Magnesium oxide has been used in many trials but it is not absorbed very well. Sakaguchi et al (2019) conducted a 2-year, open label, randomized, controlled trial on patients with stage 3-4 chronic kidney disease with risk factors for CAC (diabetes mellitus, history of cardiovascular disease or smoking). The incease in CAC score was 11.3% in the high dose magnesium oxide group and 39.5% in the control group.

The proportion of patients with an annualized percentage change in CAC score less than 15% was 62% in the high dose magnesium oxide group compared with 23.9% in the control group.The trial clearly showed magnesium supplementation slowed the annual increase in calcium scores.

Magnesium deficiency is only one of the major drivers of heart disease.

Magnesium oxide did not suppress the progression of thoracic aorta calcification.

https://www.ncbi.nlm.nih.gov/pubmed/31036759

A woman who took magnesium supplements for 10 weeks  had an ultrasound scan of her carotid artery which showed a 30-40% reduction in calcified plaque. (n=1)