Humans make their own vitamin D from exposure to sunlight. However in recent years, with a thinning/perforated ozone layer and rising skin cancer rates, people don't spend as much time in direct sunlight as they used to. Today, Americans get most of their vitamin D via fortified foods, which may not nearly be enough (e.g. milk is fortified w/ only 100IU/cup in US).
See:
Office of Dietary Supplements fact sheet: Vitamin D
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