The ultimate guide to vitamin D

The ultimate guide to vitamin D

We’re all encouraged to get enough vitamin D to stay healthy, either through our diet, by going outside or through supplements.

There are many benefits to taking a vitamin D supplement, and it’s essential during autumn and winter when we aren’t getting enough sunlight here in the UK.

There’s lots to learn about vitamin D, so if you’re curious, feast your eyes upon our guide.

We’ve created the ultimate guide for vitamin D to discuss everything you need to know about this sunny little vitamin.

What is vitamin D?

Vitamin D, otherwise known as the sunshine vitamin, is an essential nutrient your body needs for normal functioning, offering numerous health benefits if taken correctly.

You can get vitamin D from food, but it’s advised to get your intake in the form of supplements and sunlight to ensure your vitamin D levels stay in a normal range.

When your skin is exposed to natural sunlight, your body converts it into a chemical called calciferol, which then reacts with your body to create vitamin D.

The quantity of vitamin D your skin produces will depend on many things, such as:

  • Time of day: When the sun is at its highest point at noon, your body will be producing the most vitamin D.
  • Season: The sun is at its strongest during the spring and summer months and it’s much harder to get enough vitamin D during the autumn and winter here in the UK.
  • Latitude: This refers to how far north or south you are from the equator. People in more northern countries, like the UK, tend to get less vitamin D than those nearer the equator.
  • Skin pigmentation: People with darker skin, such as those from African, African-Caribbean or south Asian backgrounds, don’t produce as much vitamin D as those with lighter skin.

Vitamin D3

You may have noticed vitamin D3 on the label of supplements and wondered what the difference is between vitamin D and vitamin D3?

There are actually two kinds of vitamin D - D2 and D3 - and studies suggest the latter is more effective at raising your blood levels.

So, you’ll find that most vitamin D supplements will contain vitamin D3, regardless of whether it says ‘vitamin D’ or ‘vitamin D3’ on the bottle.

What are the benefits of vitamin D?

We all love being out in the sun, but do you know about all the health benefits you can reap from the sunshine vitamin?

Taking a sufficient amount of vitamin D contributes to the normal function of your immune system, strengthens your bones and teeth, aids muscle function and helps your body to absorb minerals such as calcium.

Some studies suggest that vitamin D may help to improve symptoms of seasonal affective disorder (SAD), otherwise known as seasonal depression.

It’s claimed that sufferers have a lower amount of vitamin D in their body compared to those without the condition; however, more research needs to be done to further understand the correlation.

We know that vitamin D can give your immune system a well needed boost, so by increasing your vitamin D intake, you could contract fewer viruses like the common cold or flu.

Can you have too much vitamin D?

You can take too much vitamin D, but in order to reach a toxic, dangerous level a person would have to take an extremely high dose of vitamin D over a period of time.

Vitamin D supplements are, however, considered to be very safe provided they are taken as per the instructions.

The season, the weather, skin pigmentation and your age are all factors to consider when taking vitamin D.

According to the NHS, if you’re an adult you should take 10 μg (micrograms), which is the same as 400 IU per day, but you can also take a higher strength vitamin D supplement once a week or once a month instead if necessary.

This recommended dose also applies to children from the age of 1, pregnant and breastfeeding women, and people at risk of vitamin D deficiency.

The maximum amount of vitamin D per day for adults is 100 μg (4,000 IU) and taking any more than this may result in side effects.

Many people make the mistake of taking an excessive amount of vitamin D by unknowingly taking a monthly dose every day, so remember to always read the label carefully.

Vitamin D strengths are sometimes presented differently, which can be confusing - see the table below which converts common vitamin D strengths.

Micrograms (μg) International Units (IU)
10 μg (NHS recommended dose) 400 IU
15 μg 600 IU
20 μg 800 IU
25 μg 1000 IU
50 μg 2000 IU
100 μg 4000 IU

Water-soluble vs fat-soluble

There are two types of vitamins: water-soluble and fat-soluble.

The majority of vitamins are water-soluble, such as vitamin C and vitamin B complex.

Water-soluble vitamins dissolve in water and can’t stay in the body for long, so we regularly need to replace them.

If we take too much of a water-soluble vitamin, the body will only absorb what it needs and flush out the rest.

Fat-soluble vitamins, however, have a consistency similar to oil and cannot dissolve in water.

Vitamin D is a fat-soluble vitamin and our body easily retains it so there’s a higher chance of build-up, leading to possible side effects if we take too much.

Signs of vitamin D deficiency

If you’re lacking in vitamin D, you might experience certain symptoms.

Low vitamin D is very common in the UK - in fact, around 1 in 5 adults and 1 in 6 children are understood to be deficient in vitamin D.

The main cause of low vitamin D is lack of sunlight, so the NHS advice is to take 10μg (400 IU) of vitamin D in a supplement each day during autumn and winter if you live in the UK.

Frequently unwell

If you’re frequently unwell or seem to catch infections more than the average person, your body may be deficient in vitamin D.

Vitamin D is essential to keep your immune system strong so it's able to fight viruses and bacteria that make you unwell.

So, if you often become unwell, especially with colds or the flu, your body may have a lack of vitamin D.


Feeling tired can be caused by many things such as lifestyle factors, physical health conditions and vitamin deficiencies.

Some studies suggest that those with very low blood levels of vitamin D are frequently fatigued, resulting in a negative quality of life, but by supplementing vitamin D, it can reduce the severity of tiredness in those who are deficient.

Slow wound healing

If you’ve got an injury or wound that’s taking a long time to heal, it might be a sign that you’re lacking in vitamin D.

In fact, studies suggest that vitamin D increases the production of compounds that are essential for forming new skin as part of the wound-healing process.

Muscle pain

Research suggests that vitamin D deficiency is a possible cause of chronic muscle pain in children and adults.

A potential link has been identified between taking vitamin D supplements and a reduction in muscle pain, so, if you’re suffering with pain, it’s definitely worth incorporating a vitamin D supplement into your daily routine if you aren’t already.

Mental health

As we mentioned earlier with SAD (seasonal affective disorder), there’s research to suggest a link between insufficient vitamin D and mental health issues.


It’s normal to feel low from time to time, but depression is more than just feeling unhappy for a short time period.

Depression is a low mood that persists for prolonged periods of time, and it can be debilitating.

Depression doesn’t have a singular cause and can be triggered by a wide range of reasons, such as bereavement, illness, divorce and money or job worries.

To understand whether there’s a connection between low blood levels of vitamin D and depression, one study found that 65% of people who suffered from depression were also vitamin D deficient.

There’s a stronger link between vitamin D deficiency and seasonal depression, which occurs during the colder months when sunlight is scarce, which makes sense when we consider vitamin D to be the sunshine vitamin!

There’s still more research to be done around the relationship between vitamin D and depression, so it’s important not to consider vitamin D to be a miracle cure for depression.

Always visit your GP if you’re experiencing any symptoms such as low mood or thoughts of self harm.


Feeling anxious occasionally is a normal part of life - you may feel anxious before a job interview, public speaking, or even when flying on an aeroplane.

However, people with an anxiety disorder regularly have bouts of intense and persistent worry or fear about everyday situations.

The exact cause of an anxiety disorder isn’t fully understood, but it’s likely caused by a combination of different factors, such as genetics, a traumatic experience, overactivity in the brain or an imbalance of chemicals in the brain.

There may be a connection between vitamin D deficiency and anxiety disorders, too.

According to one analysis, levels of calcifediol (a form of vitamin D), was found to be reduced in those with anxiety.

Additionally, a study found that pregnant women who had normal vitamin D levels could have reduced anxiety, improved sleep quality and a lower chance of developing postpartum depression.

Calcium and vitamin D

To absorb calcium, the body needs vitamin D.

If there’s a lack of vitamin D, the body can’t make enough calcifediol which leads to an inadequate absorption of calcium.

To make up for the loss, the body takes calcium from the bones, making them weak and soft.

It’s important to remember that if you’re taking calcium supplements, ensure that they include vitamin D in order for them to be more effective.

Understanding vitamins D supplements

During autumn and winter, the NHS advises the majority of people to supplement their vitamin D due to the limited amount of sunlight.

Thankfully, vitamin D supplements are available in various forms and strengths, from tablets and capsules to gummies and sprays, there’s something available for the whole family.

It’s vital that you pay close attention to the strength and dosage instructions of your vitamin D supplements as they vary between daily, weekly and monthly dosages.

You don’t want to make the mistake of taking your monthly vitamin D daily as you might experience uncomfortable side effects.

It’s also important to note that vitamin D is present in other supplements too, such as calcium, cod liver oil and multivitamins.

However, you should always check exactly how much vitamin D is included in these supplements to make sure you’re getting the recommended intake of 10μg (400IU) a day.

Can children take vitamin D?

It’s essential that children are receiving enough vitamin D - particularly with supplements in their early years - as a lack of vitamin D can lead to bone deformities such as rickets.

Vitamin D helps to regulate the amount of calcium in the body, helping children grow strong bones and teeth.

All children aged 1-4 should take a vitamin D supplement throughout the year, as well as babies, unless they’re having more than 500ml of infant formula per day.

We know how fussy children can be when it comes to taking medicines, so that’s why we stock a range of vitamin D supplements designed for babies and children, including gummies and liquid formulas.

Vitamin D is just as essential as a warm, cosy coat during autumn and winter time, so don’t forget to take your vitamin D before you zip up!

This sunny-saviour is a very safe and versatile vitamin, offering many benefits for your health, like strengthening your immune system to snuff those sniffles.

Starting to see the bright side of vitamin D? We hope our guide has filled any gaps in your knowledge about the sunshine vitamin, but if you need more information, visit the NHS website.

Image Credit: People illustrations by Storyset

Alexandra Moses - Medical Content Writer
James O'Loan - CEO & Superintendent Pharmacist
James O'Loan , CEO & Superintendent Pharmacist on 15 March 2023
How we ensure accuracy in our content