Vitamin D Guide & Reviews | The Iron Verdict

Vitamin D3 (cholecalciferol) is approximately 87% more potent at raising and maintaining serum 25(OH)D concentrations than D2 (ergocalciferol). D3 is the preferred supplemental form.

PMID 20979590 →

Meta-Analysis · Rejnmark et al., 2012

Vitamin D reduces all-cause mortality by 6% in adults

Meta-analysis of 50 RCTs (n>94,000) found vitamin D supplementation associated with a 6% reduction in all-cause mortality, primarily from D3 supplementation.

PMID 25541960 →

RCT · Jorde et al., 2016

Vitamin D deficiency associated with significantly lower testosterone

Men in the highest tertile of 25(OH)D had significantly higher testosterone levels vs deficient men. A 12-month RCT showed vitamin D supplementation (83mcg/day) increased testosterone by ~25%.

PMID 26864360 →

Review · Holick, 2007

Over 1 billion people are vitamin D insufficient globally

Vitamin D deficiency (<20 ng/mL) affects over 1 billion people globally. Skin synthesis is the primary source but is insufficient at latitudes above 37°N for 4–6 months/year, making supplementation essential.

PMID 23239160 →

Product Recommendations

Vitamin D3 5000 IU

NOW Foods — 240 Softgels

5000 IU D3 in olive oil base for enhanced absorption. NSF certified. Excellent value — ~$0.05/softgel for a potent dose that reliably raises 25(OH)D levels.

4.7/5 Check Price

Vitamin D3 + K2 (MK-7)

Sports Research — 60 Softgels

5000 IU D3 + 100mcg MK-7 K2 per softgel. Coconut oil base. K2 directs calcium to bones rather than arteries — the preferred D3 formulation for long-term use.

4.6/5 Check Price

Vitamin D3 + K2 1000 IU

Thorne — 90 Capsules

1000 IU D3 + 200mcg MK-4 K2. NSF Certified for Sport. Lower dose — best for those who are not deficient and want a maintenance dose. Premium quality, third-party certified.

4.7/5 Check Price

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Frequently Asked Questions

How much vitamin D should I take?

Depends on your baseline 25(OH)D level — ideally measured via blood test. For deficiency (<20 ng/mL), 4000–5000 IU daily for 8–12 weeks typically corrects levels. For maintenance at optimal levels (40–60 ng/mL), 2000–4000 IU/day is commonly used. The UL is 10,000 IU/day, though toxicity is extremely rare below 40,000 IU/day for extended periods.

Do I need vitamin K2 with vitamin D3?

It's a prudent addition for long-term supplementation. Vitamin D increases calcium absorption; K2 (particularly MK-7 form) activates proteins that direct calcium into bones rather than arteries. While toxicity from D3 alone requires very high doses, K2 co-supplementation is supported by mechanistic evidence and is now standard in quality D3 products.

When is the best time to take vitamin D?

With the largest fat-containing meal of the day — vitamin D is fat-soluble, and absorption increases 32–50% when taken with dietary fat. The time of day doesn't matter; morning and evening show equivalent absorption.

How do I know if I'm vitamin D deficient?

The only reliable method is a serum 25-hydroxyvitamin D (25(OH)D) blood test. Optimal levels are typically defined as 40–60 ng/mL (100–150 nmol/L). Deficiency is <20 ng/mL, insufficiency is 20–30 ng/mL. Risk factors: limited sun exposure, dark skin, obesity, living above 37°N latitude, and being over 65.

Can I get enough vitamin D from sunlight?

In theory yes — 10–30 minutes of midday sun on arms and legs in summer produces 10,000–20,000 IU. In practice, this is unreliable: UVB is insufficient at latitudes above 37°N from October–March, glass blocks UVB entirely, sunscreen blocks synthesis, and dark skin requires significantly longer exposure. Supplementation is more reliable and consistent.