Good Sources of Vitamin D: Sunlight, Food and Supplements
Vitamin D is unusual among vitamins because your body can manufacture it. Skin exposed to ultraviolet B light converts a cholesterol derivative into vitamin D, which the liver and kidneys then activate.
That matters when choosing sources, because sunlight does most of the work for most people, and food supplies far less than people assume.
Sunlight
UVB light drives the reaction. How much you need depends on factors you cannot control and some you can.
- Latitude and season. Above roughly 37 degrees latitude, winter sunlight carries too little UVB to produce meaningful vitamin D no matter how long you stay outside.
- Time of day. UVB is strongest in the middle of the day. Early morning and late afternoon sun produces very little.
- Skin tone. Melanin absorbs UVB, so darker skin needs considerably longer exposure for the same production.
- Age. Skin becomes less efficient at producing vitamin D over time.
- Glass and sunscreen. Window glass blocks UVB almost entirely. Sunscreen reduces production, though in practice most people apply too little for it to block production completely.
A common rough guide is 10 to 30 minutes of midday sun on arms and legs several times a week for lighter skin, and longer for darker skin. This has to be balanced against skin cancer risk, and burning provides no extra vitamin D.
Food sources
Very few foods contain vitamin D naturally, and the list is short enough to memorise.
| Food | Notes |
|---|---|
| Oily fish: salmon, mackerel, sardines, herring | The richest natural source by a wide margin |
| Cod liver oil | Very high, but also high in vitamin A, so dosage matters |
| Egg yolks | Modest amounts, varying with how the hens were raised |
| Beef liver | Small amounts |
| Mushrooms exposed to UV light | Contain vitamin D2, and the label will usually say |
| Fortified milk, cereal, orange juice, plant milks | Added rather than natural, and the main dietary source in many countries |
Ordinary mushrooms grown in the dark contain very little. Only those deliberately exposed to UV light are a useful source.
D2 and D3
Vitamin D2 (ergocalciferol) comes from plants and fungi. Vitamin D3 (cholecalciferol) comes from animal sources and is what your skin makes.
Both raise blood levels, but D3 is generally more effective at raising and maintaining them. Most supplements use D3. Vegan D3 made from lichen is available for anyone avoiding animal products.
Supplements
Supplements are the practical answer for people who spend winters at high latitude, work indoors, cover their skin, have darker skin in a low sun climate, or are older.
Labels use both international units (IU) and micrograms (mcg). The conversion is straightforward: 40 IU equals 1 mcg, so 1,000 IU is 25 mcg.
Vitamin D is fat soluble, which means it is stored rather than flushed out, so it is possible to take too much over time. Toxicity is rare and comes from sustained very high doses rather than normal supplementation. The main consequence is a build up of calcium in the blood.
Because it is fat soluble, taking it with a meal containing some fat improves absorption.
Who is most likely to be low
- People living far from the equator, especially in winter
- People with darker skin living in low sunlight regions
- Older adults, particularly those who are housebound
- People who cover most of their skin
- People with conditions affecting fat absorption, such as coeliac or Crohn’s disease
- People who have had weight loss surgery
- Exclusively breastfed infants, which is why supplementation is routinely advised
Frequently asked questions
What are the symptoms of low vitamin D?
Often none at first. When symptoms appear they are vague, such as tiredness, aching muscles and bone pain. Severe deficiency causes rickets in children and osteomalacia in adults. Because the early signs are so general, a blood test is the only way to know.
Can I get enough vitamin D through a window?
No. Standard window glass blocks almost all UVB, so sitting in a sunny room produces essentially none.
How long does it take to correct a deficiency?
With supplementation it typically takes several weeks to a few months for blood levels to rise, depending on the starting level and dose. Your doctor may retest after a set period.
Is it better to take vitamin D daily or weekly?
Both work for maintaining levels. Daily dosing is often preferred for steadier levels, while weekly dosing can help people who find daily tablets hard to remember.
Does vitamin D help with sleep?
Low levels have been associated with poorer sleep in some studies, but the evidence that supplementing improves sleep in people who are not deficient is weak. For practical steps, see our guide on how to get good sleep.
Should I take vitamin K2 with vitamin D?
The combination is often marketed together on the basis that K2 directs calcium to bone. The evidence is still developing and it is not a routine medical recommendation. Ask your doctor rather than relying on supplement marketing.
This article is general information, not medical advice. Test results and doses should be discussed with a doctor.
