Age, weather, pollution, and other factors can all affect how much vitamin D someone's skin absorbs from sun exposure. Coupled with the fact that there are limited food sources of vitamin D, supplementation is a popular way for many older adults to meet their daily recommended amount of this nutrient.
Like calcium, vitamin D supplements often come in two forms in the US: D2, or ergocalciferol, and D3, or cholecalciferol. The primary difference between the two is, according to the US Office of Dietary Supplements, is that the D3 form is more likely to increase blood levels of vitamin D than the D2 form, and the D3 form may increase blood levels for longer than the D2 form as well.
As a fat-soluble substance, taking vitamin D with food increases absorption.