@peatlegal it does seem that supplementing does not have the advantages that eating real foods does. Eggshell powder is a supplement. It's not a normal part of anyone's diet. It is primarily calcium carbonate.
Boiling greens and drinking the water, or dairy, are far preferable. A bit of supplementation is fine for calcium if diet can't go far enough, but to get most calcium from supplementing is not healthy.
I'll include this study. Not the kind I really like, but it is one of several concluding what I've said above.
https://jamanetwork.com/journals/jamainternalmedicine/fullarticle/1568523
Importance Calcium intake has been promoted because of its proposed benefit on bone health, particularly among the older population. However, concerns have been raised about the potential adverse effect of high calcium intake on cardiovascular health.
Objective To investigate whether intake of dietary and supplemental calcium is associated with mortality from total cardiovascular disease (CVD), heart disease, and cerebrovascular diseases.
Design and Setting Prospective study from 1995 through 1996 in California, Florida, Louisiana, New Jersey, North Carolina, and Pennsylvania and the 2 metropolitan areas of Atlanta, Georgia, and Detroit, Michigan.
Participants A total of 388 229 men and women aged 50 to 71 years from the National Institutes of Health–AARP Diet and Health Study.
Main Outcome Measures Dietary and supplemental calcium intake was assessed at baseline (1995-1996). Supplemental calcium intake included calcium from multivitamins and individual calcium supplements. Cardiovascular disease deaths were ascertained using the National Death Index. Multivariate Cox proportional hazards regression models adjusted for demographic, lifestyle, and dietary variables were used to estimate relative risks (RRs) and 95% CIs.
Results During a mean of 12 years of follow-up, 7904 and 3874 CVD deaths in men and women, respectively, were identified. Supplements containing calcium were used by 51% of men and 70% of women. In men, supplemental calcium intake was associated with an elevated risk of CVD death (RR>1000 vs 0 mg/d, 1.20; 95% CI, 1.05-1.36), more specifically with heart disease death (RR, 1.19; 95% CI, 1.03-1.37) but not significantly with cerebrovascular disease death (RR, 1.14; 95% CI, 0.81-1.61). In women, supplemental calcium intake was not associated with CVD death (RR, 1.06; 95% CI, 0.96-1.18), heart disease death (1.05; 0.93-1.18), or cerebrovascular disease death (1.08; 0.87-1.33). Dietary calcium intake was unrelated to CVD death in either men or women.
Conclusions and Relevance Our findings suggest that high intake of supplemental calcium is associated with an excess risk of CVD death in men but not in women. Additional studies are needed to investigate the effect of supplemental calcium use beyond bone health.
In Western countries, great emphasis has been put on calcium intake because of its proposed benefit for bone health. Calcium supplementation has become widely used, especially among the elderly population. A recent study1 reported that more than 50% of older men and almost 70% of older women in the United States use supplemental calcium. However, beyond calcium's established role in prevention and treatment of osteoporosis, its health effect on nonskeletal outcomes, including cardiovascular health, remains largely unknown and has become increasingly contentious.2,3
Despite some earlier observational and interventional studies4-6 that suggested a protective role of calcium against cardiovascular diseases (CVDs) by linking supplemental calcium intake with improved blood pressure or serum lipid profiles, recent analyses of several randomized controlled trials (RCTs) found an increased risk of various cardiovascular events, including myocardial infarction, stroke, and cardiovascular deaths, in the intervention arm with calcium supplementation.7-9 Likewise, the effects of dietary calcium intake on various cardiovascular outcomes also remain controversial, with most of the observational studies revealing inverse10,11 or null associations.12-14 The heterogeneity of the aforementioned studies and inconsistency in their results warrant further investigation into the relation between calcium intake and cardiovascular health. Therefore, in a large cohort of US men and women, we investigated whether intake of dietary and supplemental calcium is associated with mortality from total CVD, heart disease, and cerebrovascular diseases.