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I've also heard about it 'hardening' in the bloodstream and that you should really be taking it with K2 as well. This is anecdotal so someone please do more research than basing anything from this comment

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Roughly, Vitamine D3 make the body effective at extracting calcium from food and dumping it into the bloodstream; vitamin k2 make it easier for the body to use that calcium. Too D3 without K2 can cause hypercalcemia; K2 make it less likely but there is still a point where too much D3 is too much!

And too much D3 without calcium supplements make the body use the calcium from it's bones !

there is also some research that suggests that the UV radiation leads to the body producing more nitrous oxide, a noted vasodilator (and occasional sports suppliment)

lots of vit D without the NO there to improve vascularity makes the arteries hard

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