<prompt> At concentrations present in the diet, which vitamin is absorbed primarily by diffusion?Vitamin CFolateVitamin DNiacin  <response>: Absorption of vitamin D increases linearly as the intraluminal concentration increases, suggesting absorption by a non saturable passive-diffusion mechanism. The term vitamin D refers to a family of essentially water-insoluble compounds involved primarily in the regulation of calcium homeostasis.At the low concentrations present in the diet 1 to 100 nM, transport of the vitamins across the brush border occurs by specialized mechanisms, such as membrane carriers, active transport systems, and membrane-binding proteins and receptors, specific for a particular vitamin. 