Vitamin D3 (+ K2)

Evidence: ★★★★★ · Status: OTC Supplement, Prescription

In plain English

A hormone your skin makes from sunlight that switches on hundreds of genes. Indoor life (very common in Singapore) leaves most people deficient, which quietly lowers testosterone, immunity, and mood.

How it works

Cholecalciferol → 25(OH)D (liver) → 1,25(OH)₂D (kidney), which binds the VDR (vitamin D receptor), a nuclear transcription factor regulating 200+ genes including those for calcium transport, immune peptides, and testosterone synthesis. K2 (MK-7) activates matrix-Gla protein to route calcium into bone, not arteries.

Molecular target & official sources

VDR vitamin D receptor (NCBI Gene) · Cholecalciferol (PubChem CID 5280795)

Protocol

2,000–4,000 IU D3 + 100–200 mcg K2 with fat; test 25-OH-D, target 75–125 nmol/L.

Bottom line

Highest-value fix if you work indoors. Almost everyone benefits.

Helps with: Build Muscle & Strength · Live Longer · Hormones & Testosterone · Stress & Anxiety · Immunity · Skin & Hair · Bone Health

The human evidence

VITAL (2019, ~25,000 adults) found vitamin D did not reduce cancer or cardiovascular events in a largely already-sufficient population. The robust evidence is for correcting deficiency (common in low-sun / darker-skinned groups), not for supplementing people who are already replete.

Stacks with

Magnesium (a cofactor that activates vitamin D) and K2 (directs the calcium to bone, not arteries).

Shares a pathway — often paired with: Omega-3 (EPA/DHA), Testosterone (TRT), Finasteride / Dutasteride, Tretinoin / Retinoids.

Avoid combining with

Mega-doses without K2 and magnesium; get a blood level rather than guessing.

Availability & where to buy

Prescription only. Prescription-only — a doctor must prescribe it. Not sold over the counter. (In Singapore: HSA-regulated.) Cheap — ~S$10–20/month. Everywhere in SG.

How it works: the Nuclear receptors pathway →

Used in these protocols

• Muscle Loss (Sarcopenia)
• Low Mood / Depression
• Low Testosterone
• Menopause Symptoms
• Bone Density / Osteoporosis
• Immunity

Compare Vitamin D3 (+ K2)

• Vitamin D3 (+ K2) vs Creatine Monohydrate
• Vitamin D3 (+ K2) vs Caffeine
• Vitamin D3 (+ K2) vs Whey / Casein Protein
• Vitamin D3 (+ K2) vs Semaglutide (Ozempic / Wegovy / Rybelsus)
• Vitamin D3 (+ K2) vs Testosterone (TRT)
• Vitamin D3 (+ K2) vs Metformin
• Vitamin D3 (+ K2) vs Beta-Alanine
• Vitamin D3 (+ K2) vs Omega-3 (EPA/DHA)