What is Vitamin D? A nutritional therapist’s complete guide
Updated: Oct 14
Most of your Vitamin D comes from sunshine, and a UK winter does not provide enough of it. Discover the best sources, when to test your levels, and how to raise them.
Vitamin D is often referred to as the ‘sunshine vitamin’ because your body produces it in response to sunlight. But its benefits go far beyond its cheery nickname. As we head into winter it is a good time to understand the importance of Vitamin D, so this guide starts with the question “what is Vitamin D?” and then covers where to get it, why your levels might be low, and how to check and increase them.
In this article, we’ll explore:
What is Vitamin D? A nutrient that acts like a hormone
Vitamin D is a nutrient that acts like a hormone in your body, playing a critical role in maintaining overall health.
Bones and teeth
Its most famous job is helping your body absorb calcium and phosphorus, two minerals that are vital for building strong bones and teeth. Without enough Vitamin D, bones can weaken, leading to conditions like rickets in children or osteomalacia and osteoporosis in adults.
Your immune system
Beyond bone health, Vitamin D supports your immune system, helping your body fend off illnesses and infections.
Mood and the darker months
Vitamin D is also linked to mental health, with low levels often associated with mood changes and disorders like seasonal affective disorder (SAD), common in the darker months of the year.
Why Vitamin D matters so much in the UK
Your skin can only make Vitamin D when the sun is high enough in the sky. In the UK, most people can make what they need from about late March or early April to the end of September, but between October and early March the sun is simply too weak.
While your body can store Vitamin D during the summer months, many people have not made and stored enough to last the whole winter. A 2025 UK Government survey found that nearly 1 in 5 adults aged 19 to 65 have low Vitamin D levels.
This is why the NHS advises everyone to consider a Vitamin D supplement during autumn and winter. As nutritional therapists, we recommend going one step further and checking your levels first, so you know whether you need a supplement at all. You will find out how in when and how to check your Vitamin D levels below.
Where can you get Vitamin D?
The three main sources of Vitamin D are sunlight, food, and supplements. We cover supplements in how to increase your Vitamin D levels below.
Sunlight
Sun exposure is the most natural and efficient way to get Vitamin D. When ultraviolet B (UVB) rays hit your skin, they react with a steroid (7-dehydrocholesterol) found within your skin. This steroid is then turned into Vitamin D3 (cholecalciferol). Vitamin D3 enters your liver and is turned into a pro-hormone (calcidiol). This pro-hormone then enters your kidneys and is turned into the active form (useful form) of Vitamin D (calcitriol).
The more skin you have exposed without clothes or sunscreen (SPF), and therefore the greater the surface area available for the sun to reach, the more Vitamin D your body will produce.
Food
Vitamin D is found naturally in certain foods, but not in very large amounts. You can find Vitamin D in:
SMASH fish (salmon, mackerel, anchovies, sardines, and herring)
eggs
red meat (ideally grass-fed)
liver (choose organic and avoid if you are pregnant)
sun-exposed mushrooms
How much of each?
As a rough guide, the following should help you towards your weekly needs:
SMASH fish: 3 servings (300g total)
eggs: 6 to 8 eggs
sun-exposed mushrooms: 3 servings (300g total)
Optional: 1 to 2 servings of red meat or liver for a small Vitamin D boost
Including a wide variety of vegetables in your diet will also support the absorption and use of Vitamin D in your body.
How to create Vitamin D rich mushrooms
Sunbathe your mushrooms! To boost Vitamin D in mushrooms, expose them to sunlight, sliced or with the gills facing up, for at least 15 minutes and ideally up to an hour. Even 10 minutes makes a difference, but there is little gain beyond 2 hours. Just like in people, UV light helps them make Vitamin D. Store them in a cool, dark place to retain levels, and cook with minimal heat or steaming to preserve the Vitamin D content. If there isn’t any sun, or you don’t have time, some supermarkets have done the job for you: look out for Vitamin D-rich mushrooms.
Dietary tip
Because Vitamin D is fat-soluble (see our article on vitamins and minerals), it needs dietary fat to be effectively absorbed. Oily fish is great because it has both, but mushrooms are best combined with a source of healthy fat.
For a simple breakfast or lunch idea, try sun-exposed mushrooms sautéed in a little olive oil or coconut oil, and add them to a bed of smashed avocado on toast with a sprinkle of pumpkin seeds.
What about fortified foods?
Some foods, like milk, cereals, and orange juice, are fortified with Vitamin D. However, they are more often than not fortified with poor quality nutrients and should not be relied upon. In addition to this, fortified foods are typically ultra-processed and are therefore low in other health-supportive nutrients, such as healthy fats, protein, fibre, vitamins, and minerals.
If you would like to learn more about how to optimise your diet to support your health and wellbeing long-term check out our self-guided online nutrition courses.
Why might your Vitamin D levels be low?
Here are the different reasons why you or someone you know might have a Vitamin D deficiency. I think most of us can relate to at least one of these.
Geography
Where you live in the world can impact your Vitamin D levels. Those who live in areas with long winters or frequent grey days will have less sun exposure compared to those with longer hours of sun exposure. As Vitamin D is created by the sun’s UV rays hitting your skin, less sunshine reduces the opportunity for this process to occur.
Lifestyle
If you spend a lot of time indoors, wear suncream, or your skin is covered with protective clothing (e.g. PPE or long-sleeved tops), this can limit the amount of sunlight that your skin is exposed to. Therefore, even in sunny regions, you can be at risk of a Vitamin D deficiency. We will take a look at how you can adapt your lifestyle to optimise your Vitamin D levels later in this article.
Skin tone
Individuals with darker skin have higher levels of melanin, a chemical that the body produces to protect itself from UV rays emitted from the sun. While this natural phenomenon serves as a great protector against sunburn, skin cancer, and premature ageing, it reduces a person’s ability to make Vitamin D.
Age
Older adults can be less efficient at manufacturing Vitamin D, and they may wrap themselves in more clothes and spend less time outside, putting them at greater risk of a Vitamin D deficiency.
Genetics
Some people have what is referred to as genetic SNPs on the genes that help them to make and utilise Vitamin D within the body. This makes them genetically more susceptible to having lower levels of Vitamin D even if they were to spend many hours outside each day. You can find out if you have a genetic SNP by speaking to a private healthcare practitioner who specialises in nutrigenomics. Check out our practitioner directory or send us a message, and we will direct you to someone who can help.
Dietary habits
A lack of Vitamin D-rich foods in your diet can increase your chances of having lower Vitamin D levels. Those who are vegetarian or vegan are at an increased risk, as the foods containing the highest levels of Vitamin D are typically animal-based.
Medical conditions
Those with certain medical conditions are at an increased risk of having lower levels of Vitamin D. Conditions that impact the digestive system, for example Crohn’s disease and coeliac disease, can impair absorption of Vitamin D. Kidney and liver diseases can also impair the body’s ability to turn Vitamin D into the active form necessary to perform its important functions. In addition to this, certain medications interfere with how Vitamin D is absorbed or used.
Signs your Vitamin D might be low
Symptoms of a Vitamin D deficiency include:
fatigue
muscle or bone discomfort
muscle weakness
frequent illnesses
slow wound healing
low mood or SAD (seasonal affective disorder)
These signs can be subtle or overlap with other conditions, so testing is key to identifying if Vitamin D deficiency might be a contributory factor. It is especially important to check your Vitamin D levels if you are showing symptoms of a deficiency. If any of these symptoms persist, speak to your GP.
When and how to check your Vitamin D levels
When to test?
Checking Vitamin D levels is beneficial for everyone. The best times to test are at the end of summer, when levels should be at their highest, and at the end of winter, when they are naturally lower, having used up stored reserves.
Vitamin D is fat-soluble and can be stored within your fatty tissues (adipose tissues). This is great, as your body is able to build up a Vitamin D store during the summer months and make use of it during the winter months. However, if you were to take a high-dose Vitamin D supplement for many months, this can lead to excessive levels of Vitamin D within your body (Vitamin D from sunlight or diet does not pose this risk).
Excessive levels of Vitamin D can have negative health consequences and are therefore something you want to avoid. It is therefore important to test your Vitamin D levels before taking any form of Vitamin D supplementation.
How to test at home
Your GP may test your Vitamin D levels if there is a medical reason to, but it is less likely to be offered as a routine check. A home test is an affordable and accessible alternative.
The most accurate way to measure Vitamin D levels is through a blood test. This is an inexpensive test that can be done at home with a simple finger-prick blood sample. Below is the at-home Vitamin D test kit that we recommend and is used by health practitioners within the UK.
The BetterYou at-home Vitamin D test kit comes with everything you need to check your Vitamin D levels. Read the instructions carefully before starting the test. It is important to note that you should ideally stop taking any Vitamin D supplementation 4 to 6 weeks before taking the test. If you have been prescribed Vitamin D by your healthcare practitioner, please check with them first before stopping supplementation. And if you want to see how well your current supplement is working, you can test while still taking it.
Send off your test in the prepaid envelope and await your test results, which will be sent via email within 10 working days.
Tips for an easy finger-prick test
Take the test first thing in the morning before you have eaten anything.
Drink lots of water at least 30 minutes before taking the test.
Shake your hand around or do some star jumps to get your blood pumping around your body.
Run your hand under warm water.
If you are really struggling to get enough blood to come out, you may want to have a warm bath or shower before taking the test.
Prick your finger on your non-dominant hand (e.g. if you are right-handed, prick your left hand).
Prick your middle or ring finger on the side, a couple of millimetres below your nail (see image below). This is where you will feel less sensitivity, but still have good blood flow.
What to do with your results
Your results will show whether you need to supplement at all, and if so, how much. There is no point in taking a supplement you do not need.
When selecting your free Vitamin D supplement from BetterYou, opt for one containing Vitamin K2 (unless you are taking any medications, in which case please check with your pharmacist first, as Vitamin K can interact with certain medications). Vitamin D and K2 work together to ensure that calcium is directed out of your arteries and into your bones, where it helps to improve bone density.
If you suspect you have a Vitamin D deficiency, it is important that you consult your primary healthcare provider (GP) or a nutritional therapist. They will help you to interpret your results properly and discuss the next steps. Check out our practitioner directory to find a healthcare practitioner.
How to increase your Vitamin D levels
You can improve your Vitamin D levels naturally by making simple diet and lifestyle changes, from spending more time outside to increasing your intake of Vitamin D rich foods. However, if your levels are low, you may want to opt for a therapeutic-grade supplement.
Getting enough sun
Spending time outside is one of the best ways to boost your Vitamin D levels naturally. To get enough Vitamin D from sunlight without risking sunburn, you will need to balance your sun exposure based on your skin type, location, and the time of day and year.
For optimal levels:
fair skin: aim to get 10 to 15 minutes of sunlight exposure per day
medium or darker skin: aim for 15 minutes to 2 hours of sunlight per day
Bare minimum:
At the very minimum, you should aim to spend 10 to 30 minutes in the midday sun with your forearms exposed, 2 to 3 times a week.
During this time, you should avoid wearing sunscreen on your exposed areas of skin so you can absorb the UV rays that help make Vitamin D. It is recommended that you increase your exposure time gradually and be sure not to stay out long enough to burn.
The shadow test
The sun isn’t always strong enough for your skin to produce Vitamin D. An easy way to check is by using the ‘shadow test’.
If you stand outside in the sun and your shadow is shorter than your height, it means that the sun is high enough in the sky and strong enough for you to make Vitamin D optimally.
If you stand outside and your shadow is taller than you, it means that the sun is not high enough in the sky and not strong enough for you to make adequate amounts of Vitamin D.
Choosing a supplement
If your levels are low, Vitamin D supplements can help restore them quickly, but it is important to choose the right supplement. Look for Vitamin D3 (cholecalciferol), as it is the most effective form for raising blood levels. You can find some great therapeutic-grade supplement options in our product directory.
Some Vitamin D supplements contain Vitamin K2, which can help with calcium balance and may improve bone health when taken alongside Vitamin D3. If you are going to take Vitamin D and K2 together, be sure to read our article on what you need to know before you buy or take supplements.
How much is too much?
It is important to avoid over-supplementing Vitamin D. Vitamin D is fat-soluble, meaning it builds up in the body, and excessive amounts can do more harm than good. High levels may cause nausea, weakness, or more serious issues like kidney damage. Read when and how to check your Vitamin D levels above before supplementing with Vitamin D.
Working with a practitioner
While you can test your Vitamin D levels at home, it is highly recommended that you seek the support of a nutritional therapist or other qualified health professional. They can help you interpret your blood test results and find the right supplement and dosage to correct your levels safely. They can also offer personalised guidance, especially if you have a restricted diet, digestive issues, or a genetic predisposition that affects your ability to absorb Vitamin D from food. Find a practitioner in our practitioner directory.
Final thoughts
Vitamin D is vital for bone strength, immune function, and mood regulation, yet many people don’t get enough. Whether through sunlight, diet, or supplements, maintaining adequate levels is essential for overall health. If you suspect a deficiency, testing your levels and taking proactive steps can make a big difference. With a combination of small lifestyle changes and informed choices, you can ensure your body gets the Vitamin D it needs to thrive.
If you would like personalised support, a nutritional therapist can help you interpret your results and build a plan that suits you. Browse our practitioner directory to find one.
Would you like to learn how to optimise your diet to support your health?
Our online nutrition courses are self-paced, created by nutritional therapist and founder of The Health Puzzle, Josie Shillabeer. You can get started with our free introduction and mini-mindset course.
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