It is not really a chink in the theory. I thought I qualified my remark with the codicil "correct nutrients"
I also should mention the other codicil before someone else spots it.
The animal needs to be physiologically able to digest the food provided in order for the nutrient to become avaliable. Therefore I don't think Tim's clever idea regarding peanut butter would work.
The above statement would also give more credence to the "obligate feeder" statements.