Per this article, 1 tsp = 800 to 1,000 mg calcium
From link:
Research suggests daily supplement dose of 1,000 mg of calcium and 600 mg of vitamin D can counteract the effects of osteoporosis. However, it is said that older adults might benefit from a slightly higher dose to reduce the risks of fractures and falls.
About 1,300 mg suggested by “Osteoporosis Australia”.
https://morelifehealth.com/articles/eggshells-calcium
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My elderly mom just got a DEXA scan that is a mix of osteopenia & actual osteoporosis so we’ll be doing eggshell version of calcium along with a bunch of other nutritional things she’s either on already (Vit D) or we’re adding plus weight bearing & resistance exercises - no Big Pharma drugs.