Mom is from Holland - she thinks re-heated spinach is toxic. I've noticed this in all Dutch natives I've met. If you cook spinach, you must eat it right away .... no spinach quiche spanakopita, or anything like that. Apparently spinach leftovers are deadly to the dutch!but not their Amercan children!
http://www.wolfstad.com/2009/05/reheating-spinach-dangerous-in-holland-fine-in-rest-of-world/
If reheating spinach really does cause nitrates, I would tend to believe this. Nitrates have been proven in many studies to cause cancer. We used to be be a big time backyard barbeque family. We loved those pretty black stripes on our meat. My FIL passed away from colon cancer. My MIL passed away from cancer. I don't have any cancer in my family history at all, but after years of living my husband's family lifestyle, I had a polyp removed that was precancerous - a very fast type growing cancer cell and was told to eliminate eating nitrates as much as possible because it was proven in multiple studies that that particular cancer cell was caused by nitrates. They strongly stressed that I avoid nitrates. Thankfully they caught it - they said had I not had an early colonoscopy, I would not have lived to see 50. I radically changed what I was eating and try not to eat foots that are laden in nitrates (blackened meats, packaged sandwich meats, deli meats, hot dogs, etc.) I was tested again 3 years later and so far so good. So, if reheating spinach really does cause nitrates, it could actually be deadly to American children, too.
We're still somewhat of a barbeque family - meats, though, now get cooked in tin foil so they don't develop any blackening at all. I avoid deli-type meats, as well, as much as I can.