Vitamin D

Yes, materially. Tripkovic 2012's meta-analysis pooled the head-to-head trials and found D3 raises serum 25(OH)D roughly 1.7× more efficiently than D2 at equivalent oral doses. The mechanism: D3 binds vitamin D binding protein (DBP) more tightly than D2, so circulating half-life is longer and conversion to the active 1,25-D form is more efficient. D2 is still widely prescribed in the US because it's the prescription-strength form (50,000 IU capsules) — clinically inferior to D3 at the same numerical IU, but adequate at the much higher dose. For self-directed supplementation, default to D3.

If you're dosing above ~2,000 IU/day chronically, yes. D3 increases intestinal calcium absorption; K2 (MK-7 form preferred — longer half-life than MK-4) activates the matrix-Gla-protein that directs that calcium to bone rather than letting it deposit in vascular tissue. The Rotterdam Study and the Geleijnse 2004 K2 trial both linked higher K2 intake to lower coronary-calcification scores. At 1,000 IU/day or less of D3, the cofactor is less critical — the calcium-mobilisation effect is modest. At 5,000+ IU/day, running the D-only protocol for years is a documented mechanism for arterial calcification in some users. The K2 add is cheap (~$5/month) and a strict upside-side bet.

Depends on baseline. The Heaney 2003 dose-response data give the operational rule: roughly +1 ng/mL of serum 25(OH)D per 100 IU/day of chronic dosing, with diminishing returns past 40 ng/mL. So someone starting at 15 ng/mL aiming for 40 ng/mL needs ~2,500 IU/day for 8-12 weeks to fill the gap, then can drop to 1,000-2,000 IU/day maintenance. Severe deficiency (under 12 ng/mL) often warrants short-term loading at 5,000-10,000 IU/day for 4-8 weeks under bloodwork monitoring, then de-escalation to maintenance. Don't guess — retest 25(OH)D 8 weeks after starting a new dose and adjust from there.

Theoretically yes — that's how the system evolved. Practically, no, for most modern adults. The dose-response rule is roughly: 10-15 minutes of midday sun on uncovered face + arms + legs at latitudes below ~35° produces ~3,000-10,000 IU of D3, depending on skin pigmentation, age, and season. Above ~35° latitude (most of the US and Europe), the UVB angle from October through March is too oblique to produce meaningful D, regardless of how long you stand outside. People with darker skin pigmentation need 3-6× the sun exposure to make equivalent D. Indoor workers in northern latitudes are functionally guaranteed to be deficient by February. Supplement.

40-60 ng/mL (100-150 nmol/L) serum 25(OH)D — the band where the bone, immune, T, and mood endpoint signals are strongest and the hypercalcemia tail risk is still near-zero. The IOM threshold of 20 ng/mL is too low — it's the rickets-prevention floor, not the optimal-function target. The Endocrine Society's clinical guideline puts the floor at 30 ng/mL. The Pludowski 2018 expert consensus (D-cardiac authorities, EU rheumatology + endocrinology bodies) settled on 40-60 ng/mL as the optimal-function range. Above 80 ng/mL, additional supplementation produces no demonstrated additional benefit and starts raising hypercalcemia signal.

In deficient men, yes — modestly. The Pilz 2011 trial (PMID 21482228) gave 3,332 IU/day to D-deficient men for 12 months and measured ~25% rises in total testosterone, bioactive T, and free T vs placebo (which moved nothing). In men who are not D-deficient at baseline, the trial signal vanishes — there's nothing to correct. The 50/50 expectation: if your 25(OH)D is below 30 ng/mL and your testosterone is low, repleting D over 12 months is one of the highest-leverage natural moves available. If your D is already at 40+ ng/mL, adding more D will not move T further — switch tools to Tongkat Ali, Ashwagandha, or address sleep + body composition.

Bloodwork moves fastest — serum 25(OH)D climbs measurably by week 4-6, near-final by week 8-12 on a stable dose. Subjective effects depend on which deficiency-driven symptom is dominant. Winter mood + recurrent infections shift in 8-12 weeks. Muscle weakness + bone discomfort in 12+ weeks. Testosterone uplift in deficient men requires 6-12 months. If you've been on 2,000 IU/day for 8 weeks and your 25(OH)D didn't move, the issue is usually magnesium deficiency (Mg is the cofactor for D activation) — add 300 mg elemental magnesium glycinate at night and retest at month 4.

Yes, and the threshold is well-characterised. Chronic dosing above 10,000 IU/day for months without bloodwork can drive serum 25(OH)D above 100 ng/mL, where hypercalcemia risk rises substantially. The toxicity case-reports cluster at sustained intake of 40,000-100,000 IU/day for months. Acute single-dose toxicity is extremely rare below 300,000 IU. The honest framing: 1,000-4,000 IU/day is universally safe; 5,000-10,000 IU/day is safe with periodic bloodwork (every 6 months); past 10,000 IU/day without bloodwork is the protocol that produces the published toxicity cases. The asymmetry favours staying at clinical doses and testing serum.