Is it safe to reheat leftover cooked spinach?

It isn't a good idea. Here's why: Certain bacteria that grow on spinach and similar nitrate-rich food can produce enzymes that convert nitrates to nitrites--substances that can be harmful to young infants. These nitrate-converting enzymes are more active during the second heating. For older children and adults, small concentrations of nitrites are harmless, though formation of nitrosamine compounds from the nitrites could be of concern.
 
Source(s):
Wikibooks.org "Cookbook: Spinach"