The short answer
- Vitamin D keeps your bones and muscles working and helps your immune system stay balanced.
- Your skin makes it when sunlight (UVB) hits it. Food and supplements can add to it.
- Older people make much less from the same sun, so they need extra care.
- In winter, far from the equator, the sun is too weak to make any. But somewhere on Earth the sun is strong enough in every month.
- That is what this site shows: which sea destinations give you strong enough sun in the month you want to travel.
What is vitamin D?
Vitamin D is a vitamin that works more like a hormone. Your body uses it to take calcium and phosphorus out of food and into your bones. Almost every organ in the body has a place to receive it, which is why it touches so many things.
It is unusual because most vitamins only come from food. Vitamin D can also be made by your own skin.
Why it matters
- Strong bones. It helps you absorb calcium and phosphorus, which keeps bone hard.
- Working muscles. It plays a part in normal muscle function.
- A balanced immune system. It helps regulate both the quick and the slower parts of your defences.
- Growing children. A serious lack in children can cause rickets, a bone-softening disease.
- Adults too. A serious lack in adults can cause osteomalacia (soft, painful bones) and add to bone loss.
Research also links low vitamin D to many other conditions. Scientists are still working out which of those links are cause and which are coincidence, so this page sticks to the well-established points.
Why it matters even more for older people
Skin ages, and aging skin makes less vitamin D. In a classic experiment, young adults (aged 20–30) and older adults (62–80) were given the same dose of UVB. The young adults' blood level of vitamin D3 rose at least three times as much.3
On top of that, many older people spend more time indoors, and some have gut, liver or kidney problems or take medicines that change how vitamin D is absorbed or used.
What this means in practice:
- Sun still helps, but the same time in the sun gives an older person less than a younger one.
- Do not rely on a holiday alone. Ask your doctor whether a supplement or a blood test makes sense for you.
- Older skin is also more fragile, so keep sun sessions short and never let it burn.
How sunlight makes vitamin D
Only one part of sunlight does the job: UVB. It reaches your skin, and a substance there (7-dehydrocholesterol) turns into previtamin D3, which then becomes vitamin D3. Your liver and kidneys finish the job.2
Three things decide how much UVB reaches you:
We use the UV Index as a simple guide. In the Sundi app and on this site, vitamin D is counted only when the UV Index is 2.5 or higher. Below that, the sun is too weak to bother with.
Sun alone does not give you too much vitamin D. If you stay out too long, the same UV that makes vitamin D also breaks the extra down into inactive by-products.1 The risk of too much sun is sunburn and skin damage, not too much vitamin D. So more sun is never better. A short, sensible session does the job.
Why winter is a problem
Far from the equator the winter sun is low, and low sun gives almost no UVB. Researchers tested this in the 1980s by putting human skin, and the skin chemical that turns into vitamin D, under real winter sunlight:4
| Place | Months with no vitamin D made, even on clear days |
|---|---|
| Boston, USA (42°N) | November to February |
| Edmonton, Canada (52°N) | October to March |
| 34°N (about Los Angeles) and 18°N (about Puerto Rico) | Vitamin D was still made in mid-winter |
The researchers called this the "vitamin D winter". A later study of 13 Canadian cities found the same: in autumn and winter, and in the north, the sun was too weak to make enough vitamin D, so people should look to other safe sources.5 Most of Europe lies at similar or higher latitudes than Boston, so the same applies.
Your body can store some vitamin D in body fat during sunny months. But the store runs down. In one study, people with no UV light at all (submarine crews) lost about half of their vitamin D in two months.6
Living in a sunny country does not guarantee a good level either. A study of 729 adults in Recife, Brazil, where the sun is strong all year, found that 31% still had low vitamin D and 63% were below 30 ng/mL.7 How much time you spend outside, your skin and your age matter as much as where you live.
What this website does about it
The sun is strong enough in every month of the year, somewhere. When it is dark winter in northern Europe, it is high summer in the southern hemisphere, and the tropics stay sunny most of the year.
But it is not the same everywhere in the same month. A beach in Greece in April gives you far less than one in Mozambique, and a rainy-season island can give you little even near the equator. The destination has to be picked for the month.
That is what the Sun Atlas map does. For each month it uses the same sun and cloud model as the Sundi app to colour every sea-holiday country and US state from None (UV Index never reaches 2.5) to Excellent (a long, strong midday window). Click a country to see the best region, the main places to visit, and book.
A sunny holiday can help you build a reserve for darker months, but it does not replace a supplement if your doctor recommends one.
Getting vitamin D from the sun, safely
- Short and regular beats long and rare. Stop well before your skin goes pink.
- Midday is best. When your shadow is shorter than you are, the sun is high and makes the most vitamin D. That is also when you burn fastest, so keep it short.
- Show more skin, for less time. Arms and legs make far more than a face alone. The more skin is uncovered, the shorter the time you need.
- Darker skin needs longer. Pale skin burns sooner, so it needs shorter sessions. Dark skin takes longer to make the same amount.
- Cover up after your dose. Use shade, a hat, sunglasses and clothing. Use SPF 30+ on skin that stays exposed.
- Water, sand and snow reflect UV. At the beach you get more than the number suggests, for the good and the bad.
- Glass blocks UVB. Sitting by a sunny window does not make vitamin D.
As a rough guide from this site's model, about 1000 IU of vitamin D takes roughly 5 to 15 minutes of midday sun on bare arms and legs at a high UV Index, for light skin. It takes much longer at a low UV Index. The Sundi app works out your own numbers.
Who is at higher risk of too little?
- Older people
- People who spend little time outdoors
- People living far from the equator in winter
- People with darker skin
- People with overweight or obesity
- People who always cover up or use strong sunscreen
- People with gut, liver or kidney disease
- People taking some medicines, such as some anti-seizure drugs
A blood test is the only way to know your level. Ask your doctor, especially if you are in one of these groups.
Common myths
"The more sun, the more vitamin D."
Not true. How much you make depends on time of day, season, latitude, skin colour and clothing. Staying longer does not give more benefit, and it raises your risk of burning.
"Sunscreen completely blocks vitamin D."
Sunscreen can reduce how much you make, but sun protection is still important. Food and supplements can cover the gap.
"The more vitamin D I take, the better."
No. Too much from supplements can raise the calcium in your blood and cause other problems. More is not automatically better.
"D2 and D3 are the same."
They are different forms. D3 comes from skin and animal foods. D2 comes from plants and fungi. D3 usually keeps your level up for longer.
"I eat healthily, so I must have enough."
Food helps, but your level also depends on sun, age, skin colour, weight and health.
"If I were low, I would feel it."
Not necessarily. Low vitamin D can have no clear symptoms. A blood test is the way to check.
"Vitamin D can replace medical treatment."
No. It has important roles, but a supplement does not replace diagnosis or treatment of a disease.
Sources
- Holick MF. The cutaneous photosynthesis of previtamin D3: a unique photoendocrine system. J Invest Dermatol 1981;77:51–58.
- Wacker M, Holick MF. Sunlight and vitamin D: a global perspective for health. Dermato-Endocrinology 2013;5(1):51–108.
- MacLaughlin J, Holick MF. Aging decreases the capacity of human skin to produce vitamin D3. J Clin Invest 1985;76:1536–8, as described in Wacker and Holick (2013).
- Webb AR, Kline L, Holick MF. Influence of season and latitude on the cutaneous synthesis of vitamin D3. J Clin Endocrinol Metab 1988;67:373–8.
- Gill P, Kalia S. Assessment of the feasibility of using sunlight exposure to obtain the recommended level of vitamin D in Canada. CMAJ Open 2015;3(3):E258–E263.
- Webb AR, Engelsen O. Calculated ultraviolet exposure levels for a healthy vitamin D status. Photochem Photobiol 2006;82:1697–1703.
- Azevedo M, et al. Vitamin D deficiency, skin phototype, sun index, and metabolic risk among patients with high rates of sun exposure living in the tropics. J Clin Aesthet Dermatol 2018;11(8):15–18.
Source titles are in English. Also used: the Sundi project's vitamin D fact sheet, and Miyauchi and Nakajima (Photochem Photobiol 2016) on UV exposure time, Dowdy, Sayre and Holick (J Steroid Biochem Mol Biol 2010) on "Holick's rule", and Blumthaler et al. (J Photochem Photobiol B 1997) on UV and altitude.
Education and wellness only. This is an estimate for education and wellness purposes only. It is not medical advice, a diagnosis, or a treatment recommendation. Always consult a qualified healthcare professional before making decisions about sun exposure or vitamin D intake. Sun exposure carries a risk of sunburn and long-term skin damage, so wear sun protection and seek shade when UV is high.