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Vitamin D Guide

Vitamin D is essential for bone health and supporting the immune system, yet it is one of the most common deficiencies in the UK. Read our guide to understand how vitamin D can affect you.

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Dr Sam Rodgers - Chief Medical Officer

Reviewed by Dr Sam Rodgers

18th May 2020


What is vitamin D?

Chapter 1

What is vitamin D?

What are the causes of vitamin D deficiency?

Chapter 2

What are the causes of vitamin D deficiency?

What are the symptoms of vitamin D deficiency?

Chapter 3

What are the symptoms of vitamin D deficiency?

What are the effects of vitamin D deficiency on our health?

Chapter 4

What are the effects of vitamin D deficiency on our health?

How is vitamin D deficiency diagnosed?

Chapter 5

How is vitamin D deficiency diagnosed?

What are the treatments for vitamin D deficiency?

Chapter 6

What are the treatments for vitamin D deficiency?

What is vitamin D?

Chapter 1

What is vitamin D?

With the current risk of deficiency higher than usual, we take a look at the important role vitamin D plays in our body’s major systems and how we can easily improve our levels. 

Referred to as the 'sunshine vitamin,' vitamin D is a fat-soluble vitamin produced in your skin in response to sunlight. Although your body produces vitamin D naturally when exposed to sunlight, you can also get it through certain foods and as well as supplements.

Vitamin D is one of the most common deficiencies in the UK, with around 1 in 5 people having low levels. During the summer months, we can get sufficient amounts of vitamin D from being outdoors. However, we can’t get adequate amounts during winter, leading to high numbers of people suffering from vitamin D deficiency.

Due to the recent lockdown measures put in place to stop the spread of COVID-19, we have been spending even less time in natural sunlight than usual, so the risk of deficiency could be even higher.

Public Health England previously advised that a daily vitamin D supplement of 10 micrograms (mcg) for children over one and adults should be taken during the winter months between October and March to avoid deficiency. They have now updated the guidelines to recommend taking a supplement throughout the year to ensure levels are optimal.

However, a recent Medichecks study found that nearly two-thirds of Brits tested during lockdown had good levels of the vitamin, a 10% improvement on the previous year.

The study examined the findings of over 11,000 people from the period March to June, in both 2019 and 2020.

Optimum levels in lockdown could be attributed to spending more time outside due to factors including better work life balance, being furloughed, not being preoccupied by closed services such as gyms, bars and restaurants and those taking early advice from Public Health England on supplementing the vitamin.

Let's take a look at why it's important to have a good level of vitamin D.

What are the functions of vitamin D?

The two primary dietary forms of vitamin D that exist are:

  • Vitamin D3 (cholecalciferol) – found in some animal foods such a fatty fish and egg yolks.
  • Vitamin D2 (ergocalciferol) – found in some plants, mushrooms, and yeasts.

When vitamin D enters your body, it needs to undergo two conversion steps to become active. Firstly, the liver converts it to calcidiol or 25(OH)D - this is the storage form of the vitamin. Secondly, the kidneys convert it to calcitriol, or 1,25(OH)2D, which is the active form of vitamin D. The active form of vitamin D then binds to the vitamin D receptor found in almost every cell in the body and changes those cells.

Vitamin D is essential for many processes in the body. The two main functions of vitamin D are:

  • Regulating the absorption of calcium and phosphorus for healthy bones, teeth, and muscles.
  • Supporting the functioning of the immune system to help the body fight diseases.

As vitamin D is necessary for calcium absorption, if your body isn’t getting enough, there is a risk of developing bone disorders where the bones become weak and deformed (osteomalacia in adults and rickets in children) or porous and brittle (osteopenia and osteoporosis). Vitamin D deficiency can also increase the risk of the body getting infections as it affects the immune system.

Studies have linked vitamin D deficiency to several diseases, including breast cancer, colon cancer, prostate cancer, heart disease, depression, and obesity.

What are the causes of vitamin D deficiency?

Chapter 2

What are the causes of vitamin D deficiency?

The amount of vitamin D in the body depends on a variety of factors, find out what could be causing your low levels.

The amount of vitamin D in the body will depend on a variety of factors.

Here are some of them:

  • Time spent outdoors - Due to the unpredictable nature of the UK weather, it is no surprise that inadequate exposure to sunlight is one of the most common causes of vitamin D deficiency in the UK.
  • Diet – vitamin D is found in foods such as fatty fish like tuna and salmon, fortified foods like certain dairy products, cereals, and soy milk, beef liver, egg yolks, and some mushrooms. Therefore, a poor diet lacking these foods can result in low vitamin D levels. However, getting adequate levels of vitamin D through food alone is difficult, so we should not rely on it solely as our source of vitamin D.
  • Age - older people have increased rates of vitamin D deficiency, which can be down to several factors, including a decrease in the body’s ability to absorb, synthesise and convert vitamin D into its active form. Other factors may also play a role, including older people being less likely to spend time outside in the sun.
  • Skin tone – people with darker skin tones may also be at increased risk of vitamin D deficiency as they have higher levels of melanin, which causes skin pigmentation, and lowers the skin's ability to make vitamin D in response to sunlight. As a result, people with darker skin tones may need to spend longer in the sun to produce the same amount of vitamin D as people with lighter skin [1].
  • Malabsorption – certain medical conditions affect the body’s ability to absorb vitamins and minerals from the food we eat. With celiac and Chron’s disease, the lining of the small intestine becomes damaged either as a result of the person eating gluten or through inflammation. This damage affects the ability of the small intestine to absorb nutrients from the foods we consume, including vitamin D. As a result, we often see vitamin D deficiency can in people suffering from celiac and Crohn's disease.
  • Medical conditions - other conditions such as kidney disease can also affect vitamin D levels. As the kidneys are responsible for converting vitamin D into the active form our body needs, if they are damaged, they are less able to make this conversion, resulting in low levels in the body. Liver disease can also affect the absorption and synthesis of vitamin D, causing a deficiency [2]. Certain medications, such as anti-epileptic drugs, can also affect how the body processes vitamin D. Finally, women who are pregnant or breastfeeding will have increased nutritional demands so they may be at increased risk of deficiencies, including vitamin D [3].
  • Lifestyle factors – obesity has been linked to Vitamin D deficiency. Scientists are still studying the reasons for this, however, it could be because vitamin D may become 'trapped' inside the fat tissue, so less is available to circulate in the blood throughout the body [4]. Studies have also shown smoking to be detrimental to vitamin D levels in the body [5].
What are the symptoms of vitamin D deficiency?

Chapter 3

What are the symptoms of vitamin D deficiency?

The signs of low vitamin D can be subtle; we discuss the symptoms that you may recognise if you have vitamin D deficiency.

The symptoms of low vitamin D can be subtle, so most of us won’t know if we are deficient.

Symptoms include:

  • Tiredness
  • Low mood
  • Irritability
  • Brain fog
  • Hair loss
  • Weight gain
  • Joint and bone pain
What are the effects of vitamin D deficiency on our health?

Chapter 4

What are the effects of vitamin D deficiency on our health?

Without vitamin D, our bodies cannot function properly. Find out the effects that vitamin D deficiency can have on your health.

Bone Health

Vitamin D's primary role is to ensure healthy absorption of calcium and phosphate so that our bones and teeth remain healthy. Without vitamin D, our bodies only absorb 10-15% of calcium and 60% of phosphate from our food.

As a result of low vitamin D, the body cannot take in adequate amounts of calcium and phosphate, which triggers an increase in a hormone called parathyroid hormone (PTH). To maintain balance, PTH will then increase bone breakdown to release more calcium and phosphate, causing a decrease in bone mineral density (BMD). This breakdown in the bones leads to osteopenia and osteoporosis, placing a person at a higher risk of fractures.

Also, a severe lack of calcium and phosphate in the bones can lead to weakness and bone deformity, also known as rickets (in children) and osteomalacia (in adults).

Immune system function

Vitamin D deficiency is associated with increased autoimmunity as well as increased susceptibility to infection as vitamin D plays a crucial role in immune function. Vitamin D is involved in the activation of specific cells in the immune system called 'T cells’. These cells play a significant role in the immune system by killing infected host cells, activating other immune cells, producing cytokines (signalling molecules), and regulating the immune response.

One study suggests the benefits of supplementing with vitamin D goes beyond just bone health, and could also have significant benefits for the immune system [6].

Other diseases

Research is still ongoing, but having low vitamin D levels is associated with increased risk of many chronic diseases, including cardiovascular disease, cancer, and diabetes, as well as depression, obesity, pregnancy complications, and severe asthma and autism in children [7]. Research also suggests that vitamin D could play a role in the prevention and treatment of particular conditions, including diabetes [8], glucose intolerance [9], and multiple sclerosis [10]. However, more research is needed to confirm this.

How is vitamin D deficiency diagnosed?

Chapter 5

How is vitamin D deficiency diagnosed?

Diagnosing a vitamin deficiency usually starts with a blood test.  Here we look at what is commonly tested and what you can do to manage your levels.

The most accurate way to measure how much vitamin D is in your body is with a 25(OH) vitamin D blood test, also known as a 25-hydroxy-cholecalciferol or calcidiol test.

The amount of 25(OH)D in your blood is a good indication of how much vitamin D your body has as it is the primary circulating form of vitamin D, so this test is currently considered the best indicator of vitamin D supply to the body.

The 25-hydroxy-cholecalciferol test may not be a reliable measure of vitamin D status in people with severe kidney disease.

Vitamin D toxicity

Although rare, it is also possible to have too much vitamin D. Vitamin D toxicity is usually caused by taking substantial doses of vitamin D supplements, rather than by diet or sun exposure. Our bodies regulate the amount of vitamin D produced by sun exposure, and fortified foods don't usually contain large enough amounts of vitamin D to be dangerous, making toxicity unlikely.

The main consequence of vitamin D toxicity is a build-up of calcium in your blood (hypercalcemia), which can cause nausea, vomiting, weakness, and frequent urination. Vitamin D toxicity might also progress to bone pain and kidney problems, such as the formation of calcium stones.

Treatment for vitamin D toxicity includes stopping vitamin D intake from food and supplements and restricting dietary calcium. Medications may also be required. If you are taking vitamin D supplements, be sure to read the label to check you aren’t taking too much – 10 micrograms a day should be enough to maintain proper levels.

What are the treatments for vitamin D deficiency?

Chapter 6

What are the treatments for vitamin D deficiency?

Vitamin D deficiency is easy to treat, and many of the symptoms can be improved once levels in the body recover. Here we explore what treatment options are available to you.

Vitamin D deficiency is easy to treat, and many of the symptoms can be improved or reversed once levels in the body recover.


The dose of vitamin D required will depend on whether you are treating a vitamin D deficiency or insufficiency.

If your blood results show you are deficient in vitamin D, you will need to take a supplement. Here at Medichecks, we advise supplementing with 80 micrograms (3200 IU) per day for twelve weeks.

If your vitamin D level is in the insufficient range, we will advise you to consider supplementing with 20-50 micrograms of vitamin D per day for twelve weeks.

We encourage re-checking your vitamin D level after 12 weeks of supplementation. If your level is then healthy, you can reduce your vitamin D to a maintenance dose of 10 micrograms per day.

Prevention is always better than cure and we may suggest ways in which you can optimise factors such as diet, sun exposure, and supplementation to ensure your levels don't slip.

When choosing to supplement, vitamin D3 is preferred over vitamin D2 as it is more efficient in increasing levels in the body as the liver metabolises it better. You can find vitamin D supplements in most supermarkets, pharmacies, and health shops, as well as online. Pregnant women, women with a child under 12 months, and eligible children up to four years old are entitled to free vitamin tablets through the NHS Healthy Start Scheme.

If you have been supplementing, but your vitamin D levels have not improved, or you are still experiencing symptoms, it is a good idea to discuss this with your GP. You may require further investigations for malabsorption or other treatments.

Vitamin D supplements should be taken with caution if you have bone disease, renal impairment, kidney stones, or any conditions affecting calcium levels. We also advise discussing with your doctor if you have increased sensitivity to vitamin D due to certain conditions (such as lymphoma, sarcoidosis, or hyperparathyroidism). If you are pregnant, breastfeeding, or are taking any medications, it would also be a good idea to check with your GP before starting any new supplements.

Spend time in the sun

One of the best ways to improve vitamin D levels is to spend time in the sun. Around 15-20 minutes outside every day between 11 am, and 3 pm when the sun is at its highest is best. Wearing a short sleeve top and shorts will help to increase absorption, and those with darker skin may need to stay out longer to ensure they are getting enough vitamin D.

It may be challenging to get outside during lockdown, however, try to get outdoors every day if you can. Reading a book in the garden or going for a daily walk with your family will help ensure your vitamin D levels are optimal.

Many people worry about sun damage from spending too long in the sun. However, as long as we are safe, the sun can be beneficial to our health. If you have fairer skin that is more sensitive to burning, make sure to start with short periods outside and build up if you can. It is also important to note that prolonged exposure to sunlight will not lead to excess vitamin D as there is a regulatory mechanism in the body to prevent this. However, spending too long in the sun will increase the risk of skin cancer, so around 15-20 minutes is all you need. According to studies, sun cream does not generally affect vitamin D levels, so it is fine to wear it [11]. We don’t recommend you use sunbeds as they emit high levels of UVA, which will not increase vitamin D levels but will increase the risk of skin cancer.

Eat good food sources of vitamin D

The richest sources of natural vitamin D can is found in oily fish. Typically, a 3oz salmon fillet will contain 11-17 micrograms per portion. Mushrooms, especially those exposed to UV light, also contain vitamin D providing around 7.9 micrograms per half a cup of mushrooms.

Dairy products, cereals, eggs, and dairy-free milk, such as soy and almond, are often fortified with vitamin D. Incorporating these into your diet is a good way to boost your levels. However, as our body isn’t the best at absorbing vitamin D from food, and the amount of vitamin D can vary in these products, we shouldn't rely on them as our only source of vitamin D.

As well as ensuring healthy vitamin D levels, it is also essential to maintain adequate dietary calcium intake as we need both for healthy bones and teeth. Good sources of dietary calcium include fortified cereals, cheese, almond milk, and tofu. If you aren’t eating much of these foods, then you may also want to consider a calcium supplement.

How can I ensure my vitamin D levels are optimal?

The number of people with vitamin D deficiency in the UK remains high, and now more than ever, it is important we think about our vitamin D levels as we spend more time indoors. While studies are still investigating the role of vitamin D in the prevention of chronic disease, what we do know for sure is that it has significant benefits to our bone health and immune function.

Get tested

Taking the guesswork out by checking your vitamin D level is a great place to start. A simple at-home blood test will show you how much vitamin D is in your body and whether or not you are deficient. With Medichecks, you can also monitor your levels over time to see how they improve with supplementation.

You can find out more about vitamin D via the British Dietetic Association factsheet on vitamin D and the NHS website.