Whenever I have a food that I'm questioning for my beardies, I run it through the USDA Nutrient Database. That's where I got all my info for my food chart.
Here's what the USDA says about Kellogg's Corn Flakes...
http://www.nal.usda.gov/fnic/cgi-bin/list_nut.pl
Basically the calcium to phosphorus ratio is way off, it's 1:7.1. I don't think corn flakes would harm your beardie as a rare treat, but I'd limit his intake of them because of the phosphorus ratio. And I'm no expert, but it looks like corn flakes contain a good amount of sodium.
Hope this helps.
Veronica
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Beautiful Dragons