How does the body get vitamin D in a sunny country?

Thailand has sun almost year-round, but vitamin D does not follow from how much sun the country has. It follows from how much reaches skin.

How is vitamin D made?

The body makes vitamin D when skin is exposed to UVB from the sun. Ordinary window glass blocks UVB, so sunlight through a window does not produce it. It also comes from some foods and from supplements.

What reduces how much reaches skin?

Time spent indoors, in vehicles and in shade reduces how much UVB reaches skin.

Sunscreen and covering clothing also reduce what reaches skin, which is a trade-off against looking after skin.

How long should you spend in the sun?

There is no single figure that applies to everyone. It depends on skin tone, time of day, season and how much skin is exposed.

Longer exposure carries its own trade-offs for skin, so it is not something to give as a fixed formula.

Where does it come from in food?

Oily fish such as salmon, mackerel and sardines, plus egg yolk and fortified foods such as some milks.

Natural food sources are limited compared with other nutrients.

What does vitamin D do?

Under the annex to Thai Ministry of Public Health Notification No. 447 (B.E. 2566), vitamin D contributes to normal absorption of calcium and phosphorus, to normal blood calcium levels, to the maintenance of normal bones and teeth, to the maintenance of normal muscle function, and to the normal function of the immune system.

How much per day?

The recommended intake set by the US National Academies' Food and Nutrition Board, as published by the NIH Office of Dietary Supplements, for adults is 15 micrograms, or 600 IU, a day, rising to 20 micrograms or 800 IU for people over 70.

The tolerable upper intake level for adults is 100 micrograms or 4,000 IU a day. Vitamin D is fat-soluble and accumulates, so do not exceed the amount on the label.

Dietary supplements are not intended to prevent, treat or cure any disease.