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 !