Certain green leafy vegetables have a very low calcium bioavailability due to the presence of low calcium content. low vitamin d content. naturally occurring binders. high magnesium content, which lowers calcium absorption.

The answer would be: naturally occurring binders.

Some leafy vegetables like spinach have high in calcium. But spinach has a substance that binds calcium called oxalic acid. This substance will prevent the absorption of the calcium, makes it wasted. There are other calcium binders that can be found like phytate or magnesium.