Why do cheap calcium carbonate supplements work as well as expensive ones?

Why do cheap calcium carbonate supplements work as well as expensive ones?
Clinical evidence dating back to a 1989 New England Journal study shows no statistically significant difference in absorption rates among different calcium salts in healthy adults. What truly determines calcium utilization is the body's own calcium status and vitamin D levels, not price. For calcium carbonate specifically, single doses above 500 mg see sharply declining absorption—at 200 mg intake absorption is about 45%, but above 2000 mg it drops to roughly 15%. Splitting daily needs into 200-300 mg doses taken multiple times is more effective than one large pill. However, extremely cheap products warrant caution: raw material quality and heavy metal control vary widely. Major mines in Hubei's Changyang area can have ore grades above 99%, yet heavy metal content differs dramatically between layers. Reputable manufacturers with strict quality systems keep lead levels far lower than small factories with limited testing capability.