many things can keep the skin from making enough d3. winter sunlight may be too weak. melanin, which protects skin from damage, also keeps it from making d3, which is why people with darker skin are more prone to deficiency. older people are also prone, because aging skin makes less d3, even with enough sunlight. clothing, glass, sunscreens and sunblocks also shield the skin from getting enough ultraviolet light to make d3.