Daily Requirements:
An adult under the age of 70 requires 600 International Units (IU) of Vitamin Dper day. Adults over the age of 70 require 800 IU daily.
Vitamin D from Sunlight:
Your body has an incredible capacity to manufacture Vitamin D from sunlight. Under ideal conditions, a single session of full-body exposure to midday sun (until the skin just begins to turn pink) can produce between 10,000 to 25,000 IUof Vitamin D.
However, because you do not need that much every day, routine, sensible sun exposure is sufficient to hit your daily goals:
Light skin: 5 to 15 minutes of midday sun on the arms and legs, 2–3 times a week, is usually enough.
Dark skin: Melanin acts as a natural sunblock. Individuals with darker skin tones may need anywhere from 30 minutes to a few hours of exposure to produce the same amount of Vitamin D.
Note: Your body automatically stops producing Vitamin D once it has enough, meaning you cannot "overdose" or get Vitamin D toxicity from too much sunlight.
Does Synthesis Only Happen Through Exposed Skin?
Yes, Vitamin D synthesis from sunlight only happens through directly exposed, bare skin.
The chemical process relies entirely on Ultraviolet B (UVB) radiation directly striking the epidermis. The rays must physically penetrate the skin cells to convert a cholesterol-like compound (7-dehydrocholesterol) into Vitamin D3. Because of this requirement, production is blocked or severely limited by:
Clothing: Fabric physically blocks UVB waves from reaching the skin.
Sunscreen: Broad-spectrum sunscreens (especially SPF 30 or higher) absorb or reflect UVB rays, reducing the skin's Vitamin D production by 95% to 98% if applied heavily and perfectly.
Glass: Standard window glass (in homes or cars) completely absorbs UVB radiation. Sitting in a sunny indoor room will not trigger Vitamin D synthesis.
While you do not need your entire body exposed—getting sun on just your face, hands, and arms is often enough—the specific patch of skin must be bare and in direct contact with sunlight to create the vitamin.
N.B: It is recommended to have a bone mineral density (DEXA) scanto check for osteopenia or osteoporosis, and have serum 25-hydroxyvitamin D levels tested to ensure whether D3 is needed.
Source: Google.