A few years back, around 2007-2008, I started hearing a lot of people with concerns about added synthetic Vitamin K in dog food. Apparently there are some studies that point to it being toxic in large amounts, but I don't have an idea of what counts as a "large amount" and it isn't clear how definitive those studies are. For about a year we had to be aware of which foods had the synthetic K because we had many customers who wouldn't feed it, but that seemed to die down after a while. Possibly this is because the better brands either stopped adding Vitamin K or switched to a natural source; I'm not sure. But it was extremely difficult, at the time, for us to sift through various sources to find actual unbiased information on the issue.