At concentrations present in the diet, which vitamin is absorbed primarily by diffusion?

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.