Vitamin D: the test, the target, the dose

Why it's low even in sunny countries

Vitamin D is made in skin exposed to strong midday sun — which is precisely the exposure modern life removes. Indoor work, sunscreen (correctly used for skin-cancer prevention), darker skin at higher latitudes, and covering clothing all cut production dramatically. Israel's health funds report widespread insufficiency despite the climate: sunshine outside the window doesn't reach your bloodstream.

The test: 25(OH)D

The storage form, 25-hydroxyvitamin D, is what labs measure and what targets refer to. Watch the units — ng/mL and nmol/L differ by a factor of 2.5 and both are common on Israeli reports:

• < 20 ng/mL (< 50 nmol/L) — deficient by essentially every guideline.
• 20–30 ng/mL (50–75 nmol/L) — "insufficient"; many people report feeling better above this band.
• 40–70 ng/mL (100–175 nmol/L) — the optimal band Bariva targets, consistent with much of the functional-range literature.
• > 100 ng/mL (> 250 nmol/L) — approaching toxicity risk (hypercalcemia); back off.

How to supplement well

• D3 (cholecalciferol), not D2. D3 raises and holds blood levels more effectively.
• Dose to your level. Common practice: 1,000–2,000 IU/day for maintenance; 2,000–4,000 IU/day to correct insufficiency. Genuinely deficient values are often treated with higher short-term protocols — that's a decision to make with your doctor.
• Take it with fat. It's fat-soluble; absorption is meaningfully better with your largest meal.
• Consider K2 (MK-7) alongside — it directs calcium toward bone rather than soft tissue. The evidence is suggestive rather than definitive, which is why many D3 products now include it.
• Retest after ~12 weeks. Levels move slowly; retesting sooner mostly measures noise.

Cautions

Vitamin D can interact with conditions involving calcium handling (sarcoidosis, hyperparathyroidism, kidney stones) and belongs in a doctor conversation if you have any of them. Don't stack multiple D-containing products unknowingly — multivitamins, calcium+D combos, and fortified foods add up.