I just got back from NRT and took a multi-vitamin and fish oil while I was there. It can't hurt.
There are several web sites out there that are tracking radiation in Tokyo and crowd sourced results are pretty consistent at .1 - .2 micro-Sieverts around Narita and Tokyo.
Last week the arrivals were coasting in around Sendai and transiting Honsuhu to the west of Fukushima on the way to Narita. This week, we went R580-OATIS-FACER-JAMES-SWAMP which took us about 95 miles to the east of Fukushima at the closest tangent. Today we departed 34L and got an immediate turn to the southeast which kept us at least 100 miles south of Fukushima. Surface winds were 010/15 so I'd watch the radiation readings for sure. If you're a UA type, you'll be headed back downtown instead of staying in Naritaville.
To be safe, I'm skipping the local fish and dairy products for a while. Tanker bottled water to the hotel as it's soldout everywhere in town. Also, the downtown layover hotel is much better than the airport hotel for dampening earthquakes. There were two yesterday and I barely felt them, at the airport people were running for the door-frames, so hopefully you'll get better rest downtown.