I like the idea of algae-based calcium, hoping it comes from clean waters though. I kind of worry about animal-sourced calcium because I don't know how much hormones or pesticides or other drugs that livestock are frequently given end up in the calcium.

For those not taking vitamin K as part of a bone-support regime, there are studies behind the value of vitamin K2 in helping bones use the calcium and also helping prevent arterial calcium build-up. That's if my cardiologist is correct. I've been taking calcium for several years and have a very low calcium score and a recent echocardiogram result was good despite chronic highish cholesterol.