Podcast episode 61 (in German)
Vitamin D & K2: the sunshine duo fact-checked
August 31, 2026
Vitamin D is the lab value that a large part of the population fails in winter — and at the same time the dietary supplement with the biggest gap between hope and study data. This episode brings the two together and adds the forgotten partner K2.
What it is about
Strictly speaking, vitamin D is not a vitamin but the precursor of a steroid hormone that helps regulate more than 1,000 genes and whose receptors sit in immune cells, muscles, brain and vessels. North of the Alps, from October to March the sun is too low for the body’s own production. The episode explains this starting point, then goes through the sobering large intervention study along with its remarkable secondary findings and resolves the apparent contradiction with bathtub logic. Then comes the calcium paradox and with it K2 as the traffic cop that decides where the calcium ends up. Finally, it covers risk groups, food sources, the sun itself and the role of magnesium in the background.
Key points
- Vitamin D is the precursor of a steroid hormone that helps regulate more than 1,000 genes; production in the skin roughly halves between the ages of 20 and 70.
- The VITAL study with more than 25,000 participants gave 2,000 units daily for just over 5 years and found no protection against the primary endpoints of heart attack and new cancer.
- The secondary analyses of the same study contained two signals: just over 20 percent fewer new autoimmune diseases and indications of fewer cancer deaths, especially in people of normal weight.
- The calcium paradox explains the partner: osteocalcin and matrix GLA protein are only activated by vitamin K2 — in the Rotterdam Study, the group with the highest K2 intake had about half the risk of severe vascular calcification.
- Knapen and colleagues gave postmenopausal women 180 micrograms of MK-7 for 3 years: bone density was better preserved and vascular stiffness improved compared with placebo.
- The common regimen: measure first, then 1,000 to 4,000 units of vitamin D with a fat-containing meal plus 100 to 200 micrograms of K2 as MK-7, with a follow-up measurement after 8 to 12 weeks.
- Only 10 to 20 percent of requirements can be covered through diet, while 15 to 30 minutes of midday sun on arms and legs in summer produce several thousand units.
Where the data stop
Most of the positive vitamin D literature consists of observational data in which low levels are linked to almost everything bad. The intervention studies put this into perspective: those who are already well supplied do not benefit from more, and precisely such people were predominantly enrolled. The benefit is concentrated in those who start out deficient — which is why measuring is not optional here. K2 too comes with an honest counterpoint: a newer Danish study in men with existing calcification could not significantly slow its progression. K2 apparently works better at preventing than at reversing.
Two warnings belong here. Vitamin D is toxic in extreme doses over long periods, and the known cases of harm practically always arise from dosing errors with high-dose preparations. And K vitamins are off-limits without medical consultation for anyone taking vitamin K antagonists such as Marcumar.
Listen (in German)
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Published on August 31, 2026.
The topic in the database
- The “sunshine vitamin” – essential for the immune system, bones, mood and more than 1,000 genes.Vitamin D3
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Information only, not medical advice and not a usage or dosage recommendation. Prescription-only and unapproved substances belong in the hands of a physician. Last updated: 2026-09-19.