D3 (cholecalciferol) is metabolized better by the body and raises the 25(OH)D level more reliably than D2 (ergocalciferol). At the same dose, D3 is about 1.5-2× more effective for blood-test level. D2 is primarily used in pharmaceutical substitution for malabsorption — for standard supplementation, prefer D3.

How much without a blood test?

2000-4000 IU/day year-round is safe for adults and brings ~80 % into the sufficient range (25(OH)D 30-60 ng/ml). Anyone chronically on > 4000 IU: annual blood test for 25(OH)D + calcium. Above 100 ng/ml, hypercalcemia risk rises.

Is summer sun enough?

In Central European latitudes from October to April, virtually no vitamin D is produced via UVB (sun angle too low). Even in summer you need 15-30 min direct sun on arms and legs without sunscreen, multiple times/week — reality: most don't reach that. Supplementation is the more reliable source.

What about K2 with D3?

K2 (MK-7) is often combined with D3. Mechanism: K2 activates osteocalcin and matrix Gla-protein, directing calcium into bones rather than arteries. Human evidence is mixed — solid for bone density in postmenopausal women, less clear for arterial calcification in healthy adults. Doesn't hurt at moderate dose (90-180 µg MK-7/day), not mandatory.

If someone has no vitamin D deficiency — what then?

Then the next-recommended choice is goal-oriented: strength training → creatine 3-5 g, elevated triglycerides → omega-3 EPA/DHA 2-4 g, sleep problems → magnesium bisglycinate 300-500 mg in the evening, vegetarian → B12 (methylcobalamin 500-1000 µg/day). 'Best' supplement doesn't exist without context.

What does a multivitamin do?

Little — the Physicians' Health Study II (PHS-II, n=14,641, 11 years, PMID 23117275) found no significant effect on cardiovascular endpoints or mortality. As insurance against multiple subclinical deficiencies it may not hurt, but targeted single supplements are almost always superior.

Best supplement for self-optimization: vitamin D

Direct answer

If you could only choose ONE supplement: vitamin D3 2000-4000 IU/day. Rationale: 40-80 % of Central Europeans have suboptimal 25(OH)D levels depending on season, supplementation is cheap (~$10/year), safe, and corrects a documented deficit. But "best" is context-dependent — if you have no deficiency, creatine (strength + cognition), omega-3 (for elevated triglycerides), or magnesium bisglycinate (sleep) yield more. Multivitamins almost never beat single-target supplements.

Why vitamin D3 is the default recommendation

Deficiency prevalence is high

Studies from Central Europe show 40-80 % of adults with suboptimal 25(OH)D levels (< 30 ng/ml) — seasonally dependent, even higher in darker skin types and older adults. Deficiency consequences: reduced bone mineral density, increased fall/fracture risk in the elderly, possible immune modulation.

Substitution is safe and cheap

2000-4000 IU/day year-round for adults is considered safe. A 60-day pack of high-dose drops costs $5-15, lasts 6-12 months. A one-time 25(OH)D blood test (~$30) gives certainty about your personal level.

Efficacy in deficiency is clear

With documented deficiency, supplementation improves bone mineral density, reduces fall and fracture risk in older adults, and normalizes calcium homeostasis. In non-deficient adults, the VITAL trial (n=25,871, 5.3 years, Manson 2019, PMID 30415629) found no additional reduction in cancer or cardiovascular events — vitamin D is therefore no longevity wonder for healthy adults, but a reliable deficit corrector.

When vitamin D is already sufficient — what next?

The "best supplement" question has no universal answer. It depends on the goal:

Strength / sport / cognition under sleep deprivation → creatine monohydrate 3-5 g/day (Kreider 2017, PMID 28615987)
Elevated triglycerides > 150 mg/dl → omega-3 EPA/DHA 2-4 g/day (Skulas-Ray 2019, PMID 31476893)
Sleep issues with documented Mg deficit → magnesium bisglycinate 300-500 mg evenings (Abbasi 2012, PMID 23853635)
Vegetarian / vegan → B12 methylcobalamin 500-1000 µg/day (B12 is almost exclusively animal-sourced)
Women of childbearing age planning pregnancy → folate 400 µg/day (neural tube defect prevention, evidence-based classic)

What you should NOT let be sold to you as "best supplement"

Multivitamins — no mortality or disease effect in healthy adults (PHS-II, PMID 23117275). May not hurt, but no performance or longevity lever. If you want to know exactly what you need: blood test + targeted single supplements.

NMN, NR (NAD+ precursors) — interesting mechanism, encouraging animal data, but: no human studies on hard longevity endpoints. Currently an expensive experiment, not an evidence-based "best" supplement.

Greens powders — convenience, no RCT data on relevant endpoints. Fresh plant food is cheaper and more bioavailable.

Adaptogens (ashwagandha, rhodiola, eleuthero) — moderate evidence for stress/sleep in small short-term studies. Not "best" supplement, but defensible as a cycle for specific indication.

Methodology — how we define "best"

Three filters: a) How common is the deficit that the supplement corrects? b) How large is the effect size on supplementation (clinical vs. statistical)? c) How safe and cheap is its use?

Vitamin D wins on these three filters for the general German/Central European population. In a sunny country like Spain or Italy, the default recommendation would differ.

Which supplement best supports self-optimization?