Vitamin D3
Sunshine in a capsule. The vitamin most people in northern latitudes need more of.
The UK gets less sunlight than most people realise, and skin synthesis of Vitamin D drops off sharply from October to March. D3 (cholecalciferol) is the form your body makes in response to UVB light. It is better retained than D2, and it works alongside Vitamin K2 to support bone strength, immune function, and mood regulation. Most adults who supplement are making up for something their geography quietly takes away.
Reference values
Daily reference values from official nutrition authorities. These are population reference figures, not personal recommendations.
| Group | RDA | Upper limit | Source |
|---|---|---|---|
| General adult | 10 mcg | 100 mcg | UK SACN Vitamin D and Health (PHE, 2016); EFSA UL (2018) |
| General adult | 15 mcg | 100 mcg | US NIH ODS Vitamin D DRI (IOM 2011) |
| General adult | 20 mcg | 100 mcg | US NIH ODS Vitamin D DRI (IOM 2011) — 71+ raised to 20 mcg |
Forms and bioavailability
Not all forms are absorbed equally. The form on your supplement label affects how much your body can actually use.
| Form | Absorption | With food | Timing | Evidence |
|---|---|---|---|---|
| cholecalciferol (D3) dry tablet | Lower | With food | Either | Likely |
| cholecalciferol (D3) in oil | Typical | With food | Either | Strong |
How it pairs with other supplements
Works well with
Vitamin D3 and K2 work together for calcium handling: D3 raises calcium absorption, K2 activates osteocalcin and MGP to direct calcium to bone rather than soft tissue.
Magnesium is required as a cofactor for the enzymes that activate vitamin D in the liver and kidney. Low magnesium status can blunt vitamin D effectiveness.
Vitamin D3 and K1 (phylloquinone) work together for calcium handling: D3 increases intestinal calcium absorption, K1 activates gamma-carboxylation of osteocalcin and matrix Gla protein (MGP) to direct calcium to bone rather than soft tissue. Often co-supplemented in bone health stacks.
Omega-3 fatty acids and vitamin D share complementary anti-inflammatory pathways and are commonly co-supplemented for immune and cardiovascular health. Some evidence suggests omega-3 enhances vitamin D receptor signalling in immune cells.
Vitamins A and D share the RXR nuclear receptor heterodimerisation pathway and co-regulate gene expression. At normal supplement doses they are complementary. At high preformed retinol doses (>3000 mcg RAE/day) vitamin A may competitively impair vitamin D receptor signalling -- avoid very high retinol stacking with vitamin D.
If you track D3 in Stack Almanac, the advisor will compare your total daily intake across all supplements against the SACN reference value for your life stage. This is particularly useful when you stack a multivitamin alongside a standalone D3 capsule and want to know your combined dose.
Track your Vitamin D3 in Stack Almanac
Last reviewed: 2026-05-21. Reference values are adult-general RDAs from UK, US. Individual needs may vary. This page is for informational purposes only and is not a substitute for professional medical advice.