ORIGINAL ARTICLE Concentrations of retinol, 3,4-didehydroretinol, and retinyl esters in plasma of free-ranging birds of prey K. Mu ¨ ller 1 , J. Raila 2 , R. Altenkamp 3 , D. Schmidt 4 , R. Dietrich 5 , A. Hurtienne 2 , M. Wink 6 , O. Krone 7 , L. Brunnberg 1 and F. J. Schweigert 2 1 College of Veterinary Medicine, Small Animal Clinic, Freie Universita ¨ t Berlin, Germany 2 Institute of Nutritional Science, University of Potsdam, Potsdam-Rehbru ¨ cke, Germany 3 Institute of Biology and Zoology (Section Evolutionary Biology), Freie Universita ¨ t Berlin, Germany 4 Bird Conservation Centre of the Nature and Biodiversity Conservation Union (NABU), Mo ¨ ssingen, Germany 5 Berlin, Germany 6 Institute of Pharmacy & Molecular Biotechnology (IPMB), Heidelberg University, Heidelberg, Germany 7 Leibniz-Institute for Zoo and Wildlife Research, Berlin, Germany Introduction Vitamin A is a group of fat-soluble substances pos- sessing the biological activity of all-trans-retinol, which occurs naturally only in animals including humans. To this group belong retinol (vitamin A 1 ), 3,4-didehydroretinol (vitamin A 2 ), as well as their retinyl esters and aldehyde (retinal, 3,4-didehydrore- tinal). The biological functions of vitamin A are manifold and involve sensory performances like vision, the embryonic development, cell differentia- tion, haematopoesis, growth, reproduction, immune functions, and development of tumour resistances (Moore, 1957; Olson, 1984; Wolf, 1984). Herbivores can meet their vitamin A requirements from provitamin A carotenoids such as a- and b-carotene or b-cryptoxanthin found in the vegeta- ble diet, which are converted via retinal to retinol and retinyl esters in the enterocytes (Wolf, 1984). Carnivores ingest preformed vitamin A as retinol Keywords birds of prey, plasma, retinol, 3,4-didehydroretinol, retinyl esters Correspondence Kerstin Mu ¨ ller, Freie Universita ¨ t Berlin, College of Veterinary Medicine, Small Animal Clinic, Oertzenweg 19b, D-14163 Berlin, Germany. Tel: + 49-30-83-86-23-88; Fax: + 49-30-83-86- 25-21; E-mail: Muellerkerstin@gmx.de Received: 13 April 2011; accepted: 10 July 2011 Summary This study investigated vitamin A compounds in the plasma of healthy free-ranging Central European raptors with different feeding strategies. Plasma samples of nestlings of white-tailed sea eagle [white-tailed sea eagle (WTSE), Haliaeetus albicilla)(n = 32), osprey (Pandion haliaetus) (n = 39), northern goshawk (Accipiter gentilis)(n = 25), common buzzard (Buteo buteo)(n = 31), and honey buzzard (Pernis apivorus)(n = 18) and adults of WTSE (n = 10), osprey (n = 31), and northern goshawk (n = 45) were investigated with reversed-phase-high-performance liquid chromatography (RP-HPLC). In WTSE, northern goshawks and common buzzards retinol were the main plasma component of vitamin A, whilst in ospreys and honey buzzards, 3,4-didehydroretinol predominated. The median of the retinol plasma concentration in the nestlings group ran- ged from 0.12 to 3.80 lM and in the adult group from 0.15 to 6.13 lM. Median plasma concentrations of 3,4-didehydroretinol in nestlings ran- ged from 0.06 to 3.55 lM. In adults, northern goshawks had the lowest plasma concentration of 3,4-didehydroretinol followed by WTSE and ospreys. The plasma of all investigated species contained retinyl esters (palmitate, oleate, and stearate). The results show considerable species- specific differences in the vitamin A plasma concentrations that might be caused by different nutrition strategies. DOI: 10.1111/j.1439-0396.2011.01219.x 1044 Journal of Animal Physiology and Animal Nutrition 96 (2012) 1044–1053 ª 2011 Blackwell Verlag GmbH