Rat metabolism is 3x that of humans, so of course they can nibble all day, other factors must be considered though, (e.g. certain drugs are metabolized faster in rats).

A COMPREHENSIVE QUANTITATIVE AND QUALITATIVE EVALUATION OF EXTRAPOLATION OF INTRAVENOUS PHARMACOKINETIC PARAMETERS FROM RAT, DOG, AND MONKEY TO HUMANS. I. CLEARANCE
http://dmd.aspetjournals.org/content/32/6/603.full