Calcium from food sources is the answer. Calcium supplements are associated with heart and coronary artery diseases. Supposedly, calcium as an isolated nutrient, does not help fortify bones or prevent osteoporosis. Instead, it gets into the arteries, stiffening up the endothelium and increasing the risk of heart disease. It's believed to be atherogenic.
People were also commenting about the "calcium paradox" eons ago. In countries that supposedly take in the highest calcium, you find the highest rates of osteoporosis and hip fractures. Why is that the case? Well, because how calcium operates once ingested is complex and is not a one-way street. It has to do with K-2, according to one prominent cardiologist. http://www.prescription2000.com/Interview-Transcripts/2011-02-18-william-castelli-heart-disease-lipids-transcript.html
[T]his came about with the study of these calcium trials where the ladies in New Zealand and Finland put on the calcium supplements ran a higher heart attack rate and the ones on the placebo did not get that and the calcium was going into their arteries instead of their bones. And then this revived this old WHO trial where they went around the world, country by country, measuring hip fracture and came up with the WHO paradox. That the countries that eat the most calcium have the most osteoporosis and the highest hip fracture rate, and these poor countries, they eat hardly any calcium, have hardly any osteoporosis and no hip fracture hardly. Well, they said this demands an explanation. Well it took a while before we learned from our molecular friends who started studying the entry and exit of calcium into your artery wall. And let's face it, you live in a country where 70% of us have calcifications in our major arteries. They found these proteins that control the entry and exit of calcium into your artery wall and then they stimulated these proteins and the thing that worked the best was vitamin K2. And where would you get vitamin K2 in your diet? Well, this is not K1 by the way, it's K2. Well you actually would have to go out on the range and eat all these plants by the gazillions to get enough K2, or you had to live in one of those poor countries where the meats they eat are from animals raised on the range and their meats are loaded with K2 and their milks and their cheeses.
Dr. Davis also comments about the calcium paradox.
Also, Paul Jaminet mentions that Vit K2 strengthens bones while calcium has been found to be dangerous in his PHD. (p. 192 / p. 220). Kudos to Paul.