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The main objective of the present study was to examine and optimize the housing conditions of marmosets with regard to nutrition composition and nutrient uptake, as well as to ultraviolet radiation, at F. Hoffmann-La Roche Ltd.'s animal husbandry unit in Basel, Switzerland. Particular emphasis focused on bone health protection. The investigation comprised three studies: 1. In the first study, peripheral quantitative computed tomography (pQCT) was applied to 149 female (n=64) and male (n=85) marmosets aged between 1.5 months and 7.5 years. This study was conducted in order to describe the bone mineral density status (BMD) of the colony and to facilitate the choice of animals for the main study. To the author's knowledge, this is the first reported age-dependent characterization of bone mineral density in marmosets using pQCT. The study showed an increase in cortical and total BMD (BMDcort, BMDtot) in adolescent marmosets and the achievement of peak BMD at the age of approx. 2 years. In contrast to the low BMD measured by dual-energy X-ray absorptiometry (DEXA) in other primate species, <1 year old marmosets showed the highest trabecular bone mineral density (BMDtrab) of the study population. Subsequently the BMDtrab decreased until the age of 2 years, the cause of which could not be completely determined. 2. In the second study, standard methods and commercially available kits for detecting urinary (u) and serum (s) markers of bone formation (serum osteocalcin [sOC], serum Nterminal pro-peptide of human pro-collagen type I [sPINP]), bone resorption (urinary pyridinoline [uPYD], urinary deoxypyridinoline [uDPD], serum C-terminal crosslinks of human collagen type I (C-telopeptide) [sCTX]) and calciotropic hormones (intact serum parathyroid hormone [siPTH]) were investigated for their suitability in detecting and monitoring changes in the bone turnover of marmosets. For this purpose either isotonic saline or the bone resorption-antagonizing bisphosphonate ibandronate (0.1 mg/kg) was administered subcutaneously to two groups of animals. Blood and urine were collected before and 5 days after administration and the above mentioned parameters determined. Following the administration of ibandronate, all tested biochemical markers of bone turnover and siPTH responded appropriately in the expected direction. On the basis of the attained results, the studied test kits were deemed suitable for monitoring and evaluating changes in bone turnover in captive marmosets. 3. After assessment and choice of an appropriate commercial basal feed, a new diet (ModF) and feeding regime were implemented in the third study. The impact of ModF, as well as of UV-radiation on bone metabolism and bone density, was evaluated. For this purpose 48 animals were randomly assigned into 4 groups, as follows: group 1: standard diet (StdH), group 2: StdH and UV-radiation, group 3: ModF and group 4: ModF and UV-radiation. Weight gain and feed intake, as well as biochemical markers of bone turnover, vitamin D3- metabolites, siPTH and routine blood biochemistry for general health monitoring were determined at the beginning of the study and then in monthly intervals for the duration of 6 months. Bone mineral density was determined with pQCT and DEXA at the beginning and the end of the study. The ModF resulted in a significant weight gain and reduced feed intake in the respective groups. The lesser amounts that were consumed were sufficient to maintain the significantly higher body weight compared to the StdH-groups. Depending on the measured serum parameters, a possible nutrient deficiency (Vitamin D3 3, Ca, P or others) could be excluded. The bone turnover rate decreased in all groups in the course of the study and was comparable for both diets. The siPTH values for all animals decreased during the study period. Serum- 25(OH)D3 concentrations showed a rise and, like the 1,25(OH)2 D3-values, a decrease in variation over the course of the study. The exposure to artificial UV-irradiation seemed to be of secondary importance. This might be due to a too short irradiation exposure of 30 min/day and/or insufficient emitting of the relevant UVB-wavelength. As obtained from the DEXA and pQCT-measurements, a positive influence of the ModF could be assumed. An overlap with growth effects could not be completely excluded. Presumably due to the short study duration, small animal numbers and the subtle changes in bone metabolism, explicit results could not be achieved. The animals with the ModF showed a significant weight gain and a stronger decrease of bone turnover, probably due to the better digestion, absorption and utilization of the ModF. This is with the utmost probability due to the supplemented gum arabic as part of the ModF. The lack of distinct significant differences in the bone turnover of the StdH- and ModF-groups can be explained with the two diets evidently fulfilling the requirements of marmosets. The feed fractionation over the course of the day is in itself an important contributing factor as well, underlining the importance of the feeding regime for bone metabolism. From the above studies, the following conclusions can be drawn: I . The ModF is well suited to replace the standard colony feed and had a positive influence on bone metabolism and health. Gum arabic appears to be an essential dietary fiber for marmosets and should constitute at least 10 % of the provided diet. 2. Particular attention should be drawn to fractionating the feed by a minimum of three meals per day. Thus postprandial bone resorption will be minimized and, in the long term, adolescent animals will achieve a higher bone mineral density. The basal feed should always be given as the first meal in the morning in order to achieve a sufficient nutrient uptake. 3. Although UV-radiation failed to show a distinct positive effect, appropriate artificial light sources, emitting in the desired UVB-wavelengths, should be supplied in the marmoset colony. Thus variations in food- and consequently in Vitamin D3-intake can be securely equated. 4. A vitamin D3-intake of approx. 87 to 55 IU/kg body weight appears sufficient in order to maintain serum-25(OH) D3 and 1,25(OH)2 D3-values in the physiological range (140-220 nmol/l and 280-400 pg/ml respectively). Thus the 25(OH)D3 ascertained span is in accordance with the range of other new world primates, while the measured 1,25(OH)2 D3-concentrations are slightly lower than when compared with the literature. 5. A siPTH-concentration extending between 82 and 225 ng/ml is presumably the physiological range for marmosets. 6. The tested biochemical markers of bone turnover could be applied successfully not only in pharmacological studies, but in the monitoring of bone health status in marmosets as well. 7. DEXA and pQCT are well suited to determine the bone mineral density of marmosets. Because of the different results obtained with DEXA and pQCT (DEXA provides no differentiation between trabecular and cortical bone; pQCT provides a differentiation of trabecular and cortical bone in a single slice only) neither of the methods could be given preference over the other. Future applications in marmosets should have study durations of at least 1, preferably 2-3 years in order to achieve significant results with respect to changes in BMD.