"Expose Your Skin Around Midday
Midday, especially during summer, is the best time to get sunlight.
At noon, the sun is at its highest point, and its UVB rays are most intense. That means you need less time in the sun to make sufficient vitamin D (5Trusted Source).
Many studies also show that the body is most efficient at making vitamin D at noon (6Trusted Source, 7Trusted Source).
For example, in the UK, 13 minutes of midday sunlight exposure during summer three times per week is enough to maintain healthy levels among Caucasian adults (5Trusted Source).
Another study found that 30 minutes of midday summer sun exposure in Oslo, Norway was equivalent to consuming 10,000–20,000 IU of vitamin D (8Trusted Source).
The commonly recommended daily dose of vitamin D is 600 IU (15 mcg) (3Trusted Source).
Not only is getting vitamin D around midday more efficient, but it might also be safer than getting sun later in the day. One study found that afternoon sun exposure may increase the risk of dangerous skin cancers (9Trusted Source)."
https://www.healthline.com/nutrition/vitamin-d-from-sun#time-of-day
What is vitamin D?
Vitamin D is not one chemical but many. The natural type is produced in the skin from a universally present form of cholesterol, 7-dehydrocholesterol. Sunlight is the key: Its ultraviolet B (UVB)
energy converts the precursor to vitamin D3. In contrast, most dietary supplements are manufactured by exposing a plant sterol to ultraviolet energy, thus producing vitamin D2. Because their function is almost identical, D2 and D3 are lumped together under the name vitamin D — but neither will function until the body works its magic (see figure).
The sun's energy turns a chemical in your skin into vitamin D3, which is carried to your liver and then your kidneys to transform it to active vitamin D.
The first stop is in the liver, where vitamin D picks up extra oxygen and hydrogen molecules to become 25-hydroxyvitamin D, or 25(OH)D. This is the chemical that doctors usually measure to diagnose vitamin D deficiencies. But although 25(OH)D is used for diagnosis, it can't function until it travels to the kidney. There it acquires a final pair of oxygen and hydrogen molecules to become 1,25 dihydroxyvitamin D; scientists know this active form of the vitamin as 1,25(OH)2D, or calcitriol, but for ordinary folks the name vitamin D is accurate enough.
Although standards vary, most experts agree that levels of 25(OH)D below 20 ng/ml (nanograms per milliliter) reflect clear-cut vitamin D inadequacy, while levels between 20 and 30 ng/ml are borderline.
A number of factors can play a role. Limited exposure to sunlight heads the list. Except during the short summer months, people who live at latitudes above 37 degrees north or below 37 degrees south of the equator don't get enough UVB energy from the sun to make all the vitamin D they need. The same is true for people who spend most of their time indoors and for those of us who avoid sunshine and use sunscreens to protect our skin from the harmful effects of ultraviolet radiation (see box below). It's an example of an unforeseen consequence of wise behavior, but you can enjoy sun protection and strong bones, too, by taking vitamin supplements.
These many factors explain why vitamin D deficiencies are shockingly common in the United States. Although standards vary, most experts agree that levels of 25(OH)D below 20 ng/ml (nanograms per milliliter) reflect clear-cut vitamin D inadequacy, while levels between 20 and 30 ng/ml are borderline. Using similar criteria, American researchers have reported deficiencies in 42% of African American women aged 15 to 49, in 41% of non-hospitalized patients aged 49 to 83, and in up to 57% of hospitalized patients. And low levels of vitamin D are common even in apparently healthy young adults; in one study, more than a third of people between the ages of 18 and 29 were deficient.
Numbers can never tell the whole story, but in this case, "D-ficiencies" add up to a wide range of health concerns.
"D" right amount
Until 1997, the recommended dietary allowance (RDA) for vitamin D was 200 IU for all adults. Faced with growing evidence of vitamin D deficiencies in Americans, the RDA for 51- to 70-year-olds was increased to 400 IU, and to 600 IU for people older than 70.
Is more better? New research suggests that it is, and many authorities are recommending 800 or even 1,000 IU a day. Remember, though, that you can get too much of a good thing. Like the other fat-soluble vitamins, vitamin D is stored in the body's adipose (fat) tissue. That means your body can mobilize its own reserves if your daily intake falters temporarily — but it also means that excessive doses of vitamin D can build up to toxic levels. At those extremes, vitamin D can raise blood calcium to levels that can cause grogginess, constipation, and even death. But it takes massive overdosing to produce toxicity, and doses up to 2,000 IU a day are considered safe.
Delivering D
You can make your vitamin D the old-fashioned way, by exposing your skin to UVB radiation in sunlight. It doesn't take much, but people living north of the 37-degree-latitude line — roughly the imaginary line between Philadelphia and San Francisco — can't get enough UVB in winter to do the trick. And many others will find it all too easy to overdose on UVB, increasing their risk of malignant melanomas and other skin cancers, as well as wrinkles and premature skin aging. All in all, most doctors recommend avoiding sunlight (see box) and getting vitamin D by mouth.
Diet can help, but it's very hard to approach the new goals with food alone. Fish and shellfish provide natural vitamin D (oily fish are best), but you'll have to eat about 5 ounces of salmon, 7 ounces of halibut, 30 ounces of cod, or nearly two 8-ounce cans of tuna to get just 400 IU. An egg yolk will provide about 20 IU, but since it also contains nearly a day's quota of cholesterol, you can't very well use eggs to fill your tank with D. Other foods have even less D, which is why manufacturers fortify milk, some yogurt, some orange juice, and many cereals with vitamin D. In general, a serving will provide about 100 IU; that means drinking a quart of fortified milk to get 400 IU.
Most people require supplements to get the vitamin D they need. It's the main benefit of a daily multivitamin; most provide 400 IU. Remember to read the labels carefully so you won't get too little or too much. And although cod liver oil is rich in vitamin D, it has too much vitamin A for regular use.
https://www.health.harvard.edu/staying-healthy/vitamin-d-and-your-health-breaking-old-rules-raising-new-hopes
From the UK:
How long should we spend in the sun?
Most people can make enough vitamin D from being out in the sun daily for short periods with their forearms, hands or lower legs uncovered and without sunscreen from late March or early April to the end of September, especially from 11am to 3pm.
It's not known exactly how much time is needed in the sun to make enough vitamin D to meet the body's requirements.
This is because there are a number of factors that can affect how vitamin D is made, such as your skin colour or how much skin you have exposed.
But you should be careful not to burn in the sun, so take care to cover up or protect your skin with sunscreen before your skin starts to turn red or burn.
People with dark skin, such as those of African, African-Caribbean or south Asian origin, will need to spend longer in the sun to produce the same amount of vitamin D as someone with lighter skin.
How long it takes for your skin to go red or burn varies from person to person. Cancer Research UK has tips to help you protect your skin in the sun.
Your body can't make vitamin D if you're sitting indoors by a sunny window because ultraviolet B (UVB) rays (the ones your body needs to make vitamin D) can't get through the glass.
The longer you stay in the sun, especially for prolonged periods without sun protection, the greater your risk of skin cancer.
If you plan to be out in the sun for long, cover up with suitable clothing, wrap-around sunglasses, seeking shade and applying at least SPF15 sunscreen.
Winter sunlight
In the UK, sunlight doesn't contain enough UVB radiation in winter (October to early March) for our skin to be able to make vitamin D.
During these months, we rely on getting our vitamin D from food sources (including fortified foods) and supplements.
https://www.nhs.uk/live-well/healthy-body/how-to-get-vitamin-d-from-sunlight/
Vitamin D comes from the synthesis of UVB radiation, which is less than 5% of the sunshine. Depending on the time of day and time of year, there may not be enough UVB to produce vitamin D. The UVB strength is dependent on your latitude, and thus all cities in the same latitude have similar UVB exposure (barring pollution or fog).
The map and calendar below can help determine which months you can get vitamin D from the sun where you live.
Find your location; which latitude lines are you within?
Read across from January to December to see what months you are able to produce vitamin D. Red dots mean good UVB availability, Orange is moderate availability, Yellow is low, and Clear is non-existent.
Also, remember that you need to be out in prime hours, 10 am – 2 pm, or when your shadow is shorter than you.
https://www.grassrootshealth.net/document/sunshine-calendar/