Discussions of 'calcium absorption' helped me appreciate the complexity associated with calcium! Oh boy! Let's see what's going on.

NIH Article on Calcium explains:

1. Calcium absorption rate is different for different food sources! What does that mean? Let's say two foods are listed as containing 100 mg of calcium. Then it's possible that we absorb 30 mg from the first food source but only 10 mg from the second source. What?? How may we operationalize this knowledge? Do we have tables listing calcium absorption rates of hundreds of foods? I couldn't find one. In his Optimum Nutrition guidelines, Dr Greger simply says,

   Calcium: At least 600 mg daily via calcium-rich plant foods — preferably low-oxalate dark green leafy vegetables, which includes all greens except spinach, chard, and beet greens (all very healthy foods, but not good calcium sources due to their oxalate content).

   Turns out that low-oxalate dark leafy greens do have high absorption rates. But Dr Greger doesn't encourage us to go and study calcium absorption rates. He has simplified the recommendations to: (a) consume ≥600 mg calcium daily, and (b) prefer low-oxalate dark leafy greens.

2. Presence of other foods (with phytic acid or oxalic acid) may inhibit absorption.
3. Our body increases / decreases absorption as a function of our age, our life stage (childhood, teenage, pregnancy, …) and in response to low / adequate calcium in our body.
4. Absorption is also a function of Vitamin D.
5. Absorption is also a function of how much total calcium we're consuming!
6. (to make matters more complicated) Some absorbed calcium is eliminated from our body in urine, feces and sweat… and this elimination is a function of many factors (sodium intake, protein intake, caffeine intake, alcohol intake, phosphorus intake, fruit and veggie intake, ..)!

Note: The points listed above are not the complete story! Please see NIH Article on Calcium for details.

After learning that so many factors influence calcium absorption, I thought, "maybe the complexity outlined above is the reason behind non-consensus for calcium RDA across the world — different countries have different guidelines!"