As of 2015, the recommended daily adult dose of vitamin D-3 ranges from 600 to 800 IU, pending on age over or under 70, states Mayo Clinic. Dosage varies widely for different purposes such as cancer prevention, immune function and cognitive improvement.
Patients may take a 100,000 IU dose of vitamin D-3 every four months for cancer prevention, while a daily dose up to 9,000 IU is recommended for cognitive purposes, according to Mayo Clinic. For immune function, patients may take 40 IU of vitamin D-3 daily.
Vitamin D-3 can be synthesized by human skin once it is exposed to sunlight. The body needs as little as 10 minutes of daily sunlight to synthesize enough of the vitamin. The vitamin helps the body absorb calcium and can protect against numerous diseases, including osteoporosis.