Vitamin D: what it does and how to get it

What is vitamin D for?

Vitamin D helps the intestine absorb calcium and contributes to maintaining bone health and muscle function [1]. A prolonged deficiency can impair bone mineralisation. In adults, it can cause osteomalacia; in children, rickets [3]. For dietary guidance on osteoporosis, see the osteoporosis diet guide. See “Vitamin D and iodine in a vegan diet”.

Vitamin D also plays a role in immune processes. This does not mean that taking more prevents depression or cancer in healthy adults [3] [4]. The winter defences guide explains what is known about colds. If you are concerned about muscle strength, the sarcopenia guide covers diet and exercise.

What reduces vitamin D production in the skin?

The skin produces vitamin D when exposed to UVB radiation [5]. The amount varies with the season, latitude, time of day, age, skin tone and clothing [3]. Someone who rarely goes outdoors or covers nearly all their skin receives less radiation. Window glass also blocks the UVB needed for production [3].

Sunscreen reduces the UVB radiation reaching the skin when applied correctly. In real life, using it does not completely stop vitamin D synthesis [3]. Keep protecting yourself against sunburn and excessive exposure: ultraviolet radiation increases the risk of skin cancer [5].

You can read the recommendations on ultraviolet radiation. There is no number of minutes that works for everyone.

A low test result needs context; it is not an automatic prescription for sun exposure or supplements.

Which foods provide vitamin D?

Oily fish is one of the most useful dietary sources. Egg yolk and liver provide smaller amounts [2]. The figures are approximate because they vary with the food and how it is prepared [2].

Food	Reference serving	Approximate vitamin D
Canned sardines	One drained can	Amount varies depending on the product and drained weight [2].
Grilled salmon	150 g	12 µg [2]
Raw egg yolk	20 g, approximately one yolk	0.9 µg [2]
Raw pork liver	100 g before cooking	2.2 µg [2]
Fortified dairy product	One serving or glass	Variable; check the label
Fortified plant-based drink	One glass	Variable; check the label

The values for liver and egg yolk refer to the raw product; they cannot predict exactly how much remains after cooking. The table does not mean that you should seek out liver for its vitamin D content.

The guide to plant-based drinks can help you compare products. If you are looking for more fish options, see foods rich in omega-3.

How much vitamin D do you need?

In 2016, EFSA set an adequate intake of 15 µg per day from the age of one year. This includes pregnancy and breastfeeding. For infants aged 7 to 11 months, the value is 10 µg per day [1]. These values were calculated for people who receive little sun and apply to the whole population [1].

In 2023, the same body updated the tolerable upper intake level for adults. It remains at 100 µg per day, including during pregnancy and breastfeeding [6]. This is a safety ceiling for sustained intake. Individual needs may vary; micrograms and international units should not be turned into a do-it-yourself plan.

Who is at greater risk of vitamin D deficiency?

Older age reduces the skin’s ability to produce vitamin D. The risk also increases if you get very little sun, have dark skin and live somewhere with little solar radiation, or have intestinal malabsorption [3]. After surgery that affects absorption, assessment requires even more medical context [3].

Obesity is associated with lower blood concentrations, although this association does not show that everyone with obesity needs a test or a supplement [3] [4]. During pregnancy and breastfeeding, talk to your healthcare team before changing your intake.

What are the symptoms of vitamin D deficiency?

Most of the time, there are none; that is why it is found through a blood test [3]. When the deficiency is severe and lasts for months, it can cause bone pain and muscle weakness [3]. These symptoms can have many other causes, so your doctor should assess them.

What does the blood test measure, and why do cut-offs vary?

The usual test measures 25-hydroxyvitamin D, or 25(OH)D, in the blood. It reflects vitamin D from the skin, food and supplements [3]. The result may be expressed in ng/ml or nmol/l. The active form of the vitamin is not used in the same way to assess usual vitamin D stores [3].

There is no single cut-off accepted for everyone and every purpose. Below 12 ng/ml, the risk of deficiency is high, while 20 ng/ml or more is enough for most people, according to the United States National Institutes of Health [3]. Other guidelines do not set a single figure for disease prevention [4]. They also do not recommend routine testing in healthy people [4].

Therefore, a result that is slightly below the laboratory range does not in itself mean you have a disease. When appropriate, your doctor can interpret it alongside your medical history, calcium levels, bone health and other tests.

When might a supplement be needed?

Your healthcare team decides whether you need a vitamin D supplement. They may consider one if you have a deficiency or absorption problems. For infants, the paediatrician gives the advice. During pregnancy and from the age of 75, supplementation may be recommended without routine testing [4]. Needs and exceptions are assessed individually. Your diet, sun exposure and any supplements you already take will also be considered [3]. Avoid starting one on your own, especially at high doses.

Sustained excess vitamin D from supplements can raise blood calcium and harm the kidneys [6] [3]. Review any products you already take with your healthcare team: several products can add vitamin D without it being obvious from looking at just one box. The tolerable upper intake level does not tell you what dose to buy.

What mistakes do we see in consultations?

The question we hear most often in consultations is what to do after getting a result marked in red. We see three rushed decisions that often make things more complicated:

• Treating the result in isolation without checking why the test was ordered.
• Stopping sunscreen in an attempt to raise the result.
• Buying several supplements without adding up the amount in each one.

We also often see people confuse “contains vitamin D” with “provides a useful amount”. In consultations, we start by understanding why the test was done and what someone’s habits are really like, and we coordinate any clinical intervention.

Myths worth leaving behind

“More sun always means more vitamin D.” Production does not increase without limit, and excessive exposure damages the skin [5] [3]. A recommendation giving everyone the same number of minutes ignores the season, skin and location.