Holick and his colleagues have basically highlighted our point:1 although it can be clearly shown that a sunscreen applied to subjects in a laboratory certainly will prevent an increase in 25-OH-D levels (as you would expect), there is a major difference between what happens in the laboratory and what happens in the real world. We have stated quite clearly that part of the explanation for the increase in serum 25-OH-D levels in both groups is because of an individual's lack of use of sunscreens in the manner recommended in public health education programs.
We confirmed this finding by multiple measurements concerning the amount and frequency with which our subjects were using the product. We also have other behavioral studies showing that it would be almost impossible
Marks R. Regular Use of Sunscreen on Vitamin D Levels-Reply. Arch Dermatol. 1995;131(11):1338-1339. doi:10.1001/archderm.1995.01690230120024