Vitamin d deficiency - " your digestive tract can not adequately absorb vitamin d", what does that mean?

Rare: Not to absorb a fat soluble vitamin like d. Its mostly produced by skin cells and sunlight.

Vit D: Vitamin d is well absorbed from the GI tract usually. In vitamin d deficiency there is not enough vitamin d in the diet and because of poor sun exposure and decrease intensity of sun rays in areas where the the rays are oblique ( north os south ) the skin cannot make enough. The production of vitamin d by the skin decreases with age.