Vitamin D: The Sunshine Vitamin with Big Health Benefits
Vitamin D, often called the “sunshine vitamin”, is a fat-soluble nutrient that plays a crucial role in our overall health. Unlike most vitamins, your body can actually make vitamin D when your skin is exposed to sunlight, specifically ultraviolet B (UVB) rays. Despite this natural advantage, vitamin D deficiency is extremely common. It affects people in regions with long winters, those who spend most of their time indoors, or anyone who has little direct skin exposure to sunlight. Worldwide, it’s estimated that around 1 billion people have either vitamin D deficiency or insufficiency.
What Exactly Is Vitamin D?
Vitamin D exists in two main forms: D₂ and D₃, with both available in supplements and fortified foods. Once in the body, vitamin D goes through two steps to become “active”:
First, it’s converted in the liver to 25-hydroxyvitamin D (the form usually measured in blood tests).
Then, it’s converted in the kidneys to its active form, 1,25-dihydroxyvitamin D (which I’ll be referring to as vitamin D during the rest of this post), which is tightly regulated by calcium, phosphorus, and hormone levels.
Experts generally agree that:
Deficiency = below 20 nanograms per millilitre (ng/mL)
Sufficient = 30 ng/mL or higher
Toxicity = over 150 ng/mL (which is rare, and usually from extreme supplementation)
Who’s at Risk of Deficiency?
You might think vitamin D deficiency only affects people who avoid the sun or never drink milk. But studies show it’s actually widespread, even among those who eat well and take supplements. Potential deficient groups include:
Older adults: Between 40% and 100% of seniors in the U.S. and Europe don’t get enough vitamin D.
Postmenopausal women: More than half of women on osteoporosis treatment still had low vitamin D levels.
Children & teens: In one study, over half of Hispanic and Black teens in Boston and almost half of young white girls in Maine had low levels.
Women of colour: In the U.S., 42% of Black women and girls aged 15–49 were deficient, especially in winter.
Healthcare professionals: Even one-third of doctors, residents, and medical students in Boston were deficient, despite eating fish, taking vitamins, and drinking milk regularly.
And this problem isn’t limited to North America. In Europe, where fewer foods are fortified, deficiency is even more common. Surprisingly, even in sunny countries like Saudi Arabia, Turkey, India, and Australia, 30–50% of children and adults were deficient, often because most skin was covered when outdoors.
Pregnant and breastfeeding women are also vulnerable. One study found that 73% of mothers and 80% of newborns were vitamin D deficient at birth, even though most women took prenatal vitamins and ate a diet including fish and milk.
Why Vitamin D Matters?
1. Stronger Bones and Fracture Prevention
Vitamin D is essential for absorbing calcium and phosphorus from food. With enough vitamin D, your body absorbs 30–40% of dietary calcium and about 80% of phosphorus. Without it, absorption drops by half or more. A large study found that people of all races had the best bone density when vitamin D levels were 40 ng/mL or higher. When levels dropped below 30 ng/mL, calcium absorption decreased, and the body compensated by releasing parathyroid hormone (PTH).
Here’s why that matters:
PTH tells the kidneys to make more active vitamin D.
PTH also triggers bone breakdown, releasing calcium into the blood.
Over time, this weakens the skeleton, leading to osteopenia, osteoporosis, and higher fracture risk.
PTH also lowers phosphorus levels, and without enough calcium and phosphorus, bones can’t properly harden. This results in Rickets in children (soft, deformed bones) and osteomalacia in adults (weak, aching bones). To put it in perspective: 1 in 3 women over 60 already has osteoporosis. By age 50, about 47% of women and 22% of men will experience a fracture linked to weak bones.
2. Muscle Strength and Fall Prevention
Vitamin D is also vital for muscles. Muscle cells have vitamin D receptors, and when activated, these help muscles contract, coordinate, and stay strong. Research shows:
People with very low vitamin D (around 4 ng/mL) had poor muscle performance, but muscle strength improved as levels rose above 16 ng/mL, with the best results above 40 ng/mL.
A review of five clinical trials (with 1,237 people- which is a lot) found that having enough vitamin D lowered the risk of falls by 22%.
400 IU/day wasn’t enough to prevent falls. But 800 IU/day plus calcium significantly reduced the risk of falling.
In one nursing home study, residents taking 800 IU of vitamin D₂ + calcium daily for 5 months had a 72% lower risk of falls compared to those on placebo.
3. Roles Beyond Bones and Muscles
Scientists are also uncovering links between vitamin D and many other health conditions:
Cancer prevention: Higher vitamin D levels are associated with lower risk of colon, breast, prostate, and pancreatic cancers. Vitamin D helps regulate genes that control cell growth and prevents tumors from growing new blood vessels.
Autoimmune diseases: Adequate vitamin D may reduce risk of multiple sclerosis, rheumatoid arthritis, and Crohn’s disease. Children supplemented with vitamin D early in life also have a lower risk of type 1 diabetes.
Diabetes: Vitamin D improves insulin production and sensitivity, helping prevent type 2 diabetes. Pregnant women with healthy levels also have reduced risk of type 1 diabetes in their children.
Heart health: Deficiency has been linked to high blood pressure, heart disease, and heart failure, and supplementation may help lower blood pressure and reduce inflammation.
Why Deficiency Happens
There are many reasons why vitamin D deficiency is so common:
Less sunlight: Sunscreen, darker skin, aging, and long winters all reduce vitamin D production in skin.
Limited diet: Very few foods naturally contain vitamin D.
Absorption problems: Conditions like Crohn’s, celiac disease, obesity, or gastric surgery can limit absorption.
Medications: Some drugs (like anticonvulsants and steroids) speed up vitamin D breakdown.
Breastfeeding: Breast milk is naturally low in vitamin D, which puts infants at risk unless supplemented.
Chronic disease: Kidney or liver disease can block conversion to active vitamin D
How to Maintain Healthy Vitamin D Levels — Safely
Getting vitamin D from sunlight is efficient, but it's all about the right amount. As little as 5–15 minutes of casual sun exposure to your face, arms, and hands 2–3 times per week during summer can be enough to keep vitamin D levels healthy. However, depending on your skin tone and where you live, light-skinned people may typically need just 10–15 minutes of sun per day, while darker-skinned individuals may need 25–40 minutes to produce the same amount of vitamin D.
In sunnier regions, even less may be needed: For example, someone with light skin might only need 3 minutes in Miami during summer, compared to 23 minutes in Boston in winter, remembering that in cold places winter clothing limits how much skin you expose.
It is important to be aware that sun exposure has benefits like boosting vitamin D, improving mood, and regulating sleep, but too much UV can damage your skin and increase skin cancer risk. Dermatologists agree there’s no completely safe level of UV exposure that lets your skin make vitamin D without increasing cancer risk. So, it may be safer to get plenty of vitamin D from your diet and supplements to protect your skin from the sun.
In fact, Vitamin D is one of only two supplements recommended by the NHS for the general popluation. According to NHS and Public Health England, everyone aged 1 year and older (including pregnant and breastfeeding women) should consider taking a daily supplement containing 10 micrograms (µg) of vitamin D (equivalent to 400 IU) during autumn and winter. Additionally, people who don’t get much sun exposure (e.g., housebound or living in care homes), usually cover most of their skin outdoors for cultural or other reasons or have dark skin, and children aged 1-4 years should take 10 µg/day supplement all year round. Babies under 1 year old: Should have 8.5 to 10 µg/day if breastfed (or if formula-fed and consuming less than 500 ml/day of formula).
When safe sun exposure isn’t enough you can also use diet and fortified foods, to obtain adequate levels of vitamin D. Vitamin D-rich foods include fatty fish (salmon, trout, sardines), egg yolks, and liver. However, for vegans and vegetarians, vitamin D can be harder to come by since it’s mainly found in animal products with limited plant-based options available. These include:
Fortified plant-based milks: Many brands of soy, oat, almond, and coconut milk are fortified with vitamin D.
Fortified breakfast cereals: Some cereals (check the label) contain added vitamin D.
Fortified spreads: Certain dairy-free margarines and spreads are enriched with vitamin D.
Mushrooms: Especially those exposed to UV light (look for labels like “UV-exposed mushrooms”), which naturally increase their vitamin D₂ content.
When it comes to supplements, vitamin D₂ is always vegan, though it’s slightly less effective than D₃ at raising blood levels. Vegan vitamin D₃ (derived from lichen) is now widely available and is considered just as effective as the traditional lanolin-based D₃.
For those following a vegan or vegetarian diet, fortified foods plus a daily supplement (10 µg / 400 IU) (especially in autumn and winter), is often the most reliable way to maintain healthy vitamin D levels.
Is there Vitamin D toxicity?
While vitamin D is essential for good health, too much can be harmful. Unlike water-soluble vitamins, vitamin D is stored in fat and can build up in the body, which means very high intakes over time can cause toxicity. This may lead to high calcium levels (hypercalcaemia), which can damage the kidneys, bones, and heart. According to the NHS, the safe upper daily limit is: 100 micrograms (µg) / 4,000 IU for adults and children over 11, and 50 µg / 2,000 IU for children aged 1–10, and 25 µg / 1,000 IU for infants under 12 months. Toxicity is rare and usually occurs only from excessive supplement use (not sunlight or food). That’s why it’s important to follow recommended doses and avoid “megadoses” unless specifically prescribed by a healthcare professional.
Final thoughts
Vitamin D is essential for bones, muscles, immunity, and long-term health. Deficiency is common worldwide, even in sunny climates, and increases the risk of fractures, muscle weakness, chronic illness, and falls. However, there is good news: Sensible sun exposure, fortified foods, and supplements can help you maintain healthy vitamin D levels. If you’re unsure about your vitamin D status, ask your healthcare provider about a simple blood test. A little “sunshine vitamin” goes a long way in protecting your health.
References
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Bouillon, R., Marcocci, C., Carmeliet, G., et al. (2019). Skeletal and extra-skeletal actions of vitamin D: Current evidence and outstanding questions. Endocrine Reviews, 40(4), 1109–1151.
Holick, M. F. (2007). Vitamin D deficiency. New England Journal of Medicine, 357(3), 266–281.
Holick, M. F., Binkley, N. C., Bischoff-Ferrari, H. A., et al. (2011). Evaluation, treatment, and prevention of vitamin D deficiency: An Endocrine Society clinical practice guideline. Journal of Clinical Endocrinology & Metabolism, 96(7), 1911–1930.
Institute of Medicine (US). (2011). Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: National Academies Press.
Martineau, A. R., Jolliffe, D. A., Hooper, R. L., et al. (2017). Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ, 356, i6583.
Tripkovic, L., Lambert, H., Hart, K., et al. (2012). Comparison of vitamin D₂ and vitamin D₃ supplementation in raising serum 25-hydroxyvitamin D status: A systematic review and meta-analysis. American Journal of Clinical Nutrition, 95(6), 1357–1364.
