This depends where and also when the measurements are done on the food supply. The disaster is still ongoing, and they also recently purposely dumped large quantities of contaminated water into the ocean for which the real impact is truely unknown a priori.
Also, as mentioned, it is unlikely decay readings are effective since it is doubtful alpha decay is accounted for from such food screenings using radiation detectors. And even still, even with analytical techniques like mass spec, they are still only sampling food which maybe prone to errors. E.g. i am doubtful heath authorities consider the uneven contamination distribution due to bioaccumulation into their already limited statistical sampling.