Is pet food safe for humans?

I've often read and seen reports on TV that old people often resort to eating canned cat food because they're too poor to buy any human food. It's quite sad, really. I guess it's safe enough, but I can't imagine it's all that healthy for a human to eat as their main diet! I remember when I was very little I tried eating my dog's Milk Bone treat since he loved them so much! It wasn't any good though! LOL