Vitamin D3
Fat-soluble vitamin that contributes to normal absorption of calcium and phosphorus, and to the normal function of the immune system.
Also known as Cholecalciferol, Vitamin D, Vit D, D3
Common doses
1000-5000 IU
Best timing
Morning
Food
Take with food
Interactions
6 known
Vitamin D3 (cholecalciferol) is a fat-soluble vitamin that your body produces when skin is exposed to UVB sunlight. It plays a central role in calcium absorption, bone health, immune function, and mood regulation. Despite being one of the most studied nutrients, deficiency is remarkably common, affecting an estimated 1 billion people worldwide. This is partly because modern lifestyles involve less sun exposure, and very few foods contain meaningful amounts of Vitamin D.
Supplementation is widely recommended for people living at northern latitudes, those with darker skin (melanin reduces UVB conversion), and anyone who spends most of their time indoors. Blood levels of 25-hydroxyvitamin D between 30-50 ng/mL are generally considered optimal, though debate continues about the upper end of that range.
Vitamin D3 is the preferred supplement form over D2 (ergocalciferol), as it raises blood levels more effectively and is the form naturally produced by human skin.
Key benefits
Bone health
Vitamin D contributes to normal absorption of calcium and phosphorus, and to the maintenance of normal bones. Without adequate Vitamin D, the body absorbs only 10-15% of dietary calcium.
Immune function
Vitamin D contributes to the normal function of the immune system. It activates innate immune responses and modulates adaptive immunity.
Muscle function
Vitamin D contributes to the maintenance of normal muscle function. Deficiency is associated with muscle weakness and increased fall risk in older adults.
Mood regulation
Low Vitamin D levels are consistently associated with depression in observational studies, though the evidence for supplementation improving mood is mixed.
Available forms
D3 (Cholecalciferol)
The preferred form. Identical to what your skin produces. Raises blood levels more effectively than D2. Available in softgels, drops, and tablets.
D2 (Ergocalciferol)
Plant-derived form (from fungi). Less effective at raising blood levels. Sometimes used in prescription mega-doses.
Calcifediol (25-hydroxyvitamin D)
The pre-activated form. Raises blood levels faster than D3 and may be better absorbed in people with fat malabsorption.
Food sources
- Fatty fish (salmon, mackerel, sardines)
- Cod liver oil
- Egg yolks
- Fortified milk and orange juice
- Mushrooms exposed to UV light
Signs of deficiency
- Fatigue and tiredness
- Bone pain or tenderness
- Muscle weakness
- Frequent infections
- Low mood, especially in winter
- Slow wound healing
Upper intake limit
4,000 IU/day (NIH). Some researchers argue up to 10,000 IU/day is safe, but long-term studies at that dose are limited.
Research summary
Strong evidence for bone health and calcium absorption. Good evidence for immune function and reducing respiratory infections. Mixed evidence for mood, cancer prevention, and cardiovascular health. The VITAL trial (2019, n=25,871) found no significant reduction in cancer or cardiovascular events at 2,000 IU/day, but subgroup analyses showed benefits for certain populations.
Known interactions (6)
High-dose retinol may antagonise Vitamin D
D3 and K2 direct calcium to bone
Magnesium activates Vitamin D
Both are fat-soluble with no meaningful conflict
Vitamin D is essential for calcium absorption
Fish oil provides the fat D3 needs for absorption
Check your full stack
for interactions.
See what competes, what combines well, and when to take everything. Every interaction cites a published source.
Stack
6 supplements
Stack review
1 flagZinc and Iron compete for the same absorption pathway. Take at least 2 hours apart.
Supplements
This information is for educational purposes only. It is not medical advice and does not diagnose, treat, cure, or prevent any disease. Always consult a qualified healthcare professional.