The recommended daily allowance, or RDA, of vitamin D for adults as of 2010 is 600 international units, or IUs, for adults up to 70 years of age and 800 IUs for those 71 and older, according to Mayo Clinic. Most people do not need to supplement vitamin D.
Daily exposure to sunlight is generally sufficient to get the RDA of vitamin D, notes Mayo Clinic. However, for treatment of certain conditions, additional vitamin D is sometimes recommended. For instance, 400 to 1,100 IUs of vitamin D or a monthly dose of 100,000 IUs of vitamin D3 is sometimes recommended for cancer prevention. High doses are sometimes recommended for treatment of cystic fibrosis, heart disease, prostate cancer and autoimmune disease.