Vitamin D3 Distribution and Status in the Body

Objective: To estimate the amount, type, and tissue distribution of vitamin D in the adult body under typical inputs.

Methods: Review and reanalysis of published measurements and analysis of tissue samples from growing pigs raised in confinement on diets providing about 2000 IU vitamin D/day. Cholecalciferol and 25-hydroxyvitamin D [25(OH)D] concentration measured by HPLC.

Results: Mean serum 25(OH)D in all studies combined was 45 nmol/L. At the level of vitamin D repletion represented by this concentration, total body vitamin D would be 14,665 IU for a 70 kg adult woman. 65% of this total was present as native cholecalciferol and 35% as 25(OH)D. Nearly three-quarters of the cholecalciferol was in fat, while 25(OH)D was more evenly distributed throughout the body (20% in muscle, 30% in serum, 35% in fat, and 15% in all other tissues). At the daily vitamin D consumption rates in these animals total body stores provided only a approximately 7-day reserve.

Conclusions: At total intakes on the order of 2000 IU/day, an adult has very little vitamin D reserve, despite intakes 10x the current recommendations. Those recommended inputs need to be increased by at least an order of magnitude. Food tables that fail to take into account 25(OH)D content of various meat products lead to underestimation of dietary vitamin D intake.