As of 2015, the recommended daily allowance of vitamin D is 800 international units for people over age 71. The recommended daily allowance for people under age 71 is 600 international units, reports Mayo Clinic.
Vitamin D helps the body to maintain blood levels of calcium and phosphorus. The human body produces vitamin D when exposed to sunlight and can absorb vitamin D from plant foods. The vitamin can also protect people from developing certain heart conditions and cancers. In children, vitamin D deficiency can cause rickets, a disease characterized by soft bones, according to Mayo Clinic. In adults, vitamin D deficiency can also lead to soft bones.