(this isn't a study, but I've read other articles by this author and she has been reliable)
bok choy .54
rhubarb .08 (high oxalate content)
Interesting how kale, broccoli & bok choy are all around the same figure, a good on at that. Kinda implies that green vegetables are fairly similar and then spinach blows that idea completely out of the water. I recall there was something that's in spinach, might of been vitamin B12 that requires a trick to get it out, adding salt comes to mind but I'm sure that's wrong.
BTW I see broccoli as the wonder vegetable, its good for you on so many fronts and you can get people to eat it, OK not all of them...
On the home front I'm struggling a little to hold the family vegetable intake level. I need to make them more appealing to them.