Many things can keep the skin from making enough D. Winter sunlight may be too weak. Melanin, which protects skin from damage, also keeps it from making D, which is why people with darker skin are more prone to deficiency. Older people are also prone, because aging skin makes less D, even with enough sunlight. Clothing, glass, sunscreens and sunblocks also shield the skin from getting enough ultraviolet light to make D.