What's the optimal vitamin D level and how do I get it?

Wondering how much vitamin D you should have in your blood? Find out why an “optimal” level beats “sufficient” and how to get enough.

Medical review by

Dr Alasdair Scott MBBS FRCS PhD

gut doctor, PhD and our research director


Key Takeaways

  • In the UK most of us don’t have an optimal vitamin D level, which can leave us feeling tired.
  • If you’re fatigued, have insomnia or increased illness or infections you might have vitamin D deficiency.
  • Take our Quick and Easy Vitamin D Blood Test today.

Vitamin D plays a crucial role in your health but nearly half of us don’t have enough of it1! But how much even is “enough” and how do you know if you’ve got it? We’ll talk you through what the optimal vitamin D blood level is, how to test your level and how to make sure you’re getting enough vitamin D to get in, and stay in, the optimal range.

What is vitamin D and what does it do?

As the name suggests, vitamin D is a vitamin, and an essential one at that. Vitamin D comes in two main types. Vitamin D3, also called cholecalciferol, is the type of vitamin D made by your skin from sunlight exposure and is also found in oily fish. The other type is vitamin D2, or ergocalciferol, which is found in some plants and mushrooms.

Vitamin D has many roles in your body and we’re learning more about its importance all the time. One of its key functions is to regulate calcium and phosphate levels in your body. These minerals are key building blocks for your bones, teeth and muscles.

Vitamin D is also essential for a strong immune system that fights off infection. Vitamin D deficiency is associated with an increased risk of infections, particularly respiratory tract infections, or colds2. If you have a low level of vitamin D, you’re more likely to be infected with coronavirus3. Not only that, but if you are infected with coronavirus and have vitamin D deficiency, you’re more likely to have a worse infection4.

Who’s at risk of vitamin D deficiency?

Vitamin D deficiency means you don’t have enough vitamin D for your body to function as well as it should and is among the most common nutrient deficiencies in the world. Vitamin D deficiency is so common that it has actually been labelled a “pandemic”. In Europe, recent large studies have shown that as many as 2 in every 5 people could be deficient in vitamin D5.

You’re at higher likelihood of vitamin D deficiency as you get older or if you have darker skin. As you get older, your skin isn’t as good at making vitamin D from sunlight6. If you have darker skin you’ll have more melanin. The more melanin you have in your skin, the less vitamin D your skin makes in response to the sun. If you don’t get much sunlight on your skin because you keep it covered or spend a lot of time indoors, then you’ll also be at higher risk of vitamin D deficiency. You’re also less likely to have enough vitamin D if you’re on a plant-based diet as you won’t be eating vitamin D-rich foods like eggs and oily fish. Compared to meat eaters, vegans had lower levels of vitamin D in their blood, with 20% lower levels in summer and 38% lower in winter7.

A lot of people are deficient or insufficient in vitamin D and don’t even know it. Even if you do have symptoms of vitamin D deficiency, they tend to be quite vague like tiredness, difficulty sleeping or aching bones. You might find that you are more prone to infections such as cold and flu. If you’re severely low on vitamin D you can even get bone softening which is called osteomalacia8.

Woman in sunshine

So how much vitamin D is enough “Optimal” vs. “Sufficient”

We can use a blood test to measure your vitamin D level and we have a scale to classify you depending on how much you have:

  • less than 25nmol/L – deficient
  • 25 - 50nmol/L – insufficient (may also be classed as “deficient” in many studies)
  • 50 - 75nmol/L – sufficient according to NICE, insufficient according to the Endocrine Society
  • 75 - 175nmol/L – optimal
  • 175 - 200nmol/L - high normal
  • over 200nmol/L - very high

If you’re within 50 - 75nmol/L you’re classed as being vitamin D “sufficient”. This is the level that is considered “sufficient” for the general population and is enough vitamin D for most people, most of the time. But when it comes to vitamin D, you probably shouldn’t be aiming for the bare minimum - it’s an okay starting point but not a goal to strive for.

The National Institute of Clinical Evidence (NICE) suggests that blood levels of vitamin D should be above 50nmol/L to be sufficient9. However the Endocrine Society recommends that the minimum level to aim for is 75nmol/L10. We therefore suggest that a target level of 75-100nmol/L is optimal. Let’s explain why.

Why should you aim for an optimal vitamin D level?

As we’ve said, vitamin D doesn’t just keep your bones, teeth and muscles strong and healthy, although that’s a lot of benefits already! It also plays an important role in your immune system, helping you fight infection2. There are two main arguments for aiming for a higher, “optimal” level. Firstly, is a “sufficient” level of vitamin D enough for you? Secondly, there may be benefits to having a higher level of vitamin D over and above what you’d get from being “sufficient”.

Being only “sufficient” puts you at risk of becoming “insufficient” or “deficient” if your vitamin D input from sunlight or diet drops or you start needing more. For example, during the winter months in the UK, when you’re getting less sunlight, you could well become vitamin D deficient if you were only borderline to begin with. Also, there are times in life when you might need more vitamin D than most people, such as pregnancy, breastfeeding, vigorous training or undergoing the stress of an infection. Being at the lower end of the "sufficient" 50-75nmol/L range puts you at risk of deficiency at times when your body needs more. It's easy to top up your vitamin D, so there’s no need to cut it fine.

There is debate on the impacts that an optimal vitamin D level can have, but some studies have shown that an optimal level could maximise the beneficial effects vitamin D has on your health. These include your bone and muscle health along with calcium metabolism10. This is one of the main reasons why The Endocrine Society recommends raising the blood vitamin D level above 75nmol/L. Studies which currently need further research have suggested that a level above 75nmol/L may give health benefits for various common cancers, autoimmune and infectious diseases along with type 2 diabetes and heart conditions like cardiovascular disease10.

Imagine it like this: you’ve been told that you should exercise 150 minutes per week to get the most health benefits available. You’re only exercising for 40 minutes per week. It’s still much better than nothing, but you wouldn’t expect the same benefits or results from if you were doing the full 150. That’s why you need to aim for the optimal amount. There’s nothing to lose and lots you could gain.

How to check your vitamin D level

The only way to check your vitamin D level is with a blood test. We offer an Iron, Vitamin D and B12 Deficiency Blood Test which is a simple finger-prick blood test you can do at home. It not only tests for vitamin D but also your iron and vitamin B12 levels. These are all essential to make sure you have optimal health.

We recommend checking your vitamin D level each year to make sure it’s optimal. If it's not, you can try and get more vitamin D in your diet or consider taking supplements. In fact, Public Health England recommends everyone in the UK take a supplement during the winter months11.

You’ll want to monitor your levels afterwards to make sure any dietary changes or supplements are paying off. There’s no “best time” to check your vitamin D level, but you’re more likely to get deficient in the winter months because your skin isn’t getting as much sunlight.

Symptoms of low vitamin D can include fatigue and poor sleep but these can also be caused by lots of other conditions. If you’re tired all the time and are wondering if it’s vitamin D deficiency or something else, take our Tiredness & Low Energy Blood Test which will help you get to the root of the problem. This is a comprehensive blood test panel which tests for the common causes of low energy and tiredness, including vitamin D deficiency.

How to get an optimal vitamin D level

If you’re not at the optimal level for vitamin D yet, there are ways to get there.


Vitamin D is nicknamed the “sunshine vitamin”. Your body makes vitamin D naturally in response to UVB radiation from the sun. During the spring and summer months, 15 minutes of sun on your skin can get you 1000 International Units (IU) of vitamin D12, which is above the recommended dietary allowance (RDA) of 600 IU of vitamin D13. However, those of us with darker skin and older individuals6 will need more time to reach these levels14.

Now, don’t go putting away your suncream just yet! You need to strike a balance between getting enough vitamin D from sunlight and protecting yourself from sunburn and skin cancer. The NHS has some great guidance on sun safety for you and your family that’ll help. Given how easy it is to gain enough vitamin D from a low-cost daily supplement, the benefits of good sun protection usually outweigh the risks of not getting enough vitamin D.


Of course, you can also get vitamin D from your diet. A good source of vitamin D is oily fish, like salmon, and you could take fish oil or capsules too. 100g of salmon contains 412 IU of vitamin D15. Egg yolks also contain vitamin D and this could suit vegetarians. For those on a plant-based diet, there is some vitamin D in its D2 form in plants and mushrooms. Lastly, there are some vitamin D-fortified foods, such as margarine and some breakfast cereals. However, it’s difficult to get all the vitamin D you need from your diet alone.

Foods containing vitamin D


If you’re not able to reach and maintain an optimal vitamin D level through safe sun exposure and diet, you should think about taking a vitamin D supplement. This could be with tablets, capsules, sprays or drops. The UK Government and Public Health England advise that everyone living in the UK in winter months should take a supplement of 400-1000 IU a day from October until March11. The UV light isn’t strong enough for us to generate enough vitamin D during these darker months of the year. Vitamin D supplements can be an inexpensive and easy way to boost your health, making sure you get the daily RDA of 600 IU13.

Can I have too much vitamin D?

As vitamin D is stored in the body, it is possible to take too much. Taking very high levels of vitamin D can make you quite unwell. Calcium is a component of a healthy, balanced diet. The more vitamin D you take, the more calcium you’ll absorb. Taking too much vitamin D causes the body to absorb more calcium than it needs which can be harmful.

How can you avoid this? Firstly, you can’t overdose on vitamin D from sun exposure. Make sure you’re not overdoing supplementation by taking 400-1000 IU per day. Do not take more than 4000 IU daily unless prescribed by a doctor. If you take more than 4000 IU you’re more likely to have negative health impacts16.

Stick to safe levels of vitamin D supplementation and don’t take more than 4000 IU. Test your levels annually to make sure you’re not taking too much.

Can I order a vitamin D test today?

Want to find out how far off you are from the optimal vitamin D level? You can order your Quick and Easy Vitamin D Blood Test from us here at Selph. You’re responsible for your own health. We give you the information, the resources, the support and the confidence to put yourSelph first.

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