What Is Vitamin D?

What Is Vitamin D?

Vitamin D is a fat soluble vitamin that plays a vital role in your overall health. Unlike most vitamins, your body can actually make it yourself — but only when your skin is exposed to direct sunlight. That is why it is often called the "sunshine vitamin."

Vitamin D helps regulate the amount of calcium and phosphate in your body. These nutrients are essential for keeping your bones, teeth, and muscles healthy. Without enough vitamin D, your body cannot absorb calcium properly. In fact, without vitamin D, as little as 10% of dietary calcium may be absorbed. With enough vitamin D, that figure can rise to 70%.

Vitamin D also plays a role in the normal functioning of your immune system and is important for maintaining muscle strength.

Why Is Vitamin D So Important?

Vitamin D is not just about bones. It affects many systems in your body.

Bone Health

Vitamin D is essential for the mineralisation of bones. Without it, bones can become soft, weak, and more likely to fracture. A lack of vitamin D can lead to bone deformities such as rickets in children, and bone pain caused by a condition called osteomalacia in adults.

Muscle Function

Vitamin D is important for maintaining muscle strength. Deficiency can cause muscle weakness, aches, and difficulty with everyday movements like standing up from a chair or walking.

Immune System

Vitamin D enhances the control and function of the immune system.

Other Potential Roles

Research has suggested roles for vitamin D in preventing heart disease and some cancers, though more evidence is needed. There have also been reports about vitamin D reducing the risk of COVID-19, but there is currently not enough evidence to support taking vitamin D solely to prevent or treat COVID-19.

What Happens If You Do Not Get Enough?

Vitamin D deficiency can be subtle at first. Lesser deficiency may be symptomless. But over time, the effects can become serious.

Symptoms of Vitamin D Deficiency

In children: Bone pain or "growing pains", muscle weakness, delays in walking, soft leg bones (bow legged), poor growth, dental caries (tooth decay), rickets — thickened wrists, bowed legs, and other bone deformities.

In adults: Bone discomfort or pain, often throbbing, in the lower back, pelvis, and lower legs; muscle aches and weakness, particularly in the thighs and buttocks; difficulty rising from a seated position or a waddling gait; muscle twitches or spasms, especially in the face; unexplained fractures; depression or seasonal affective disorder.

If you have vitamin D deficiency for a long time, it can make your bones weak and may cause permanent problems with bone strength.

How To Get Vitamin D Naturally

Sunlight

From about late March or early April to the end of September, most people should be able to make all the vitamin D they need from sunlight. The body creates vitamin D from direct sunlight on the skin when outdoors.

In the UK, during the spring and summer months, only about 30 minutes of exposure of the hands and face to the midday sun allows production of the daily vitamin D requirement. It is important that this is without sunblock, as even low factor sunblocks block most of the UVB from reaching the skin. Sunburn should not occur in so short a time.

But between October and early March, we do not make enough vitamin D from sunlight in the UK. The sun is simply not strong enough for the body to make vitamin D.

Food Sources

Vitamin D is found in only a small number of foods. The best natural sources are:

Oily fish — such as salmon, sardines, trout, herring, or mackerel. 100g of any of these would provide most of your daily requirement.

Other foods: Red meat, egg yolks, liver (avoid if pregnant), mushrooms — their vitamin D content can be raised 10 times by exposure to the sun for 30 minutes, fortified foods such as some fat spreads and breakfast cereals.

Important note: In the UK, cows' milk is generally not a good source of vitamin D because it is not fortified, as it is in some other countries.

If Natural Sources Are Not Enough: Supplement Options

The Scientific Advisory Committee on Nutrition (SACN) has recognised that it is difficult to achieve recommended intakes of vitamin D from natural food sources alone. This is why supplements are recommended for many people.

Who Should Take a Supplement?

Everyone during autumn and winter — Government advice is that everyone should consider taking a daily vitamin D supplement during the autumn and winter. The NHS recommends that adults and children over 4 years old take a daily supplement containing 10 micrograms (400 IU) of vitamin D during the autumn and winter.

People at higher risk all year round — The Department of Health and Social Care recommends that adults and children over 4 take a daily supplement containing 10 micrograms of vitamin D throughout the year if they are not often outdoors, frail or housebound, in a care home, usually wear clothes that cover up most of their skin, or have darker skin.

Babies and children — All babies from birth to 1 year should receive 8.5 to 10 micrograms per day. Children aged 1 to 4 should take 10 micrograms daily throughout the year.

Pregnant and breastfeeding women — NHS guidelines recommend that all pregnant women take a daily vitamin D supplement.

How Much Should You Take?

• Adults and children over 4: 10 micrograms (400 IU) per day
• Children aged 1 to 4: 10 micrograms (400 IU) per day
• Babies up to 1 year: 8.5 to 10 micrograms (340–400 IU) per day
• Do not take more than 100 micrograms (4,000 IU) a day as it could be harmful

Converting Units: 1 microgram of vitamin D is equal to 40 IU. So 10 micrograms of vitamin D is equal to 400 IU.

Who Should Be Careful?

People with kidney conditions, sarcoidosis, hypercalcaemia, or those taking certain medications (phenytoin, barbiturates, glucocorticoids, cardiac glycosides, some epilepsy medications) should seek medical advice before supplementing.

Risk of Taking Too Much

In a few people, taking too many vitamin D supplements over a long period can cause too much calcium to build up in the body (hypercalcaemia). The NHS advises not to take more than 100 micrograms (4,000 IU) a day.

The UK Picture: Where We Are Now

Population surveys in the UK have shown that many people have suboptimal levels of vitamin D, especially in winter. Particularly at risk are babies and children, pregnant and breastfeeding mothers, people unable to get outside for 30 minutes a day, those with conditions affecting absorption (coeliac disease, Crohn's, stomach surgery, liver disease, chronic pancreatitis), people on certain medications, and those with darker skin.

Current UK government advice on vitamin D supplementation is based on recommendations made by the Scientific Advisory Committee on Nutrition (SACN) in its report entitled 'Vitamin D and Health' (SACN, 2016). The SACN is currently conducting a rapid review of whether current recommendations are adequate for people with higher melanin concentrations, aiming to publish conclusions later in 2026.

The Bottom Line

Vitamin D is essential for your bones, teeth, muscles, and immune system. In the UK, most people do not get enough from sunlight between October and March, and it is difficult to get enough from food alone.

The simple steps:

• Get safe sun exposure — about 30 minutes on your hands and face during spring and summer
• Eat oily fish weekly — salmon, mackerel, sardines, or trout
• Choose fortified foods — breakfast cereals and margarines
• Take a supplement — 10 micrograms (400 IU) daily during autumn and winter
• Consider year-round supplements if you are in an at-risk group
• Consult your GP before starting supplements if you have kidney problems, sarcoidosis, or are on certain medications

Remember: It is about getting enough, not getting too much. Stick to the recommended 10 micrograms a day unless your doctor advises otherwise.

Disclaimer

This content is for educational and informational purposes only. It is not medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional with any questions about your health, especially before starting supplements or making significant dietary changes. Individual responses to supplements vary.