In recommending intakes for vitamin D, it must be recognized that in most locations in the world in a broad band around the equator (latitudes 42°N - 42°S), the most physiologically relevant and efficient way of acquiring vitamin D is to synthesize it endogenously in the skin from 7-dehydrocholesterol by sun (UV) light exposure.
In most situations, ~ 30 minutes of skin exposure of the arms and face to sunlight can provide all the daily vitamin D needs of the body.
Skin synthesis of vitamin D is negatively influenced by factors which may reduce the ability of the skin to provide the total needs of the individual:
- latitude and season - both influence the amount of UV light reaching the skin;
- ageing process - thinning of the skin reduces the efficiency of this synthetic process;
- skin pigmentation - the presence of darker pigments in the skin interferes with the synthetic process because UV light cannot reach the appropriate layer of the skin;
- clothing - virtually complete covering of the skin for medical, social, cultural, or religious reasons leaves insufficient skin exposed to sunlight;
- sunscreen use - widespread and liberal use of sunscreen, though reducing skin damage by the sun deleteriously affects synthesis of vitamin D.
Because not all of these problems can be solved in all geographic locations, particularly during winter at latitudes higher than 42° where synthesis is virtually zero, it is recommended that individuals not synthesizing vitamin D should correct their vitamin D status by consuming the amounts of vitamin D.
Recommended nutrient intakes (RNIs) for vitamin D, by group, in milligrams (1/1000 g):
Infants, children, adolescents, and adults 19–50 years, pregnant and lactating women - 5 mg a day RNI;
Adults 51–65 years - 10 mg a day RNI
Adults 65+ years - 15 mg a day RNI