October 26, 2026 · Assay Supply Co.
Did You Know? Vitamin D Is Not Really a Vitamin (and K2 Is Its Traffic Cop)
Did you know vitamin D isn't really a vitamin in the usual sense? Vitamins are things your body can't make. Vitamin D, your skin manufactures from sunlight — it behaves more like a hormone your body produces when the sun shows up. Which raises a question: what happens in winter, up north, or after months of working indoors? Production drops. That's the whole story of why D is in a bottle.
D3 has a partner, and the partner has the map
Here's the part most people never hear. Vitamin D3 helps your body absorb calcium from food. But absorbing calcium is only half the job — the calcium still has to end up in the right places. That's vitamin K2's department: it supports normal calcium metabolism, the system that keeps calcium moving where the body uses it.
Think of D3 as the delivery driver who picks up the calcium, and K2 as the dispatcher who tells it where to go. The two jobs are different — which is why the pair travels together.
A quick word about natto
Did you know the richest natural source of K2 (in the long-lasting MK-7 form) is natto — sticky, stringy, fermented Japanese soybeans that even many fans describe as an acquired taste? If "acquired" sounds like work, a capsule is the other way to get MK-7. Ours has 100 mcg per capsule.
Full strength, honestly labeled
Our Vitamin D3 + K2 capsule is a full 5,000 IU of D3. We don't hide that — and because 5,000 IU is above the standard 4,000 IU daily upper limit for adults, our label says one capsule daily, do not exceed, right on the bottle. One a day also means one bottle is a 60-day supply. And if you take blood-thinning medication like warfarin, our label says to talk to your physician first — K2 and warfarin genuinely interact, and we'd rather print that in plain sight than in fine print.
Supports normal calcium metabolism.*
Sources
- National Institutes of Health, Office of Dietary Supplements — Vitamin D: Fact Sheet for Health Professionals (skin synthesis, calcium absorption, 4,000 IU adult upper limit).
- National Institutes of Health, Office of Dietary Supplements — Vitamin K: Fact Sheet for Health Professionals (K2 forms including MK-7, and the warfarin interaction).
*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.
*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.
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