What is Vitamin D?
Despite being called a vitamin, Vitamin D functions more like a hormone. Your body can produce it on its own — but only when your skin is directly exposed to UVB radiation from sunlight. This makes it unique among vitamins and explains why deficiency is so widespread in modern life.
There are two main forms: D2 (ergocalciferol), found in some plant foods and fungi, and D3 (cholecalciferol), produced by your skin and found in animal foods. D3 is the form you want — it's significantly more effective at raising blood levels of Vitamin D.
An estimated 1 billion people worldwide have Vitamin D deficiency or insufficiency. It affects people in virtually every country — not just those in northern latitudes. Even in sunny countries like Brazil, India and Saudi Arabia, deficiency rates are surprisingly high due to indoor lifestyles, sun avoidance and darker skin tones that reduce UVB absorption.
How Vitamin D works inside your body
Skin or supplement → liver
Whether from sunlight or a capsule, Vitamin D3 travels to your liver where it's converted into 25-hydroxyvitamin D (calcidiol) — the form measured in blood tests when checking your Vitamin D status.
Liver → kidneys → active form
The liver converts it into its active form: 1,25-dihydroxyvitamin D (calcitriol). This is the biologically active hormone that actually does the work in your cells.
Acts as a hormone across hundreds of processes
Active Vitamin D binds to receptors found in nearly every tissue in the human body — bones, muscles, immune cells, brain, heart, pancreas. It regulates over 200 genes, influencing calcium absorption, immune response, cell growth and much more.
Critical for calcium absorption
Without adequate Vitamin D, your gut can only absorb about 10–15% of dietary calcium. With sufficient Vitamin D, that rises to 30–40%. This is why Vitamin D is inseparable from bone health — no D, no calcium uptake.
Why so many people are deficient
The answer is mostly modern lifestyle. Your skin needs direct UVB exposure — not through glass, not through sunscreen, and only during certain hours of the day when the sun is high enough. Most people today:
Work indoors during peak sunlight hours. Wear sunscreen when outdoors. Live at latitudes where UVB doesn't reach the earth's surface for months of the year. Have darker skin tones that require significantly more sun exposure to produce the same Vitamin D as lighter skin.
Diet alone rarely solves the problem — very few foods are naturally rich in Vitamin D. Fatty fish, egg yolks and beef liver contain some, but amounts are too small to maintain adequate levels without either sun exposure or supplementation.
What the research shows
Bone health
Essential for calcium absorption. Deficiency causes rickets in children and increases osteoporosis risk in adults. One of the most established benefits.
Immune function
Vitamin D receptors are present on immune cells. Studies link deficiency to increased respiratory infections and autoimmune conditions.
Muscle function
Deficiency is associated with muscle weakness and increased fall risk in older adults. Adequate levels support muscle protein synthesis and strength.
Mood & cognition
Low Vitamin D is associated with depression and cognitive decline. Evidence is observational but consistent across many populations worldwide.
Cardiovascular health
Observational studies link low Vitamin D to higher rates of heart disease. The causal relationship is still being studied but the association is strong.
Energy levels
Fatigue is one of the most commonly reported symptoms of Vitamin D deficiency. Many people report improved energy after correcting a deficiency.
Much of the research on Vitamin D beyond bone health is observational — meaning we know that people with low Vitamin D have worse health outcomes, but supplementation trials don't always show the same dramatic benefits. The most certain benefits are for bone health and calcium absorption. Other benefits are real but require adequate levels to be maintained over time, not a quick fix.
How much do you need — by goal
D3 vs D2 — which one to take
Always choose D3 (cholecalciferol) over D2 (ergocalciferol). Multiple studies show D3 is approximately twice as effective at raising and maintaining blood levels of Vitamin D compared to D2. D3 is also the form your body naturally produces from sunlight.
Many cheaper products use D2. Check the label — it should clearly say "cholecalciferol" or "Vitamin D3." If it just says "Vitamin D" without specifying the form, it may be D2.
Pair with Vitamin K2: Many experts recommend taking Vitamin D with Vitamin K2 (MK-7 form). D3 increases calcium absorption, and K2 helps direct that calcium into bones rather than soft tissues like arteries. Products combining D3+K2 are increasingly popular and well-supported by the research.
Best Vitamin D3 products — available globally
All products below are available on iHerb, which ships to 180+ countries. We prioritize D3 form, clean ingredients, and value per IU.
Vitamin D3 + K2
2,000 IU D3 paired with 100mcg K2 (MK-7) in organic coconut oil for better absorption. The most complete D3 option — covers both deficiency and cardiovascular protection.
Vitamin D3 2000 IU
Simple, clean, very affordable. 2,000 IU D3 per softgel, in soybean oil. Reliable brand with consistent quality control. Best for those just starting supplementation.
Vitamin D3 5000 IU
5,000 IU per capsule for people correcting a confirmed deficiency or with very low sun exposure. Thorne is one of the most trusted brands for supplement quality and purity.