Vitamin A + D3 exhibit synchronized physiological effects that enhance their individual roles 

Enhanced bioavailabiliiy and activation
Vitamin A activates vitamin D3
Vitamin D3 absorption triples when paired with vitamin A
Balanced ratios vs. taken in isolation, each vitamin reduces the other's levels.
 
Bone and calcium metabolism: Vitamin D3 enhances calcium level in the blood and phosphorus absorption via the small intestine, while vitamin A directs calcium to bones and teeth, preventing soft-tissue calcification.
Vitamin A highest amount is found in the liver, mucous membranes, eyes, lungs
Helps healthy mucous membranes, supports normal skin, iron metabolism
Contributes to cell specialization and immune function.
Essential for vision: in form of retinal, vitamin A is part of the photoreceptors in our retina, aiding in color and low-light vision.