I feed nothing as a main food, variety is key.But not regularly or as the main food?
Regularly as a small part of a mixed diet is okay, I believe.
Look at the Oxalic Acid and Goitrogenic Compounds sections at the top of the following.
http://www.chelonia.org/Articles/nutrientanalysis.htm
My little personal experience and large amounts of reading, including on this forum, lends me to agree.