How much sun exposure is enough? Lot of sources say 10-40 minutes. But nothing is said about how much skin should be exposed for that duration. (recommendations that are so vague are dismissed as useless)
.. and since i'm venting, diet experts use formula "1 cal = 1 kilo cal". why oh why?
The amount of sun exposure necessary depends on latitude, time of day, weather, skin color, diet, etc. There's no simple answer. Probably the best approach for most people is to get a periodic blood test and then adjust sun exposure and supplementation up or down as needed.
It is interesting that in Iran the ratio of people with Vitamin D deficiency is high compared to other countries. Probably behaviour (clothing, staying at home) plays quite a large role.
.. and since i'm venting, diet experts use formula "1 cal = 1 kilo cal". why oh why?