Determination of serum 25-hydroxyvitamin D3 by HPLC using 25-hydroxyvitamin D2 as an internal standard

J. Micronutr. Anal.

(1987)

S.A. Clark et al.

Somatomedin-C/insulin like growth factor I and vitamin induced growth

Endocrine

(1986)

H.F. DeLuca

The metabolism, physiology and function of vitamin D

H.F. DeLuca et al.

Vitamin D: its role and uses in immunology

FASEB J.

(2001)

E.S. Dierenfeld et al.

Plasma fat-soluble vitamin and mineral concentrations in relation to diet in captive pteropodid bats

J. Zoo Wildlife Med.

(2000)

T.B. Doumas et al.

Albumen standards and the measurement of serum albumin with bromcresel green

Clin. Chim. Acta

(1971)

M.Z. Gardener et al.

Biological assessment of vitamin D metabolites produced by rumen bacteria

J. Steroid Biochem.

(1988)

J.S. Garvey et al.

Methods of Immunology

(1977)

Cited by (24)

The diets of bats
2023, A Natural History of Bat Foraging: Evolution, Physiology, Ecology, Behavior, and Conservation
Bats consume an extraordinary diversity of foods including insects, fruits, nectar, arthropods, seeds, frogs, fish, small mammals, birds, crustaceans, and blood. They are often classified into one of six broad trophic guilds: frugivores, nectarivores, insectivores, carnivores, piscivores, and sanguivores, which allow us to generalize about their ecological roll. However, increasing evidence from new analytical methods suggests that assigning many species to these broad categories is inaccurate at best and may compromise hypothesis testing and conservation planning. While most biologists will generally agree that strict guild categories are problematic, we still rely on guilds in scientific analysis and most bat biologists likely to not realize the extent of the ecological flexibility of many species. The implications of this overgeneralization are not well considered. For most species, there is limited dietary and ecological data and exceptions from their expected guild behavior are generally passed off as “incidental.” However, when these cases are considered in a broader context, we are beginning to see a picture of omnivory begin common, raising extremely interesting questions about how these behaviors may be achieved. We may even consider this as the collapse of our traditional view of individual guilds. In this chapter we examine each “guild,” one at a time, and present an alternative view of the ecological structure of bat diets. We conclude with an argument for the conservation importance of full dietary analysis and the risks associated with under-reporting or overgeneralizing diet.
Evidence of chronic cadmium exposure identified in the critically endangered Christmas Island flying-fox (Pteropus natalis)
2021, Science of the Total Environment
Citation Excerpt :
Other possible causes for the observed bone lesions include dietary vitamin D deficiency, calcium deficiency or elevated dietary phosphorus. It is not entirely clear how bats meet their vitamin D requirements, however studies of frugivorous bats reported vitamin D can be synthesized from exposure to sunlight (Southworth et al., 2013) or through consumption of fruit skins (Cavaleros et al., 2003). It is therefore unlikely that the Christmas Island flying-fox would be deficient in vitamin D as it is constantly exposed to sunlight while roosting during the day, however further studies are necessary to confirm this.
The Christmas Island flying-fox (Pteropus natalis) is the last native mammal on Christmas Island and its population is in decline. Phosphate mining occurs across much of the eastern side of Christmas Island. The phosphate deposits are naturally rich in cadmium, and potentially other metals, which may be threatening the Christmas Island flying-fox population. To test this, concentrations of metals (cadmium, copper, iron, mercury, lead, and zinc) were measured in fur and urine collected from Christmas Island flying-foxes and interpreted concurrently with urinalysis and serum biochemistry data. In addition, metal concentrations in liver and kidney samples from two Christmas Island flying-foxes and associated histological findings from one of these individuals are reported. Fur cadmium concentrations were significantly higher in the Christmas Island flying-fox compared to concentrations found in flying-foxes in mainland Australia. Additionally, 30% of Christmas Island flying-foxes had urine cadmium concentrations exceeding maximum concentrations previously reported in flying-foxes in mainland Australia. Glucosuria and proteinuria were identified in two Christmas Island flying-foxes, suggestive of renal dysfunction. In one aged flying-fox, kidney cadmium concentrations were four-fold higher than toxic thresholds reported for domestic mammals. Microscopic evaluation of this individual identified bone lesions consistent with those described in laboratory animals with chronic cadmium poisoning. These results suggest that Christmas Island flying-foxes are being exposed to cadmium and identification of these sources is recommended as a focus of future research. Unexpectedly, urine iron concentrations in Christmas Island flying-foxes were higher compared to previous studies of Australian mainland flying-foxes, which suggests that urinary excretion of iron may be an important aspect of iron homeostasis in this species whose diet is iron rich.
The nocturnal leopard gecko (Eublepharis macularius) uses UVb radiation for vitamin D<inf>3</inf> synthesis
2020, Comparative Biochemistry and Physiology Part - B: Biochemistry and Molecular Biology
Citation Excerpt :
Animals can obtain vitamin D either from the diet or via conversion of endogenous 7-dehydrocholesterol (7-DHC) to vitamin D3 in the skin by exposure to UVb radiation (280–320 nm) and subsequent thermal isomerisation. The latter pathway is active in the great majority of animal species studied to date, including mammals (Brot et al., 2001; Cavaleros et al., 2003; Cooper et al., 1997; Hymøller & Jensen, 2010; Jakobsen et al., 2020; Kwiecinski et al., 2001; Southworth et al., 2013; Watson et al., 2014), birds (Drake et al., 2017; Edwards, 2003; Stanford, 2006) insects (Oonincx et al., 2018), and reptiles (Acierno et al., 2006; Acierno et al., 2008; Bos et al., 2018), including diurnal lizards (Ferguson et al., 2009; Gillespie et al., 2000; Laing et al., 2001; Oonincx et al., 2010; Townsend & Cole, 1985). However, this pathway is dysfunctional in some species, including cats, dogs and seals, and has limited functionality in polar bears due to absence or low levels of 7-DHC (Bouillon & Bikle, 2019; How et al., 1994; Keiver et al., 1988; Kenny et al., 1998).
Vitamin D is an important regulator of calcium and phosphorus homeostasis in animals. It can be acquired from the diet or synthesised de novo when skin is exposed to UVb. Vitamin D deficiency can lead to a complex of diseases collectively called metabolic bone disease (MBD). Diurnal lizards without access to UVb are prone to develop vitamin D deficiency, even when dietary vitamin D₃ is provided. A trial was conducted to determine whether juvenile nocturnal lizards require access to UVb to prevent vitamin D deficiency. All leopard geckos (Eublepharis macularius) were supplemented with dietary vitamin D₃. One group was exposed to low level UVb radiation (33–51 μW/cm²) from hatching until 6 months of age and a second group remained unexposed. Animals were fed ad libitum and their growth and weight gain compared with non-exposed controls. At the end of the trial, blood samples were analysed for vitamin D₃ metabolites. The concentration of the vitamin D₃ metabolite, 25(OH)D₃, was higher in UVb exposed animals (61 ± 20 vs. 38 ± 8 nmol/L), confirming cutaneous synthesis with UVb exposure. Growth and weight gain were similar in both groups, and this, together with the absence of clinical symptoms, suggests that dietary vitamin D₃ alone can meet the vitamin D requirements for growth of this nocturnal gecko, during the first six months of life. It remains to be investigated whether the higher vitamin D metabolite levels holds other health benefits for this species, such as improved bone density or immune response.
DIETARY VITAMIN D<inf>3</inf> INFLUENCE ON SERUM 25-HYDROXY VITAMIN D CONCENTRATIONS IN CAPTIVE SUGAR GLIDERS (PETAURUS BREVICEPS)
2018, Journal of Exotic Pet Medicine
A pilot study was performed to investigate the impact of dietary vitamin D on circulating 25-hydroxyvitamin D (25[OH]D) metabolite concentrations in sugar gliders (Petaurus breviceps). The study with diets containing 0, 0.2 (low), or 0.4 (moderate) International Units vitamin D₃ per gram of dry matter and fed to adults at 2 locations. Serum 25[OH]D concentrations did not differ between animals fed produce only (no added vitamin D–either D₂ or D₃) for 3 weeks (8.83 ± 0.98 nmol/L), n = 6, or low dietary levels (7.86 ± 3.80 nmol/L), n = 7, continuously for multiple years. Conversely, animals consuming diets containing moderate vitamin D₃ levels displayed increased circulating concentrations (15.00 ± 3.59), n = 8, after 3 weeks. Despite the response to diets supplemented with vitamin D, overall metabolite levels were low and may indicate minimal metabolic dependence on this nutrient in sugar gliders, similar to processes documented in other hindgut fermenters.
Slow aging in mammals—Lessons from African mole-rats and bats
2017, Seminars in Cell and Developmental Biology
Citation Excerpt :
According to a single report, T4 levels appear to be considerably lower in a species of fruit eating bats (Epomops frangueti) than in most other mammals [83]. Vitamin D signaling is involved in the regulation of cell proliferation and differentiation, mineral (including calcium) metabolism, immune responses, and brain function [84]. The pathway is highly conserved across the animal kingdom, and both deficiency and excess of vitamin D are associated with either premature aging or age-associated diseases such as osteoporosis and osteomalacia, Alzheimer’s disease, and autoimmune diseases such as diabetes in vertebrates, including humans (summarized, e.g., by Tuohimaa [85]).
Traditionally, the main mammalian models used in aging research have been mice and rats, i.e. short-lived species that obviously lack effective maintenance mechanisms to keep their soma in a functional state for prolonged periods of time. It is doubtful that life-extending mechanisms identified only in such short-lived species adequately reflect the diversity of longevity pathways that have naturally evolved in mammals, or that they have much relevance for long-lived species such as humans. Therefore, some complementary, long-lived mammalian models have been introduced to aging research in the past 15–20 years, particularly naked mole-rats (and to a lesser extent also other mole-rats) and bats. Here, I summarize and compare the most important results regarding various aspects of aging – oxidative stress, molecular homeostasis and repair, and endocrinology – that have been obtained from studies using these new mammalian models of high longevity. I argue that the inclusion of these models was an important step forward, because it drew researchers’ attention to certain oversimplifications of existing aging theories and to several features that appear to be universal components of enhanced longevity in mammals. However, even among mammals with high longevity, considerable variation exists with respect to other candidate mechanisms that also must be taken into account if inadequate generalizations are to be avoided.
Chemokine receptor requirements for epidermal T-cell trafficking
2011, American Journal of Pathology
Citation Excerpt :
Very little sunlight is likely to ever reach the skin of such animals. Nocturnal mammals obtain their vitamin D as a nutrient or through alternative metabolic pathways independent of sunlight.31 Unlike human beings, one would not expect the highest concentrations of vitamin D in these animals to occur in the skin.
Inflamed skin contains CD4 T-cell subsets that express chemokine receptors CCR4, CCR6, and/or CCR10. Prior attempts to reveal the distinct role(s) of each receptor in T-cell trafficking to skin have not produced a coherent story. Different conclusions drawn by separate research groups are difficult to reconcile because of the disparate inflammation models used. Here we directly compare CD4 T cells from wild-type, CCR4^−/−, CCR6^−/−, and CCR10^−/− mice in parallel assays of trafficking to skin. Our models require direct competition between wild-type and receptor-deficient populations for access to inflamed cutaneous sites. Major histocompatibility complex-peptide tetramers allowed us to identify antigen-specific endogenous long-term memory CD4 T cells within skin after multiple topical immunizations. We separately analyzed cells from the dermal and epidermal layers, allowing us to assess the involvement of each receptor in trafficking between dermis and epidermis. We found that CCR4 deficiency reduces accumulation of memory CD4 T cells in skin by approximately 20-fold, but neither CCR6 nor CCR10 deficiency yielded any detectable effects. Strikingly, no differences in dermal versus epidermal localization were observed for cells lacking any of these three receptors. Our findings raise the possibility that CCR6 and CCR10 play (as yet) unknown roles in cutaneous T-cell immunology, unrelated to skin-specific trafficking.

View all citing articles on Scopus

View full text