Vitamin D – The Sunshine Vitamin

What is Vitamin D?

Vitamin D (calciferol) is a fat-soluble nutrient that plays a vital role in keeping us healthy – supporting strong bones, healthy muscles, and a well-functioning immune system. It is often referred to as the “sunshine vitamin” because our bodies can make it when our skin is exposed to sunlight (specifically UVB rays). We can also get it from certain foods, fortified products (in some countries), and dietary supplements.

There are two main forms:

  • Vitamin D2 (ergocalciferol) – the plant-based form
  • Vitamin D3 (cholecalciferol) – produced by animals, including humans, and found in some animal-based foods.

Both forms can help prevent deficiency, but vitamin D3 is generally more effective at raising and maintaining vitamin D levels.

More than Just a Vitamin

Like all vitamins, vitamin D is essential for life – but it’s also unique. Once produced or consumed, it’s converted in the body into calcitriol, an active hormone that influences over 1,000 genes related to hormone balance, immune function, and other processes. In other words, vitamin D is both a nutrient and a hormone precursor, making it one of the most multifunctional players in our health toolkit.

Why Vitamin D Matters

  1. Bone Health
    Vitamin D helps your body absorb calcium and phosphorus – minerals that are vital for building and maintaining strong bones and teeth. Without enough vitamin D, bones can become thin, brittle, or misshapen. In older adults, low vitamin D increases the risk of osteoporosis and fractures.
  2. Muscle Function
    Vitamin D supports skeletal muscle performance by helping with protein synthesis, calcium and phosphate transport, and energy production within muscle cells. Deficiency can lead to muscle weakness, cramps, and poor balance – and in older adults, a greater risk of falls.
  3. Immune Support
    Vitamin D helps regulate the immune system, influencing both its “first response” (innate immunity) and its more targeted defense (adaptive immunity). Low vitamin D levels have been linked to increased susceptibility to infections and a higher risk of autoimmune diseases such as multiple sclerosis, type 1 diabetes, and lupus.
  4. Other Potential Benefits
    Emerging research suggests vitamin D may also play a role in protecting against certain cancers, cardiovascular disease, respiratory infections, diabetes, and possibly even dementia. Vitamin D receptors are found in the brain, hinting at a role in cognitive health. Studies are exploring possible links between vitamin D status and mood disorders like depression – though more evidence is needed to substantiate this.

Where Do You Get Vitamin D? (Safely)

To keep your vitamin D levels in the healthy range, you can rely on three main sources:

  1. Sunlight exposure
  2. Dietary sources
  3. Supplements

For most people, sunlight is the most reliable source of vitamin D, but food and supplements can make a big difference – especially if you live in a region with long winters, spend most of your time indoors, or your body simply doesn’t make vitamin D as efficiently as it used to.

1. Sunlight Exposure: The Body’s Own Vitamin Factory

When UVB rays from the sun hit your skin, they set off a chain reaction starting with cholesterol in the skin. This process produces vitamin D3 (cholecalciferol), which is then stored in your body’s fat cells in an inactive form – ready and waiting until it is needed. However, your body can’t actually use this vitamin D straight away. It has to be activated first…

2. Dietary Sources

Vitamin D is available in two main dietary forms: D2 (ergocalciferol) and D3 (cholecalciferol). Both forms are absorbed in the small intestine, and while having some fat in your meal helps with absorption, your body can take in some vitamin D even without dietary fat. Interestingly, ageing does not significantly affect how well your gut absorbs vitamin D.

  • Vitamin D2 is found naturally in certain mushrooms, especially those exposed to UV light. In some countries, it’s also added to fortified foods like milk, plant-based milks, and breakfast cereals.
  • Vitamin D3 is exclusively found in animal-based foods, particularly fatty fish (salmon, tuna, mackerel, and sardines), as well as egg yolks, beef and lamb organs, like liver and kidneys.

Since most people don’t eat large amounts of these foods regularly, diet alone usually isn’t enough to meet daily vitamin D needs. That’s why sunlight remains the main source, with food and supplements helping to fill in the gaps.

3. Supplementation

For those who can’t get enough from sunlight or food, vitamin D supplements can be helpful. They are available in both capsule and liquid form and are especially useful for:

  • People with fat absorption issues, lactose intolerance, or milk allergies
  • Individuals with darker skin tones living in high-latitude parts of the world
  • Anyone with health conditions or lifestyles that limit outdoor time

A few key points to keep in mind:

  • Vitamin D3 is the preferred form for supplementation – it tends to be more effective than D2.
  • Consult with a healthcare professional if you’re concerned about vitamin D deficiency, especially if you have risk factors. A simple blood test can confirm your status.
  • Don’t overdo it – while deficiency is common, too much vitamin D can be harmful. Stick with recommended dosages unless advised otherwise by your doctor.

How Does the Body Activate Vitamin D?

Vitamin D from the sun, food, or supplements is biologically “inactive” when it first enters your system. To “switch it on”, your body puts it through a two-step process called hydroxylation:

  1. In the liver – vitamin D is converted into calcifediol (25-hydroxyvitamin D), which circulates in the blood. This is the form usually measured in blood tests to check your vitamin D status.
  2. In the kidneys – calcifediol is further converted into calcitriol (1,25-dihydroxyvitamin D), the active hormone form. This is the version that helps regulate calcium, phosphorus, bone health, and many other processes in the body.

A quick note: it doesn’t matter too much whether your vitamin D comes from sunlight, diet, or supplements – whether it’s D2 or D3 – your body knows how to process and use both, although vitamin D3 is generally more effective at raising and maintaining calcifediol levels in the blood.

What is considered a Healthy Level of Vitamin D?

If you’re having blood work done at your annual checkup, it’s worth asking your doctor to include a vitamin D test. The standard measure is 25-hydroxyvitamin D (calcifediol), which shows how much vitamin D is circulating in your blood.

On your lab report, here’s what the numbers usually mean:

  • Below 30 nmol/L – Deficient. Talk to your doctor about supplements.
  • 30 to 50 nmol/L – Generally inadequate for bone and overall health.
  • 50 nmol/L and above – Adequate (but remember: more isn’t always better)
  • 125 nmol/L and above – Too high, which may cause adverse effects.

In the United States, results are often given in nanograms per millilitre (ng/mL). To convert ng/mL to nmol/L, multiply by 2.5. For example, 20 ng/mL = 50 nmol/L.

Vitamin D Deficiency

There are many factors that can contribute to vitamin D deficiency, including:

1. Inadequate Sun Exposure

Not getting enough sun exposure is the most common cause – whether it’s because you spend most of your time indoors, live at higher latitudes, cover up, or use too much sunscreen too often.

2. Diet

Since only a few foods naturally contain vitamin D, diets lacking in fatty fish, eggs, or fortified foods can make deficiency more likely. Diets that exclude animal products, cannot supply adequate amounts of vitamin D, making supplementation or sun exposure crucial. 

3. Medical Conditions

There are many medical conditions that can impact vitamin D levels, either by affecting absorption, metabolism, or increasing the risk of deficiency.

● Conditions Affecting Absorption

   Some health issues reduce your body’s ability to absorb vitamin D from food or supplements:

  • Digestive conditions like Crohn’s disease, celiac disease, or other inflammatory bowel diseases.
  • Bariatric surgery – reduces the surface area available for nutrient absorption.
  • Pancreatic insufficiency – limits the release of enzymes needed to absorb fat-soluble vitamins like vitamin D.
  • Gallbladder problems – Bile essential for the absorption of fat-soluble vitamins like vitamin D. Poor bile release or gallbladder stasis, a condition where the gallbladder doesn’t empty properly may worsen deficiency and also raise the risk of gallstones.

● Conditions Affecting Metabolism

  Even if you do absorb enough vitamin D, it still needs to be “activated” by your liver and kidneys.

  • Liver disease (e.g., cirrhosis) can impair the first step of activation.
  • Kidney disease, especially advanced stages, can block the second step.

4. Obesity

Excess body fat can sequester (“trap”) vitamin D, making less of it available for your body to use. People with obesity may need higher doses of vitamin D supplements to achieve adequate levels.

5. Other Factors

  • Medications – Some drugs interfere with vitamin D absorption or metabolism. If you suspect your medications are interfering with your vitamin D levels, consult with your doctor or healthcare provider. They can assess your specific situation, identify potential interactions, and recommend appropriate adjustments to your medication regimen or vitamin D intake. Never stop or change your medication dosage without professional guidance.
  • Age – Older adults make less vitamin D from sun exposure, and may also have reduced kidney function which can impair vitamin D activation.
  • Skin tone – Darker skin pigmentation reduces the skin’s ability to make vitamin D from sunlight, raising deficiency risk.

Symptoms of Vitamin D Deficiency

Vitamin D deficiency is sometimes called a “silent problem” because its early signs can be vague, subtle, or mistaken for other health issues. Some people feel fine despite low levels, while others may notice:

  • Chronic fatigue or low energy
  • Muscle weakness, cramps, or aches
  • Bone pain or tenderness
  • Frequent colds or infections (weakened immunity)
  • Mood changes such as sadness, low mood, or depression

Because these symptoms are nonspecific, a simple blood test is the most reliable way to know if you’re deficient.

When Deficiency Becomes Severe

Long-term or severe vitamin D deficiency can lead to more serious consequences, especially for bone and muscle health:

  • Bone Disorders
    • Rickets (children) – Soft, weak bones leading to bowed legs, delayed growth, and bone pain.
    • Osteomalacia (adults) – Soft bones, bone pain, muscle weakness, and a higher risk of fractures.
    • Osteoporosis (adults) – Brittle bones, common in older adults, greatly increasing fracture risk.
  • Muscle Issues – Weakness, cramps, or spasms, which may worsen balance and coordination.
  • Increased Risk of Falls – In older adults, a combination of weaker bones and weaker muscles significantly raises the risk of falls and fractures.

Other Potential Health Concerns

Research suggests that vitamin D deficiency may also be linked to:

  • Frequent infections – due to impaired immune response.
  • Slow wound healing – Vitamin D plays a role in several processes crucial for tissue repair, including cell growth, inflammation regulation, and immune response.
  • Diabetes – possible involvement in both type 1 and type 2 diabetes.
  • Autoimmune conditions – such as multiple sclerosis, rheumatoid arthritis, and inflammatory bowel disease.
  • Cardiovascular disease – increased risk of heart disease and stroke.
  • Certain cancers – including breast and colorectal cancers (research is ongoing).
  • Neurological and cognitive issues – Low levels of vitamin D have been associated with dementia and Alzheimer’s disease. Additionally, deficiency has been linked to depression, anxiety, and even psychotic spectrum diseases such as epilepsy, autism spectrum disorder, schizophrenia and Parkinson’s disease, but more research is needed to clarify these links.
  • Hair loss – in particular, alopecia areata has been linked to low vitamin D levels.

Diagnosis and Treatment

The only reliable way to confirm deficiency is through a blood test for vitamin D levels.

  • For most people, sunlight exposure is the most effective way to increase vitamin D levels – it is the way nature intended and it’s free.
  • Supplements are often prescribed if levels are low, either in pill/liquid form or (in severe cases) as injections.
  • The dose and duration depend on your individual needs and how low your levels are, so it’s important to work with a healthcare provider rather than self-prescribing high doses.

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