I believe the way they put it is that those types of vegetables are acid producing and mildly toxic. This does include spinach by most sources. The veggies which are generally not recommended are: avocado, all potatoes, tomatoes, eggplant, peppers, asparagus, spinach. Also tropical fruits are not recommended unless you live in a tropical area where they are grown locally.
There is a more information posted on this by an gentleman named Bruce on this part of the forum:
http://www.cybermacro.com/Macrobioti...de_Vegetables/
Hope this helps!
D