How do our bodies make vitamin d outside?

Its made inside: Sun exposure is an easy, reliable way for most people to get vitamin d. Exposure of the hands, face, arms, and legs to sunlight two to three times a week for about one-fourth of the time it would take to develop a mild sunburn will cause the skin to produce enough vitamin d. The necessary exposure time varies with age, skin type, season, time of day, etc. Still, vitamin d deficiency is very common.

It's inside: Your skin's exposure to sunlight allows production of vitamin d. As your age, the skin decreases absorption of sunlight and your vitamin d levels can decease if that is your only source. Expose your midsection to sun for best exposure.