Fractional magnesium absorption is significantly lower in human subjects from a meal served with an oxalate-rich vegetable, spinach, as compared with a meal served with kale, a vegetable with a low oxalate content

Bohn T1Davidsson LWalczyk THurrell RF
Br J Nutr. 2004 Apr;91(4):601-6

https://www.ncbi.nlm.nih.gov/pubmed/15035687