Vitamin D3 (cholecalciferol) is endogenously produced in the skin of primates when exposed to the appropriate wavelengths of ultraviolet light (UV-B). Common marmosets (Callithrix jacchus) maintained indoors require dietary provision of vitamin D3 due to lack of sunlight exposure. The minimum dietary vitamin D3 requirement and the maximum amount of vitamin D3 that can be metabolized by marmosets is unknown. Observations of metabolic bone disease and gastrointestinal malabsorption have led to wide variation in dietary vitamin D3 provision amongst research institutions, with resulting variation in circulating 25-hydroxyvitamin D3 (25(OH)D3 ), the accepted marker for vitamin D sufficiency/deficiency. Multiple studies have reported serum 25(OH)D3 in captive marmosets, but 25(OH)D3 is not the final product of vitamin D3 metabolism. In addition to serum 25(OH)D3, we measured the most physiologically active metabolite, 1,25-dihydroxyvitamin D3 (1,25(OH)2 D3 ), and the less well understood metabolite, 24,25-dihydroxyvitamin D3 (24,25(OH)2 D3 ) to characterize the marmoset's ability to metabolize dietary vitamin D3 . We present vitamin D3 metabolite and related serum chemistry value colony reference ranges in marmosets provided diets with 26,367 (Colony A, N = 113) or 8,888 (Colony B, N = 52) international units (IU) of dietary vitamin D3 per kilogram of dry matter. Colony A marmosets had higher serum 25(OH)D3 (426 ng/ml SD 200] vs. 215 ng/ml SD 113]) and 24,25(OH)2 D3 (53 ng/ml SD 35] vs. 7 ng/ml SD 5]). There was no difference in serum 1,25(OH)2 D3 between the colonies. Serum 1,25(OH)2 D3 increased and 25(OH)D3 decreased with age, but the effect was weak. Marmosets tightly regulate metabolism of dietary vitamin D3 into the active metabolite 1,25(OH)2 D3 ; excess 25(OH)D3 is metabolized into 24,25(OH)2 D3 . This ability explains the tolerance of high levels of dietary vitamin D3 by marmosets, however, our data suggest that these high dietary levels are not required.
