Vitamin D (25 OH)

What is Vitamin D?

Commonly known as the sunshine vitamin, vitamin D is a fat-soluble vitamin that is crucial for the absorption of calcium and phosphorus to support bone health. It also has important roles in:

• Immunity: Helps your immune system to fight infections caused by bacteria and viruses.
• Muscle function: May increase muscle strength.
• Brain activity: Low vitamin D levels are linked to a 54% increased risk of dementia^[1].
• Better teeth: Vitamin D deficiencies have been linked with oral diseases^[2].
• Gut Health: There is growing evidence that vitamin D is linked with a healthier gut microbiome^[3].
• Vaginal microbiome: There is a link between vagina bacteria and vitamin D^[4].

There are two main forms of vitamin D: D2 and D3. The main differences are where they are produced or acquired. Vitamin D2 (ergocalciferol) is made from plants, while D3 (cholecalciferol) is naturally produced by the human body in response to sunlight exposure.

When vitamin D enters the body, either through food or supplements, or sunlight absorption, it must undergo a two-step process to become activated:

1. Vitamin D enters the liver and is converted into 25 (OH) D (calcidiol), the stored or circulating form.
2. Once it enters the kidneys, Vitamin D is converted into its active form called calcitriol or 1, 25 (OH) 2D, which is technically a hormone! It is this form which binds to the receptors in the cells and acts upon them to facilitate calcium absorption from the small intestine, reabsorption from the kidneys, and bone tissue rebuilding^[5].

How Vitamin D Impacts Your Health?

Although vitamin D is a nutrient which can be acquired through diet, it is not as readily available as other micronutrients. The synthesis of vitamin D in the body is largely dependent on sunlight exposure^[6], and this key nutrient plays a big role in your health, larger than you may have realised.

Strong, healthy bones: Vitamin D’s main role is to support bone health by aiding the absorption of calcium, the main building block of bone. Without enough vitamin D, bones become thin and brittle, increasing the risk of developing osteoarthritis^[7].

Strong muscles: It’s not just your skeletal system that benefits from adequate vitamin D levels, it’s also a key nutrient for your muscles. Mounting evidence suggests that vitamin D could have an integral role in muscle recovery following injury and that low levels are associated with an increased risk of muscle weakness and falls^[8].

Heart health: Some research has shown that vitamin D supplementation may reduce the risk of cardiovascular events^[9]. Therefore, vitamin D may have protective benefits for the heart, although more research is needed to establish the extent of these advantages.

Cognition: Because of the important role vitamin D plays in the absorption of calcium, it may also have protective benefits for the brain. Calcium is a universal messenger for extracellular signals and has key roles in nerve function and memory^[10]. There is also some evidence to suggest that higher brain concentrations of D3 are associated with improved cognitive function^[11].

Symptoms of Low Vitamin D

Vitamin D deficiency is one of the most common nutrient deficiencies in the world and symptoms include:

• Muscle cramps
• Bone pain
• Increased risk of falling and fracture
• Tiredness and fatigue

Some people who are deficient in vitamin D may have no symptoms or very minor or vague symptoms, such as tiredness, which can mean deficiency is often missed.

How To Boost Your Vitamin D Levels

For most people, much of their vitamin D will come from the sun. During sunny months, spending three to 30 minutes in the sun between 10 am and 3 pm twice a week with your face, arms, lower legs and hands uncovered should be sufficient to top up your levels^[16].

Because it is difficult to maintain levels during the winter months, you’ll need to consume vitamin D-rich foods and consider taking a supplement.

In short, the best ways to promote healthy vitamin D levels are:

• Sun exposure
• Foods
• Vitamin D supplements

Should We Use Vitamin D Supplements?

With shorter days and a lack of sunshine in the UK during the winter months, it can be challenging to get all the vitamin D your body requires to stay healthy. So, all UK adults are advised to take a 10-mcg vitamin D supplement, particularly during autumn and winter.

Some people are also at a greater risk of deficiency and should take a supplement all year round. This includes:

• Pregnant or breastfeeding women
• Anyone who wears clothes that cover up most of their skin outdoors
• People with dark skin
• Anyone who doesn’t get much exposure to sunlight^[17]

When looking for a good vitamin D supplement, it’s important to pick the best form. There are two types of vitamin D available as a supplement, D2 and D3. Both can be useful for preventing and treating deficiency, but vitamin D3 is often recommended because it is more active and better at improving vitamin D status^[18].

Oral supplements are quickly absorbed and can reach their maximum level in the body within 24 hours. Therefore, vitamin D levels peak between one to two weeks later^[19].

What Foods Are High in Vitamin D?

Few foods naturally contain vitamin D, but some sources include:

• Oily fish; mackerel, salmon, trout, tuna
• Beef liver
• Egg yolks
• Cheese
• UV light-treated mushrooms
• Fortified milk and cereals

How Does Vitamin D Work with B12?

Vitamins and minerals are like puzzle pieces. Each piece fits together nicely to build a picture of health and play unique roles to support your body. Some also work together. One example is vitamin D and B12, with some research suggesting that low levels of vitamin D may affect the absorption of B12 and increase the risk of deficiency in both nutrients^[20].

These vitamins also work synergistically to help keep you feeling at your best. For example, research shows that vitamin D levels are correlated with both vitamin B12 and folate levels and that a lack of vitamin D may impact and reduce the absorption of these nutrients^[21].

Low levels of any of these biomarkers can cause similar symptoms, like:

• Fatigue
• Muscle weakness and aches
• Mood swings
• Memory issues

Therefore, it is advisable to check your vitamin B12 and folate status alongside your vitamin D levels.

How to Check Your Vitamin D Levels

You can check your vitamin D levels using our at-home blood tests.

We have a dedicated Vitamin D blood test if you just want to monitor your vit D levels, or we have more comprehensive blood tests, such as our baseline health check, that check a whole range of markers including vitamin D, which gives you a more detailed look at your health.

You can view our full range of blood tests to find the right test for you.

Written by Leanne Edermaniger

Based in the UK, Leanne specialises in writing about health, medicine, nutrition, and fitness.

She has over 5 years of experience in writing about health and lifestyle and has a BSc (hons) Biomedical Science and an MSc Science, Communication and Society.